and ,dedicated ,to ,destanie2 ,with ,hopes ,her ,dream of ,becoming ,a veterinarian ,comes ,true ,special ,thanks to ,deborah ,furness of the ,university ,college ,london for her help1 and research1 in learning about this book1 and helping me understand it better4 ,spellchecked with www4thesolutioncafe4com file which includes the original illustrations4 ,see #bagha-;h4htm or #bagha-;h4zip3 7https3_/_/www4gutenberg4org_/dirs_/#b_/ a_/g_/h_/bagha_/bagha-;h_/#bagha-;h4htm7 or 7https3_/_/www4gutenberg4org_/dirs_/#b_/ a_/g_/h_/bagha_/bagha-;h4zip7 ,transcriber's ,note3 ,i try to edit my e-texts so they can easily be used with voice speech programs1 ,i believe blind people and children should also be able to enjoy the many books now available electronically4 ,i use the -- for an em-dash1 with a space either before or after it depending on its usage4 ,this helps to keep certain programs from squishing the words together1 such as down-stairs4 ,also to help voice speech programs ,i've enclosed upper case text between - and 7-,,upper ,,case ,,text71 and used underscores to show chapter and section headers4 ,i also added a second contents that shows the other sections of this e-text4 ,this e-text was made with a ,top can text scanner1 with a bit of correcting here and there4 ,this book is volume one of two4 ,it was later reworked by ,a4 ,m4 ,davies in #ahih under the title ,text-book of ,zoology1 then revised and rewritten by ,j4 ,t4 ,cunningham about #aiji and ,w4 ,h4 ,leigh-,sharpe around #aicb4 ,although these editions gave ,wells the main credit1 most of ,wells' writing and all his drawings were removed2 only his rough outline seems to have been used4 ,it was re-published by ,university ,tutorial ,press4 ,the ,first ,edition1 as well as ,the ,second and ,revised ,edition 7with dissections redrawn by ,miss ,a4 ,c4 ,robbins7 are used in this e-text4 ,the ,first ,edition had some small minor errors1 as well as dissection abbreviations that are shown on the ,dissection ,sheets1 but no mention of them was listed in the text4 ,certain figures on the ,dissections ,sheets are missing 7such as ,figures #a1 #b1 #d1 with no mention to a #c1 as if ,mr4 ,wells drew a ,figure #c but found it was not needed and removed it from the book74 ,rather then leaving it as is1 ,i put marks around my notes saying things like ,no ,figure #c4 ,for the ,second and ,revised ,edition ,wells was able to change some of these errors and missing parts1 but many of the same printing tablets were used and with almost each addition other things were removed1 7in one instance one entire section from a chapter71 and many of the helpful suggestions were shortened or removed so other things could be explained more4 ,in an ideal version of the book both could have been used1 but with reprinting the entire book from the first to the second editions almost as many things were lost as were gained1 so ,i've tried to indicate where both text go separate paths with the following2 ,second ,edition only text and -,first ,edition only text1- and also ,lines from ,second ,edition only4 and ,lines from ,first ,edition only4 were more then just a sentence is added or removed4 ,other things to notice is how some words are spelt or punctuated differently throughout the book1 such as2 ,blood ,vessels ,blood-,vessels ,bloodvessels ,i've tried to keep these as close to the original book as possible4 ,university ,correspondence ,college ,tutorial ,series4 -,text-,book ,of ,biology4 by ,h4 ,g4 ,wells1 ,bachelor of ,science1 ,london41 ,fellow of the ,zoological ,society4 ,lecturer in ,biology at ,university ,tutorial ,college4 ,with ,an ,introduction by ,g4 ,b4 ,howes1 ,fellow of the ,linnean ,society1 ,fellow of the ,zoological ,society4 ,assistant ,professor of ,zoology1 ,royal ,college of ,science1 ,london4 ,part #a4-- ,vertebrata4 ,contents ,introduction ,preface ,the ,rabbit-- #a4 ,external ,form and ,general ,considerations #b4 ,the ,alimentary ,canal of the ,rabbit #c4 ,the ,circulation #d4 ,the ,amoeba1 ,cells and ,tissue #e4 ,the ,skeleton #f4 ,muscle and ,nerve #g4 ,the ,nervous ,system #h4 ,renal and ,reproductive ,organs #i4 ,classificatory ,points #aj4 ,questions and ,exercises ,the ,frog-- #a4 ,general ,anatomy #b4 ,the ,skull of the ,frog 7and the ,vertebrate ,skull generally7 #c4 ,questions on the ,frog ,the ,dog-,fish-- #a4 ,general ,anatomy #b4 ,questions on the ,dog-,fish ,amphioxus-- #a4 ,anatomy #b4 ,the ,development of ,amphioxus #c4 ,questions on ,amphioxus ,development-- ,the ,development of the ,frog ,the ,development of the ,fowl ,the ,development of the ,rabbit ,the ,theory of ,evolution ,questions on ,embryology ,miscellaneous ,questions ,note on ,making ,comparisons ,syllabus of ,practical ,work ,contents part #b ,key for ,dissection ,sheets1 and ,abbreviations -,introduction ,in the year #ahhd ,i was invited to give tuition by correspondence1 in ,biology4 ,although disposed at the time to ridicule the idea of imparting instruction in natural science by letter1 ,i gladly accepted the opportunity thus afforded me of ascertaining for myself what could and could not be accomplished in that direction4 ,anyone familiar with the scope of biological enquiry1 and the methods of biological instruction1 will not need to be reminded that it is only by the most rigorous employment of precise directions for observation1 that any good results are to be looked for at the hand of the elementary student4 ,true to this principle1 ,i determined to issue to my correspondence pupils rigid instructions1 and to demand in return faithful annotated drawings of facts observed in their usage4 ,in the case of two among the few students who passed through my hands1 the result far exceeded my most sanguine anticipations4 ,the notes sent in by one of them-- a man working at a distance1 alone and unaided-- far excelled those wrung from many a student placed under the most favourable surroundings2 and their promise for the future has been fulfilled to the utmost1 the individual in question being now a recognised investigator4 ,it thus became clear that1 not-with-standing the complex conditions of work in the biological field1 tuition by correspondence would suffice to awaken the latent abilities of a naturally qualified enquirer4 ,the average members of a ,university ,correspondence ,class will be found neither better nor worse than those of any other1 and they may therefore pass unnoticed2 if however1 the correspondence system of tuition may furnish the means of arousing a latent aptitude1 when the possibilities of other methods of approach are excluded-- and in so doing1 of elevating the individual to that position for which he was by nature qualified1 ensuring him the introduction to the one sphere of labour for which he was born-- it will have created its own defence1 and have merited the confidence of all right-thinking people4 ,the plucking of one such brand from the burning is ample compensation for the energy expended on any number of average dullards1 who but require to be left alone to find their natural level4 ,mr4 ,wells' little book is avowedly written for examination purposes1 and in conformity with the requirements of the now familiar type system of teaching4 ,recent attempts have been made to depreciate this4 ,while affording a discipline in detailed observation and manipulation second to that of no other branch of learning1 it provides for that deduction and verification by which all science has been built up2 and this appears to me ample justification for its retention1 as the most rational system which can be to-day adopted4 ,evidence that its alleged shortcomings are due rather to defective handling than to any inherent weakness of its own1 would not be difficult to produce4 ,although rigid in its discipline1 it admits of commentatorial treatment which1 while heightening the interest of the student1 is calculated to stimulate alike his ambition and his imagination4 ,that the sister sciences of ,botany and ,zoology fall under one discipline1 is expressed in the ,english usage of the term ,biology4 ,experience has shown that the best work in either department has been produced by those who have acquired on all-round knowledge of at least the elementary stages of both2 and1 that the advanced morphologist and physiologist are alike the better for a familiarity with the principles-- not to say with the progressive advancement-- of each other's domain1 is to-day undeniable4 ,these and other allied considerations1 render it advisable that the elementary facts of morphology and physiology should be presented to the beginner side by side-- a principle too frequently neglected in books which1 like this one1 are specially written for the biological neophyte4 ,although the student is the wiser for the actual observation of the fact of nature1 he becomes the better only when able to apply them1 as for example1 by the judicious construction of elementary generalizations1 such as are introduced into the pages of this work4 ,so long as these generalizations1 regarded as first attempts to deduce laws in the form of generalized statement of facts based observation1 are properly introduced into an elementary text-book1 intended for the isolated worker cut off from the lecture room1 their intercalation is both healthy and desirable4 ,mr4 ,wells has kept these precepts constantly in mind in the preparation of his work1 and in the formulation of his plans for its future extension1 thereby enhancing the value of the book itself1 and at the same time1 discouraging the system of pure cram1 which is alien to the discipline of biological science4 ,g4 ,b4 ,howes ,royal ,college of ,science1 ,south ,kensington2 ,november #cj1 #ahib4 -,preface ,no method of studying-- more especially when the objects of study are tangible things-- can rival that prosecuted under the direction and in the constant presence of a teacher who has also a living and vivid knowledge of the matter which he handles with the student4 ,in the ideal world there is a plentiful supply of such teachers1 and easy access to their teaching1 but in this real world only a favoured few enjoy these advantages4 ,through causes that cannot be discussed here1 a vast number of solitary workers are scattered through the country1 to whom sustained help in this form is impossible1 or possible only in days stolen from a needed vacation2 and to such students especially does this book appeal1 as well as to those more fortunate learners who are within reach of orderly instruction1 but anxious to save their teachers' patience and their own time by some preliminary work4 ,one of the most manifest disadvantages of book-work1 under the conditions of the solitary worker1 is the rigidity of its expressions2 if the exact meaning is doubtful1 he can not ask a question4 ,this has been kept in view throughout2 the writer has1 above all1 sought to be explicit-- has1 saving over-sights1 used no uncommon or technical term without a definition or a clear indication of its meaning4 ,in this study of ,biology1 the perception and memory of form is a very important factor indeed4 ,every student should draw sketches of his dissections1 and accustom himself to copying book diagrams1 in order to train his eye to perception of details he might otherwise disregard4 ,the drawing required is within the reach of all2 but for those who are very inexperienced1 tracing figures is a useful preliminary exercise4 ,by the time the student has read the ,circulation of the ,rabbit 7,sections #cd to #di71 he will be ready to begin dissection4 ,it is possible to hunt to death even such a sound educational maxim as the thing before the name1 and we are persuaded1 by a considerable experience1 that dissection before some such preparatory reading is altogether a mistake4 ,at the end of the book is a syllabus 7with suggestions7 for practical work1 originally drawn up by the writer for his own private use with the evening classes of the ,university ,tutorial ,college-- classes of students working mainly in their spare time for the ,london examination1 and at an enormous disadvantage1 as regards the number of hours available1 in comparison with the leisurely students of a ,university laboratory4 ,this syllabus may1 perhaps by itself1 serve a useful purpose in some cases1 but in this essential part of the study the presence of some experienced overlooker to advise1 warn1 and correct1 is at first almost indispensable4 ,a few words may1 perhaps be said with respect to the design of this volume4 ,it is manifestly modelled upon the syllabus of the ,intermediate ,examination in ,science of ,london ,university4 ,that syllabus1 as at present constituted1 appears to me to afford considerable scope for fairly efficient biological study4 ,the four types dealt with in this book are extremely convenient for developing the methods of comparative anatomy and morphological embryology4 ,without any extensive reference to related organisms1 these four forms1 and especially the three vertebrata1 may be made to explain and illustrate one another in a way that cannot fail to be educational in the truest sense4 ,after dealing with the rabbit1 therefore1 as an organic mechanism1 our sections upon the frog and dog-fish1 and upon development1 are simply statements of differences1 and a commentary1 as it were1 upon the anatomy of the mammalian type4 ,in the concluding chapter1 a few suggestions of the most elementary ideas of it is hoped to make this first part of our biological course complete in itself1 and of some real and permanent value to the student4 ,and the writer is convinced that not only is a constant insistence upon resemblances and differences1 and their import1 intellectually the most valuable1 but also the most interesting1 and therefore the easiest1 way of studying animal anatomy4 ,that chaotic and breathless cramming of terms misunderstood1 tabulated statements1 formulated tips1 and lists of names1 in which so many students1 in spite of advice1 waste their youth is1 ,i sincerely hope1 as impossible with this book as it is useless for the purposes of a ,london candidate4 ,on the other hand1 our chief endeavour has been to render the matter of the book clear1 connected1 progressive1 and easily assimilable4 ,in the second part ,plants1 ,unicellular ,organisms1 and ,invertebrata will be dealt with1 in a wider and less detailed view of the entire biological province4 ,lines from ,first ,edition only4 -,in this volume1 we study four organisms1 and chiefly in their relation to each other2 in the next1 we shall study a number of organisms largely in relation to their environment4 ,in this part our key note is the evidence of inheritance2 in our second part it will be of adaptation to circumstances4- ,this book will speedily1 under the scrutiny of the critical reader1 reveal abundant weakness4 ,for these the author claims the full credit4 ,for whatever merit it may posses1 he must however1 acknowledge his profound indebtedness to his former teacher1 ,professor ,howes4 ,not only has the writer enjoyed in the past the privilege of ,professor ,howes' instruction and example1 but he has1 during the preparation of this work1 received the readiest help1 advise1 and encouragement from him-- assistance as generous as it was unmerited1 and as unaffected as it was valuable4 ,lines from ,second ,edition only4 ,the publication of a second and revised edition of this ,part affords the author an opportunity of expressing his sense of the general kindliness of his reviewers1 and the help they have him in improving this maiden effort4 ,to no one is there vouchsafed such a facility in the discovery of errors in a book as to its author1 so soon as it has passed beyond his power of correction4 ,hence the general tone of encouragement 7and in some cases the decided approval7 of the members of this termination to a period of considerable remorse and apprehension4 ,i have been able through their counsel1 and the experience ,i have had while using this book in teaching1 to correct several printer's errors and to alter various ambiguous or misleading expressions1 as well as to bring the book up to date again in one or two particulars4 ,my thanks are particularly due to my friend ,miss ,robbins1 who has very kindly redrawn the occasionally rather blottesque figures of the first edition4 ,not only have these plates gained immensely in grace and accuracy1 but the lettering is now distinct-- an improvement that any student who has had to hunt my reference letters in the first edition will at once appreciate4 ,h4 ,g4 ,wells ,november1 #ahib4 ,first ,edition4 ,december1 #ahic4 ,second ,edition4 -,the ,rabbit4 #a4 ,external ,form and ,general ,considerations4 ,section #a4 ,it is unnecessary to enter upon a description of the appearance of this familiar type1 but it is not perhaps superfluous1 as we proceed to consider its anatomy1 to call attention to one or two points in its external1 or externally apparent structure4 ,most of our readers know that it belongs to that one of two primary animal divisions which is called the vertebrata1 and that the distinctive feature which place it in this division is the possession of a spinal column or backbone1 really a series of small ring-like bones1 the vertebrae 7,figure #a v4b47 strung together1 as it were1 on the main nerve axis1 the spinal cord 7,figure #a s4c474 ,this spinal column can be felt along the neck and back to the tail4 ,this tail is small1 tilted up1 and conspicuously white beneath1 and it serves as a recognition mark to guide the young when1 during feeding1 an alarm is given and a bolt is made for the burrows4 ,in those more primitive 7older and simpler-fashioned7 vertebrata1 the fishes1 the tail is much large and far more important1 as compared with the rest of the body1 than it is in most of the air-inhabiting vertebrates4 ,in the former it is invariably a great muscular mass to propel the body forward2 in the latter it may disappear1 as in the frog1 be simply a feather-bearing stump1 as in the pigeon1 a fly flicker1 as in the cow or horse1 a fur cape in squirrel1 or be otherwise reduced and modified to meet special requirements4 ,section #b4 ,at the fore end1 or as ,english zoologists prefer to say1 anterior end1 of the vertebral column of the rabbit1 is of course the skull1 containing the anterior portion of the nerve axis1 the brain 7,figure #a br474 ,between the head and what is called the body1 in the more restricted sense of the word1 is the neck4 ,the neck gives freedom of movement to the head1 enables the animal to look this way and that1 to turn its ears about to determine the direction of a sound1 and to perform endless motions in connexion with biting and so forth easily4 ,we may note that in types which swim through the water1 the neck dose not appear-- in the fish and frog1 for instance-- and the head simply widens out as one passes back to the body4 ,the high resistance offered by water necessitates this tendency to a cigar or ship outline1 just as it has determined the cigar shape of the ordinary fish torpedo4 ,section #c4 ,in the body of the rabbit1 as examined from the outside1 we can make out by feeling two distinct regions1 just as we might in the body of a man2 anteriorily a bony cage1 having the ribs at the sides1 a rod-like bone in the front1 the sternum 7,figure #a -st4-1 stm471 and the backbone behind1 and called the chest or thorax2 and posteriorily a part called the abdomen1 which has no bony protection over its belly1 or ventral surface4 ,these parts together with the neck constitute the trunk4 ,as a consequence of these things1 in the backbone of the rabbit there are four regions3 the neck1 or cervical part1 consisting of seven vertebrae1 the thoracic part of twelve joined to ribs1 the abdominal 7also called the lumbar7 region of seven without ribs1 and the tail or caudal of about fifteen4 ,between the lumbar and caudal come four vertebrae1 the sacral1 which tend to run together into a bony mass as the animal grows old1 and which form a firm attachment for the base of the hind limb4 ,section #d4 ,the thorax and abdomen are separated by a partition1 the diaphragm 7,figure #a dia474 ,this structure is distinctive of that class of the vertebrata called mammals1 and which includes man1 most of the larger and commoner land animals1 and whales and manatee4 ,we shall find later that it is essentially connected with the perfection of the air breathing to which this group has attained4 ,another characteristic shared by all mammals1 and by no other creature1 is the presence of hair4 ,in birds we have an equally characteristic cover in the feathers1 the frog is naked1 and the fishes we find either naked skins or scales4 ,section #e4 ,the short strong fore limbs are adapted to the burrowing habit1 and have five digits2 the hind limbs are very much longer and muscular1 enable the animal to progress rapidly by short leaps1 and they have four toes4 ,if the student thinks it worth while to attempt to remember the number of digits-- it is the fault of examiners if any value dose attach to such intrinsically valueless facts-- he should associate the number #ed 7#e in front1 #d behind7 with the rabbit1 and observe that with the frog the reverse is the case4 ,section #f4 ,we may note here the meaning of certain terms we shall be constantly employing4 ,the head end of the rabbit is anterior1 the tail end posterior1 the backbone side of the body-- the upper side in life-- is dorsal1 the breast and belly side1 the lower side of the animal1 is ventral4 ,if we imagine the rabbit sawn asunder1 as it were1 by a plane passing through the head and tail1 that would be the median plane1 and parts on either side of it are lateral1 and left or right according as they lie to the animal's left or right4 ,in a limb1 or in the internal organs1 the part nearest the central organ1 or axis1 is proximal1 the more remote or terminal parts are distal4 ,for instance1 the mouth is anteriorly placed1 the tongue on its ventral wall2 the tongue is median1 the eyes are lateral1 and the fingers are distal to the elbow4 ,the student must accustom himself to these words1 and avoid1 in his descriptions1 the use of such terms as above1 below1 outside1 which vary with the position in which we conceive the animal placed4 ,section #g4 ,so much for the general form2 we may note a few facts of general knowledge1 in connection with the rabbit's life-activity4 ,in a day of the rabbit's life a considerable amount of work is done-- the animal runs hither and thither1 for instance2 in other words1 a certain mass of matter is moved through space1 and for that we know force must be exerted4 ,whence comes the force8 ,section #h4 ,we find the rabbit occupies a considerable amount of its time in taking in vegetable matter1 consisting chiefly of more or less complex combustible and unstable organic compounds4 ,it is a pure vegetarian1 and a remarkably moderate drinker4 ,some but only a small proportion1 of the vegetable matter it eats1 leaves its body comparatively unchanged1 in little pellets1 the faeces1 in the process of defaecation4 ,for the rest we have to account4 ,section #i4 ,we find1 also1 that the rabbit breathes air into its lungs1 which is returned to the atmosphere with a lessened amount of oxygen1 and the addition of a perceptible amount of carbon dioxide4 ,the rabbit also throws off1 or excretes1 a fluid1 the urine1 which consists of water with a certain partially oxydised substance containing nitrogen1 and called urea1 and other less important salts4 ,the organs within the body1 by which the urine is separated1 are called the kidneys4 ,section #aj4 ,repeating these facts in other words1 the rabbit takes into its body complex and unstable organic compounds containing nitrogen1 carbon1 hydrogen1 a certain amount of oxygen1 a small quantity of sulphur1 and still smaller amounts of other elements4 ,it also breathes in oxygen4 ,section #aa4 ,it returns a certain rejected part of its food comparatively unchanged4 ,besides this1 it returns carbon dioxide and water1 which are completely oxydised1 and very simple and stable bodies1 and urea-- a less completely oxydised compound1 but a very simple one compared with the food constituents4 ,section #ab4 ,now the chemist tells us that when a stable body is formed1 or when an unstable compound decomposes into simpler stable ones1 force is evolved4 ,the oxydation of carbon1 for instance1 in the fireplace1 is the formation of the stable compound called carbon dioxide1 and light and heat are evolved4 ,the explosion of dynamite1 again is the decomposition of an unstable compound4 ,hence1 we begin to perceive that force-- the vital force-- which keeps the rabbit moving1 is supplied by the decomposition and partial oxydation of compounds continued in its food1 to carbon dioxide1 water1 urea1 and smaller quantities of other substances4 ,section #ac4 ,this is the roughest statement of the case possible1 but it will give the general idea underlying our next chapters4 ,we shall consider how the food enters the body and is taken up into the system1 how it is conveyed to the muscles in the limbs1 to the nerve centres1 and to wherever work is done1 to be there decomposed and partially oxydised1 and finally how the products of its activity-- the katastases1 of which the three principal are carbon dioxide1 water1 and urea-- are removed from the body4 ,section #ad4 ,there are one or two comparatively modern terms that we may note here4 ,this decomposition of unstable chemical compounds1 releasing energy1 is called kataboly4 ,a reverse process1 which has a less conspicuous part in our first view of the animal's life action1 by which unstable compounds are built up and energy stored1 is called anaboly4 ,the katastases are the products of kataboly4 ,section #ae4 ,in an ordinary animal1 locomotion and other activity predominate over nutritive processes1 which fact we may express1 in the terms just given1 by saying that kataboly prevails over anaboly4 ,an animal1 as we have just explained1 is an apparatus for the decomposition and partial oxydation of certain compounds1 and these are obtained either directly or indirectly-- through other animals1 in the case of meat-eaters-- from the vegetable kingdom4 ,as the student will learn early in his botanical reading1 the typical plant has1 in its green colouring matter1 chlorophyll1 a trap to catch the radiating energy of the sun1 and to accomplish1 by the absorption of that energy1 the synthesis 7building up7 of those organic compounds which the animal destroys4 ,the typical plant is1 on whole1 passive and synthetic1 or anabolic2 the typical animal1 active and katabolic2 and the excess of kataboly over anaboly in the animal is compensated for by the anabolic work stored up1 as it were1 by the plant1 which is1 directly or indirectly1 the animal's food4 #b4 ,the ,alimentary ,canal of the ,rabbit ,section #af4 ,figure #a represents the general anatomy of the rabbit1 but is especially intended to show the alimentary 7 food7 canal1 shortened to a certain extent1 and with the proportions altered1 in order to avoid any confusing complications4 ,it is evidently simply a coiled tube-- coiled for the sake of packing-- with occasional dilatations1 and with one side-shunt1 the caecum 7cae471 into which the food enters1 and is returned to the main line1 after probably absorbent action1 imperfectly understood at present4 ,a spiral fold in this cul-de-sac bottom-of-sack1 which is marked externally by constrictions1 has a directive influence on the circulation of its contents4 ,the student should sketch ,figure #a once or twice1 and make himself familiar with the order and names of the parts before proceeding4 ,we have1 in succession1 the mouth 7,m471 separated from the nasal passage 7,na47 above the palate2 the pharynx 7ph471 where the right and left nasal passages open by the posterior nares into the mouth2 the oesophagus 7oes472 the bag-like stomach1 its left 7,section #f7 end being called the cardiac 7cd4st471 and its right the pyloric end 7py472 the ,u-shaped duodenum 7ddnm47 and the very long and greatly coiled ileum 7il474 ,the duodenum and ileum together form the small intestine2 and the ileum is dilated at its distal end into a thick-walled sacculus rotundus 7s4r471 beyond which point comes the large intestine4 ,the colon 7co47 and rectum 7r47 continue the main line of the alimentary canal2 but1 at the beginning of the large intestine1 there is also inserted a great side-shunt1 the caecum 7cae471 ending blindly in a fleshy vermiform appendix 7v4ap474 ,the figure will indicate how the parts are related better than any verbal description can4 ,between the coiling alimentary tube and the body walls is a space1 into which the student cuts when he begins dissecting2 this is the peritoneal cavity 7pt474 ,a thin1 transparent membrane1 the mesentery1 holds the intestines in place1 and binds them to the dorsal wall of this peritoneal space4 ,section #ag4 ,the food stuffs of an animal1 the unstable compounds destined ultimately to be worked into its life1 and to leave it again in the form of katastases 7,section #ac71 fall into two main divisions4 ,the first of these includes the non-nitrogenous food stuffs1 containing either carbon together with hydrogen and oxygen in the proportion of ,h#b,o 7the carbo-hydrates71 or carbon and hydrogen without oxygen 7the hydrocarbons74 ,the second division consists of the nitrogenous materials1 containing also carbon1 hydrogen1 a certain amount of oxygen1 sulphur1 and possibly other elements4 ,among the carbohydrates1 the commonest are starch and cellulose1 which are insoluble bodies1 and sugar1 which is soluble4 ,the hydrocarbons1 fats1 oils1 and so on1 form a comparatively small proportion of the rabbit's diet2 the proverb of oil and water will remind the student that these are insoluble4 ,the nitrogenous bodies have their type in the albumen of an egg2 and muscle substance and the less modified living protoplasm of plants1 a considerable proportion of the substance of seeds1 bulbs1 and so on1 are albuminous bodies1 or proteids4 ,these also are insoluble bodies1 or when soluble1 will not diffuse easily through animal membranes4 ,section #ah4 ,now the essential problem which the digestive canal of the rabbit solves is to get these insoluble1 or quasi-insoluble1 bodies into its blood and system4 ,they have to pass somehow into the circulation through the walls of the alimentary canal4 ,in order that a compound should diffuse through a membrane1 it must be both soluble and diffusible1 and therefore an essential preliminary to the absorption of nutritive matter is its conversion into a diffusible soluble form4 ,this is effected by certain fluids1 formed either by the walls of the alimentary canal or by certain organs called glands1 which open by ducts into it2 all these fluids contain small quantities of organic compounds of the class called ferments1 and these are the active agents in the change4 ,the soluble form of the carbohydrates is sugar2 proteids can be changed into the1 of course1 chemically equivalent but soluble and diffusible the peptones2 and fats and oils undergo a more complicated1 but finally similar change4 ,section #ai4 ,we shall discuss the structure and action of -a gland- glands a little more fully in a subsequent chapter4 ,here we will simply say that they are organs forming each its characteristic fluid or secretion1 and sending it by a conduit1 the duct1 to the point where its presence is required4 ,the saliva in our mouths1 tears1 and perspiration1 are examples of the secretions of glands4 ,section #bj4 ,in the month of the rabbit the food is acted upon by the teeth and saliva4 ,the saliva contains ptyalin1 a ferment converting starch into sugar1 and it also serves to moisten the food as it is ground up by the cheek teeth4 ,it does not act on fat to any appreciable extent4 ,the teeth of the rabbit are shown in ,figure ,,xviii41 ,sheet #d4 ,the incisor teeth in front1 two pairs above and one pair below 7i471 are simply employed in grasping the food2 the cheek teeth-- the premolars 7pm47 and molars 7m47 behind-- triturate the food by a complicated motion over each4 ,their crowns are flat for this purpose1 with harder ridges running across them4 ,section #ba4 ,this grinding up of the food in the mouth invariably occurs in herbivorous animals1 where there is a considerable amount of starch and comparatively little hydrocarbon in the food4 ,by finely dividing the food1 it ensures its intimate contact with the digestive ferment1 ptyalin4 ,in such meat-eaters as the cat and dog1 where little starchy matter and much fat is taken1 the saliva is1 of course1 of less importance1 and this mastication does not occur4 ,the cheek teeth of a dog 7,section #ia71 and more so of a cat1 are sharp1 and used for gnawing off fragments of food1 which are swallowed at once4 ,between the incisors and premolars of a dog come the characteristic biting teeth1 or canines1 absent in the rabbit4 ,section #bb4 ,the student will probably ask why the cheek teeth1 which are all similar in appearance1 are divided into premolars and molars4 ,the rabbit has a set of milk molars-- a milk dentition-- which are followed by the permanent teeth1 just as in man4 ,those cheek teeth of the second set1 which have predecessors in the first series1 are called premolars2 the ones posterior to these are the molars4 ,section #bc4 ,after mastication1 the food is worked by the tongue and cheeks into a saliva-soaked bolus and swallowed4 ,the passage down the oesophagus is called deglutition4 ,in the stomach it comes under the influence of the gastric juice1 formed in little glandular pits in the stomach wall-- the gastric 7,figure ,,viii4 ,sheet #c7 and pyloric glands4 ,this fluid is distinctly acid1 its acidity being due to about one-tenth per cent of a hundred of hydrochloric acid1 and it therefore stops any further action of the ptyalin1 which can act only on neutral or slightly alkaline fluids4 ,the gastric juice does not act on carbo-hydrates or hydrocarbons to any very noticeable degree4 ,its essential property is the conversion of proteids into peptones1 and the ferment by which this is effected is called pepsin4 ,milk contains a peculiar soluble proteid1 called casein1 which is precipitated by a special ferment1 the rennet-ferment1 and the insoluble proteid1 the curd1 thus obtained is then acted on by the pepsin4 ,in the manufacture of cheese1 the rennetferment obtained1 from the stomach of a calf is used to curdle the milk4 ,section #bd4 ,after the food has undergone digestion in the stomach it passes into the duodenum1 the ,u-shaped loop of intestine immediately succeeding the stomach4 ,the duodenum is separated from the stomach by a ring-like muscular valve1 the pylorus2 this valve belongs to the class of muscles called sphincters1 which1 under ordinary circumstances1 are closed1 but which relax to open the circular central aperture4 ,the valve at the anus1 which retains the faeces1 is another instance of a sphincter4 ,section #be4 ,the food at this stage is called chyme2 it is an acid and soup-like fluid-- acid through the influence of the gastric juice4 ,the temperature of the animal's body is sufficiently high to keep most of the fat in the food melted and floating in oily drops2 much of the starch1 has been changed to sugar1 and the solid proteids to soluble peptones1 but many fragments of material still float unchanged4 ,section #bf4 ,it meets now with the bile1 a greenish fluid secreted by that large and conspicuous gland the liver4 ,the bile is not simply a digestive secretion1 like the saliva or the gastric juice2 it contains matters destined to mix in1 and after a certain amount of change to be passed out of the body with1 the faeces2 among these substances1 of which some portion is doubtless excretory1 are compounds containing sulphur-- the bile salts4 ,there is also a colouring matter1 bili verdin1 which may possibly also be excretory4 ,if the student will compare ,sections #aj and #aa1 he will notice that in those paragraphs no account is taken of the sulphur among the katastases1 the account does not balance1 and he will at once see that here probably is the missing item on the outgoing side4 ,the bile1 through the presence of these salts1 is strongly alkaline1 and so stops the action of the gastric juice1 and prepares for that of the pancreas1 which can act only in an alkaline medium4 ,the fermentive action of the bile is trifling2 it dissolves fats1 to a certain extent1 and is antiseptic1 that is1 it prevents putrefaction to which the chyme might be liable2 it also seems to act as a natural purgative4 ,section #bg4 ,the bile1 as we shall see later1 is by no means the sole product of the liver4 ,section #bh4 ,the pancreatic juice1 the secretion of the pancreas is remarkable as acting on all the food stuffs that have not already become soluble4 ,it emulsifies fats1 that is1 it breaks1 the drops up into extremely small globules1 forming a milky fluid1 and it furthermore has a fermentive action upon them2 it splits them up into fatty acids1 and the soluble body glycerine4 ,the fatty acids combine with alkaline substances 7,section #bf7 to form bodies which belong to the chemical group of ,soaps1 and which are soluble also4 ,the pancreatic juice also attacks any proteids that have escaped the gastric juice1 and converts them into peptones1 and any residual starch into sugar4 ,hence by this stage1 in the duodenum1 all the food constituents noticed in ,section #ag are changed into soluble forms4 ,there are probably1 three distinct ferments in the pancreatic juice acting respectively on starch1 fat1 and proteid1 but they have not been isolated1 and the term pancreatin is sometimes used to suggest the three together4 ,section #bi4 ,a succus entericus1 a saliva-like fluid secreted by numerous small glands in the intestine wall 7,brunner's glands1 ,lieberkuhnian follicles71 probably aids1 to an unknown but comparatively small extent1 in the digestive processes4 ,section #cj4 ,the walls of the whole of the small intestine are engaged in the absorption of the soluble results of digestion4 ,in the duodenum1 especially1 small processes1 the villi project into the cavity1 and being1 like the small hairs of velvet pile1 and as thickly set1 give its inner coat a velvety appearance4 ,in a villus we find 7,figure ,,ix41 ,sheet #c7 a series of small blood-vessels and with it another vessel called a lacteal4 ,the lacteals run together into larger and larger branches until they form a main trunk1 the thoracic duct1 which opens into the blood circulation at a point near the heart2 but of this we shall speak further later4 ,they contain1 after a meal1 a fluid called chyle4 ,section #ca4 ,emulsified fats pass into the chyle4 ,water and diffusible salts certainly pass into the vein4 ,the course taken by the peptones is uncertain1 but ,professor ,foster favours the chyle in the case of the rabbit-- the student should read his ,text-book of ,physiology1 ,part #b1 ,chapter #a1 ,section #aa1 if interested in the further discussion of this question4 ,section #cb4 ,the processes that occur in the remaining portions of the alimentary canal are imperfectly understood4 ,the caecum is so large in the rabbit that it must almost certainly be of considerable importance4 ,in carnivorous animals it may be so much reduced as to be practically absent4 ,an important factor in the diet of the herbivorous animals1 and one absent from the food of the carnivora1 is that carbohydrate1 the building material of all green-meat- food1 cellulose1 and there is some ground for thinking that the caecum is probably a region of special fermentive action upon it4 ,the pancreatic juice1 it may be noted1 exercises a slight digestive activity upon this substance4 ,section #cc4 ,water is most largely absorbed in the large intestine1 and in it the rejected 7mainly insoluble7 portion of the food gradually acquires its dark colour and other faecal characteristics4 #c4 ,the ,circulation ,section #cd4 ,the next thing to consider is the distribution of the food material absorbed through the walls of the alimentary canal to the living and active parts of the body4 ,this is one of the functions of the series of structures-- heart and blood-vessels1 called the circulation1 circulatory system1 or vascular system4 ,it is not the only function4 ,the blood also carries the oxygen from the lungs to the various parts where work is done and kataboly occurs1 and it carries away the katastases to the points where they are excreted-- the carbon dioxide and some water to the lungs1 water and urea to the kidneys1 sulphur compounds of some kind to the liver4 ,section #ce4 ,the blood 7,figure #d1 ,sheet #b7 is not homogeneous2 under the low power of the microscope it may be seen to consist of-- 7#a47 a clear fluid1 the plasma1 in which float-- 7#b47 a few transparent colourless bodies of indefinite and changing shape1 and having a central brighter portion1 the nucleus with a still brighter dot therein the nucleolus-- the white corpuscles 7w4c471 and 7#c47 flat round discs1 without a nucleus1 the red corpuscles 7r4c471 greatly more numerous than the white4 ,section #cf4 ,the chyle of the lacteals passes1 as we have said1 by the thoracic duct directly into the circulation4 ,it enters the left vena cava superior 7l4v4c4s47 near where this joins the jugular vein 7ex4j47 7see ,figure #a1 ,sheet #b1 th4d47 and goes on at once with the rest of the blood to the heart4 ,the small veins of the villi1 however1 which also help suck up the soluble nutritive material1 are not directly continuous with the other body veins1 the systemic veins2 they belong to a special system1 and1 running together into larger and larger branches1 form the lieno gastric 7l4g4v47 and mesenteric 7m4v47 veins1 which unite to form the portal vein 7p4v47 which enters the liver 7l4v47 and there breaks up again into smaller and smaller branches4 ,the very finest ramifications of this spreading network are called the 7liver7 capillaries1 and these again unite to form at last the hepatic vein 7h4v47 which enters the vena cava inferior 7v4c4i471 a median vessel1 running directly to the heart4 ,this capillary network in the liver is probably connected with changes requisite before the recently absorbed materials can enter the general blood current4 ,section #cg4 ,the student has probably already heard the terms vein and artery employed4 ,in the rabbit a vein is a vessel bringing blood towards the heart1 while an artery is a vessel conducting it away4 ,veins are thin-walled1 and therefore flabby1 a conspicuous purple when full of blood1 and when empty through bleeding and collapsed sometimes difficult to make out in dissection4 ,they are formed by the union of lesser factors4 ,the portal breaks up into lesser branches within the liver4 ,arteries have thick muscular and elastic walls1 thick enough to prevent the blood showing through1 and are therefore pale pink or white and keep their round shape4 ,section #ch4 ,the heart of the rabbit is divided by partitions into four chambers3 two upper thin-walled ones1 the auricles 7au471 and two lower ones1 both1 and especially the left1 with very muscular walls1 the ventricles 7vn474 ,the right ventricle 7r4vn47 and auricle 7r4au47 communicate1 and the left ventricle 7l4vn47 and auricle 7l4au474 ,section #ci4 ,the blood coming from all parts of the body1 partly robbed of its oxygen and containing much carbon dioxide and other katastases1 enters the right auricle of the heart through three great veins1 the median vena cava inferior from the posterior parts of the body1 and the paired venae cavae superiores from the anterior4 ,with the beating of the heart1 described below1 it is forced into the right ventricle and from there through the pulmonary artery 7p4a47 seen in the figure passing under the loop of the aorta 7ao47 to the lungs4 ,section #dj4 ,the lungs 7lg4 ,figure #a1 ,sheet #a7 are moulded to the shape of the thoracic cavity and heart2 they communicate with the pharynx by the trachea 7tr4 in ,figure #a1 ,sheet #a7 or windpipe1 and are made up of a tissue of continually branching and diminishing air-tubes1 which end at last in small air-sacs1 the alveoli4 ,the final branches of the pulmonary arteries1 the lung capillaries1 lie in the walls of these air-sacs1 and are separated from the air by an extremely thin membrane through which the oxygen diffuses into1 and the carbon dioxide escapes from1 the blood4 ,section #da4 ,the mechanism of respiration will be understood by reference to ,figure #c1 ,sheet #b4 ,it will be noted1 in dissecting that the lungs have shrunk away from the walls of the thorax2 this collapse occurs directly an aperture is made in the thorax wall1 and is in part due to their extreme elasticity4 ,in life the cavity of the thorax forms an air-tight box1 between which and the lungs is a slight space1 the pleural cavity 7pl4c47 lined by a moist membrane1 which is also reflected1 over the lungs4 ,the thorax wall is muscular and bony1 and resists the atmospheric pressure on its outer side1 so that the lungs before this is cut through are kept distended to the size of the thoracic cavity by the pressure of the air within them4 ,in inspiration 7or breathing-in7 the ribs are raised by the external intercostal 7,anglice1 between-ribs1 e4i4c4m47 and other allied muscles1 and the diaphragm 7dia47 contracts and becomes flatter2 the air is consequently sucked1 in as the lungs follow the movement of the thorax wall4 ,in expiration the intercostals and diaphragm relax and allow the elastic recoil of the lungs to come into play4 ,the thoracic wall is simultaneously depressed by the muscles of the abdominal area1 the diaphragm thrust forwards1 as the result of the displacement and compression of the alimentary viscera thus brought about4 7r4r4r4 in the ,figure mark ribs47 ,section #db4 ,the oxygen and carbon dioxide are not carried in exactly the same way by the blood4 ,the student will know from his chemical reading that neither of these gases is very soluble1 but carbon dioxide is sufficiently so in an alkaline fluid to be conveyed by the liquid plasma4 ,the oxygen however1 needs a special portative mechanism in the colouring matter of the red corpuscles1 the haemoglobin1 with which it combines weakly to form oxy-haemoglobin of a bright red colour1 and decomposing easily in the capillaries 7the finest vessels between the arteries and veins71 to release the oxygen again4 ,the same compound occurs in all true vertebrata1 and in the blood-fluid of the worm2 in the crayfish a similar substance1 haemocyanin1 which when oxygenated is blue1 and when deoxydized colourless1 discharges the same function4 ,section #dc4 ,the blood returns from the lungs to the left auricle 7l4au47 by the pulmonary veins1 hidden in the ,figure by the heart1 passes thence to the thick-walled left ventricle 7l4vn471 and on into the aorta 7ao474 ,section #dd4 ,the beating of the heart is1 of course1 a succession of contractions and expansions of its muscular wall4 ,the contraction1 or systole1 commences at the base of the venae cavae and passes to the auricles1 driving the blood before it into the ventricles1 which then contract sharply and drive it on into the aorta or pulmonary artery2 a pause and then a dilatation1 the diastole follows4 ,the flow of the blood is determined in one direction by the various valves of the heart4 ,no valves occur in the opening of the superior cavae but an imperfect one1 the ,eustachian valve1 protects the inferior cava2 the direction of the heart's contraction prevents any excessive back-flow into the veins1 and the onward1 tendency is encouraged by the suck of the diastole of the ventricles4 ,between the left ventricle and auricle is a valve made up of two flaps of skin1 the mitral valve1 the edges of the flaps being connected with the walls of the ventricle through the intermediation of small muscular threads1 the chordae tendinae1 which stretch across its cavity to little muscular pillars1 the papillary muscles2 these attachments prevent the mitral valve from flapping back into the auricle1 and as the blood flows into and accumulates in the ventricle it gets behind the flaps of the valve and presses its edges together4 ,when the systole of the ventricle occurs1 the increased1 tension of the blood only closes the aperture the tighter1 and the current passes on into the aorta1 where we find three watch-pocket valves1 with the pocket turned away from the heart1 which are also closed and tightened by any attempt at regurgitation 7back-flow74 ,a similar process occurs on the right side of the heart1 but here1 instead of a mitral valve of two flaps between auricle and ventricle1 we have a tricuspid valve with three4 ,the thickness of the muscular walls1 in view of the lesser distance through which it has to force the blood1 -are- is less for the right ventricle than the left4 ,section #de4 ,the following are the chief branches of the aorta4 ,the student should be able to follow them with certainty in dissection2 they are all displayed in the ,figure2 but it must not be imagined for a moment that familiarity with this diagram will obviate the necessity for the practical work2 7in47 is the innominate artery2 it forks into 7s4cl4a47 the right subclavian1 and 7r4c4c47 the right common carotid4 ,each carotid splits at the angle of the jaw into an internal and an external branch4 ,the left common carotid1 7l4c4c47 arises from the base of the innominate199 7l4s4cl4a47 the left subclavian1 directly from the aorta4 ,the aorta now curves round to the dorsal middle line1 and runs down as seen in ,figure #a1 ,sheet #a 7d4ao47 and ,figure #a1 ,sheet #b 7d4ao474 ,small branches are given off to the ribs1 and then comes the median coeliac 7coe4a47 to the stomach and spleen1 the median superior mesenteric 7s4mes4a47 to the main portion of the intestine1 and the inferior mesenteric 7p4m4a47 to the rectum4 ,note that no veins to the inferior vena cava correspond to these arteries-- the blood they supply going back by the portal vein 7p4v474 ,the paired renal arteries 7r4a47 supply the kidneys1 and the common iliacs 7c4il4a47 the hind legs1 splitting into the internal iliacs 7i4il4a47 and the femoral 7f474 ,lines from ,second ,edition only4 ,the student should note that the only arteries in the middle line are those supplying the alimentary canal4 ,lines from ,first ,edition only4 99 -,the figure is inaccurate1 and represents the left common carotid as arising from the aortic arch4- ,section #df4 ,the distribution of the veins of the rabbit has only a superficial parallelism with arteries4 ,the chief factors of vena cava inferior are the hepatic vein 7h4v471 which receives the liver blood1 the renal veins 7r4v471 from the kidneys1 the ilaeo-lumbar1 from the abdominal wall1 and the external 7e4il4v47 and internal ilias 7i4il4v472 with the exception of the renal veins none of these run side by side with arteries4 ,the superior cavae 7r4 and l4v4c4s47 are formed by the union of internal 7i4j47 and external jugular 7e4j47 veins with a subclavian 7s4cl4v47 from the fore limb4 ,the term pre-caval vein is sometimes used for superior cava4 ,the attention1 of the student is called to the small azygos vein 7az47 running into the right vena cava superior1 and forming the only asymmetrical 7not-balancing7 feature of the veins in front of the heart2 it brings blood back from the ribs of the thorax wall1 and is of interest mainly because it answers to an enormous main vessel1 the right post-cardinal sinus1 in fishes4 ,there are spermatic arteries and veins 7s4v4 and a47 to the genital organs4 ,all these vessels should be patiently dissected out by the student1 and drawn4 ,section #dg4 ,between the final branches of the arteries and the first fine factors of the veins1 and joining them1 come the systemic capillaries4 ,these smallest and ultimate ramifications of the circulation penetrate every living part of the animal1 so that if we could isolate the vascular system we should have the complete form of the rabbit in a closely-meshed network4 ,it is in the capillaries that the exchange of gases occurs and that nutritive material passes out to the tissues and katastases in from them2 they are the essential factor in the circulatory system of the mammal-- veins1 arteries1 and heart simply exist to remove and replace their contents4 ,the details of the branching of the pulmonary artery and the pulmonary veins need not detain us now4 ,section #dh4 ,summarising the course of the circulation1 starting from the right ventricle1 we have-- pulmonary artery1 pulmonary capillaries1 pulmonary vein1 left auricle1 left ventricle1 aorta1 arteries1 and systemic capillaries4 ,after this1 from all parts except the spleen and alimentary canal1 the blood returns to systemic veins1 superior or inferior cavae1 right auricle1 and right ventricle4 ,the blood from the stomach spleen1 and intestines however1 passes via through the portal vein to the liver capillaries and then through the hepatic vein to inferior cava1 and so on4 ,material leaves the blood to be excreted in lungs1 kidneys1 by the skin 7as perspiration71 and elsewhere4 ,new material enters most conspicuously2 7a7 by the portal veins portal veins and 7b7 by the thoracic duct and left superior cava4 ,section #di4 ,the following table summarises what we have learnt up to the present of the physiology of the ,rabbit1 considered as a mechanism using up food and oxygen and disengaging energy3-- -,air ,nitrogen444 returned unchanged4 ,oxygen444 through ,pulmonary ,vein to-- see #c4 -,food ,carbo-,hydrates 7,starch1 ,sugar1 ,cellulose47 ,sugar4 ,protein4 ,peptones4 ,fat 7little in ,rabbit47 ,glycerine1 and fatty acids in soups4 ,rejected matter got rid of in ,defaecation4 #a;a4 ,chyle in ,lacteals going via through ,thoracic ,duct and ,left ,superior ,cava to-- see #b4 #a;b4 ,veins of ,villi-- ,portal ,vein-- ,liver-- ,hepatic ,vein and ,inferior ,cava to-- see #b4 #b4 ,right side of heart2 then to lungs1 and then to-- see #c4 #c4 ,left side of heart2 whence to ,systemic ,arteries and ,capillaries4 #d4 ,the tissues and -,kataboly4 #e4 ,urea 78,liver7 ,kidney and ,sweat ,glands ,,co#b ,lungs ,h#b,o ,lungs1 ,kidney1 ,sweat ,glands ,other ,substances ,mainly by ,kidney1 ,liver and ,alimentary ,canal #d4 ,the ,amoeba4 ,cells1 and ,tissue ,section #ej4 ,we have thus seen how the nutritive material is taken into the animal's system and distributed over its body1 and incidentally1 we have noted how the resultant products of the creature's activity are removed4 ,the essence of the whole process1 as we have already stated1 is the decomposition and partial oxydation of certain complex chemical compounds to water1 carbon dioxide1 a low nitrogenous body1 which finally takes the form of urea1 and other substances4 ,we may now go on to a more detailed study1 the microscopic study1 or histology1 of the tissues in which metaboly and kataboly occur1 but before we do this it will be convenient to glance for a moment at another of our animal types-- the ,amoeba1 the lowest as the rabbit is the highest1 in our series4 ,section #ea4 ,this is shown in ,figure ,,iii41 ,sheet #c1 as it would appear under the low power of the microscope4 ,we have a mass of a clear1 transparent1 greyish substance called protoplasm1 granular in places1 and with a clearer border2 within this is a denser portion called the nucleus1 or endoplast 7n471 which1 under the microscope1 by transmitted light1 appear brighter1 and within that a still denser spot1 the nucleolus 7ns47 or endoplastule4 ,the protoplasm is more or less extensively excavated by fluid spaces1 vacuoles2 one clearer circular space or vacuole1 which is invariably present1 appears at intervals1 enlarges gradually1 and then vanishes abruptly1 to reappear after a brief interval2 this is called the contractile vacuole 7c4v474 ,the amoeba is constantly changing its shape1 whence its older name of the ,proteus animalcule1 thrusting out masses of its substance in one direction1 and withdrawing from another1 and hence slowly creeping about4 ,these thrust-out parts1 in its outline1 are called pseudopodia 7ps474 ,by means of them it gradually creeps round and encloses its food4 ,little particles of nutritive matter are usually to be detected in the homogeneous protoplasm of its body2 commonly these are surrounded by a drop of water taken in with them1 and the drop of water is then called a food vacuole4 ,the process of taking in food is called ingestion4 ,the amoeba1 in all probability1 performs essentially the same chemical process as we have summarised in ,sections #aj1 #aa1 #ab2 it ingests food1 digests it in the food vacuoles and builds it up into its body protoplasm1 to undergo kataboly and furnish the force of its motion-- the contractile vacuole1 is probably respiratory and perhaps excretory1 accumulating and then1 by its systole 7compare ,section #dd71 forcing out of its body1 the water1 carbon dioxide1 urea1 and other katastases1 which are formed concomitantly with its activity4 ,the amoeba reproduces itself in the simplest way2 the nucleus occasionally divides into two portions and a widening fissure in the protoplasm of the animal's body separates one from the other4 ,it is impossible to say that one is the parent cell1 and the other the offspring2 the amoeba we merely perceive1 was one and is now two4 ,it is curious to note1 therefore1 that the amoeba is1 in a sense1 immortal-- that the living nucleus of one of these minute creatures that we examine to-day under a microscope may have conceivably drawn1 out an unbroken thread of life since the remotest epochs of the world's history4 ,although no sexual intercourse can be observed1 there is reason to believe that a process of supposed cannabalism1 in which a larger amoeba may occasionally engulph a smaller one1 is really a conjugative reproductive process1 and followed by increased vitality and division4 ,section #eb4 ,now if the student will compare ,section #ce1 he will see that in the white blood corpuscles we have a very remarkable resemblance to the amoeba2 the contractile vacuole is absent1 but we have the protoplasmic body1 the nucleus and nucleolus1 and those creeping fluctuations of shape through the thrusting out and withdrawal of pseudopodia1 which constitute amoeboid motion4 ,they also multiply1 in the same way1 by division4 ,section #ec4 ,it is not only in the white corpuscle of the blood that we find this resemblance2 in all the firmer parts of the body we find1 on microscopic examination1 similar little blebs of protoplasm1 and at an early stage of development the young rabbit is simply one mass of these protoplasmic bodies4 ,their division and multiplication is an essential condition1 of growth4 ,through an unfortunate accident1 these protoplasmic blebs1 which constitute the living basis of the animal body1 have come to be styled cells1 though the term corpuscles is far more appropriate4 ,section #ed4 ,the word is cell suggests something enclosed by firm and definite walls1 and it was first employed in vegetable histology4 ,unlike the typical cells of animals1 the cells of most plants are not naked protoplasm1 but protoplasm enclosed in a wall of substance 7cell wall7 called cellulose4 ,the presence of this cellulose cell wall1 and the consequent necessity of feeding entirely upon liquids and gases that soak through it instead of being able to ingest a portion of solid food is indeed1 the primary distinction between the vegetable and the animal kingdoms1 as ordinarily considered4 ,section #ee4 ,throughout life1 millions of these cells retain their primary characters1 and constitute the white corpuscles of blood1 phagocytes1 and connective tissue corpuscles2 others again1 engage in the formation of material round themselves1 and lie1 in such cases1 as gristle and bone1 embedded in the substance they have formed2 others again1 undergo great changes in form and internal structure1 and become permanently modified into1 for instance1 nerve fibres and muscle substance4 ,the various substances arising in this way through the activity of cells are called tissues1 the building materials of that complex thing1 the animal body4 ,since such a creature as the rabbit is formed through the co-operation of a vast multitude of cells1 it is called multicellular2 the amoeba1 on the other hand1 is unicellular4 ,the rabbit may be thus regarded as a vast community of amoeboid creatures and their products4 ,section #ef4 ,figure ,,iv41 ,sheet #c represents1 diagrammatically1 embryonic tissue1 of which1 to begin with1 the whole animal consists4 ,the cells are all living1 capable of dividing and similar1 but as development proceeds1 they differentiate1 some take on one kind of duty 7function71 and some another1 like boys taking to different trades on leaving school1 and wide differences in structure and interdependence become apparent4 ,section #eg4 ,it is convenient to divide tissues into three classes1 though the divisions are by no means clearly marked1 nor have they any scientific value4 ,the first of these comprises tissues composed wholly1 or with the exception of an almost imperceptible cementing substance1 of cells2 the second division includes the skeletal tissues1 the tissue of mesentery1 and the connective and basement tissue of most of the organs1 tissues which1 generally speaking1 consist of a matrix or embedding substance1 formed by the cells and outside of them1 as well as the cells themselves2 and1 thirdly1 muscular and nervous tissue4 ,we shall study the former two in this chapter1 and defer the third division until later4 ,section #eh4 ,the outer layer of the skin 7the epidermis71 the inmost lining of the alimentary canal1 the lining of the body cavity1 and the inner linings of blood-vessels1 glands1 and various ducts constitute our first division4 ,the general name for such tissues is epithelium4 ,when the cells are more or less flattened1 they form squamous epithelium 7,figure ,,vi47 such as we find lining the inside of a man's cheek 7from which the cells sq4ep4 were taken7 or covering the mesentery of various types-- sq4end4 are from the mesentery 7,section #af7 of a frog4 ,a short cylindroidal form of cell makes up columnar epithelium1 seen typically in the cells covering the villi of the duodenum 7,figure ,v474 ,this epithelium of the villi has the outer border curiously striated1 and this is usually spoken of as leading towards ciliated epithelium1 to be described immediately4 ,the epithelium of the epididermis is stratified-- that is to say1 has many thicknesses of cells2 the deeper layers are alive and dividing 7stratum mucosum71 while the more superficial are increasingly flattened and drier as the surface is approached 7stratum corneum7 and are continually being rubbed off and replaced from below4 ,section #ei4 ,in the branching air-tubes of the lung1 the central canal of the spinal cord1 and in the ureters of the rabbit1 and in most other types1 in various organs1 we find ciliated epithelium 7,figure ,,vii474 ,this is columnar or cubical in form1 and with the free edge curiously modified and beset with a number of hair-like processes1 the cilia1 by which1 during the life of the cell1 a waving motion is sustained in one direction4 ,this motion assists in maintaining a current in the contents of ducts which are lined with this tissue4 ,the motion is independent of the general life of the animal1 so long as the constituent cell still lives1 and so it is easy for the student to witness it himself with a microscope having a #a_/d-;inch or #a_/f-;inch objective4 ,very fine cilia may be seen by gently scraping the roof of a frog's mouth 7the cells figured are from this source71 or the gill of a recently killed mussel1 and mounting at once in water1 or1 better1 in a very weak solution of common salt4 ,section #fj4 ,the lining of glands is secretory epithelium2 the cells are usually cubical or polygonal 7#h1 g4ep471 and they display in the most characteristic form what is called metabolism4 ,anaboly 7see ,section #ad7 we have defined1 as a chemical change in an upward direction-- less stable and more complex compounds are built up in the processes of vegetable and animal activity towards protoplasm2 kataboly is a chemical running down2 metaboly is a more general term1 covering all vital chemical changes4 ,the products of the action of a glandular epithelium are metabolic products1 material derived from the blood is worked1 up within the cell1 not necessarily with conspicuous gain or loss of energy1 and discharged into the gland space4 ,the most striking case of this action is in the goblet cells that are found among the villi2 these are simply glands of one cell1 unicellular glands1 and in ,figure ,v4 we see three stages in their action3 at g4c4#a material 7secretion7 is seen forming in the cell1 at g4c4#b it approaches the outer border1 and at g4c4#c it has been discharged1 leaving a hollowed cell4 ,usually however1 the escape of secreted matter is not so conspicuous1 and the gland-cells are collected as the lining of pits1 simple1 as in the gastric1 pyloric1 and ,lieberkuhnian glands 7,figure ,,viii41 ,sections #bc1 #bi71 or branching like a tree or a bunch of grapes 7,figure r4g471 as in ,brunner's glands 7,section #bi7 the pancreas1 and the salivary glands4 ,the salivary glands1 we may mention1 are a pair internal to the posterior ventral angle of the jaw1 the sub-maxillary2 a pair anterior to these1 the sub-lingual2 a pair posterior to the jaw beneath the ear1 the parotid1 and a pair beneath the eye1 the infra orbital4 ,section #fa4 ,the liver is the most complicated gland in the body 7,figure ,x474 ,the bile duct 7b4d47 branches again and again1 and ends at last in the final pits1 the lobuli 7lb471 which are lined with secretory epithelium1 and tightly packed1 and squeeze each other into polygonal forms4 ,the blood supply from which the bile would appear to be mainly extracted1 is brought by the portal vein1 but this blood is altogether unfit for the nutrition of the liver tissue2 for this latter purpose a branch of the coeliac artery1 the hepatic serves4 ,hence in the tissue of the liver we have1 branching and interweaving among the lobuli1 the small branches of the bile duct 7b4d471 which carries away the bile formed1 the portal vein 7p4v471 the hepatic artery 7h4a471 and the hepatic vein 7h4v474 7,compare ,section #de47 ,figure ,x4b shows a lobule2 the portal vein and the artery ramify round the lobules-- are inter-lobular1 that is 7inter1 between72 the hepatic vein begins in the middle of the lobules 7intra-lobular71 and receives their blood4 7,compare ,x4a47 ,besides its function in the manufacture of the excretory1 digestive1 and auxiliary bile1 the liver performs other duties4 ,it appears to act as an inspector of the assimilation material brought in by the portal vein4 ,the villi1 for instance1 will absorb arsenic1 but this is arrested and thrown down in the liver4 ,a third function is the formation of what would seem to be a store of carbo-hydrate1 glycogen1 mainly it would appear1 from the sugar in the portal vein1 though also1 very probably1 from nitrogenous material1 though this may occur only under exceptional conditions4 ,finally1 the nitrogenous katastates1 formed in the working of muscle and nerve1 and returned by them to the blood for excretion1 are not at that stage in the form of urea4 ,whatever form they assume1 they undergo a further metabolism into urea before leaving the body1 and the presence of considerable quantities of this latter substance in the liver suggests this as a fourth function of this organ-- the elaboration of urea4 ,section #fb4 ,similar from a physiological point of view1 to the secretory glands which form the digestive fluids are those which furnish lubricating fluids1 the lachrymal gland1 and ,harderian glands in the orbit internally to the eye1 and posterior and anterior to it respectively1 the sebaceous glands 7oil glands7 connected with the hair1 and the anal and perineal glands4 ,the secretions of excretory glands are removed from the body2 chief among them are the sweat glands and kidneys4 ,the sweat glands are microscopic tubular glands1 terminating internally in a small coil 7,figure ,,viii4 s4g47 and are scattered thickly over the body1 the water of their secretion being constantly removed by evaporation1 and the small percentage of salt and urea remaining to accumulate as dirt1 and the chief reasonable excuse for washing4 ,the kidney structure is shown diagrammatically in ,figure #e1 ,sheet #g4 ,a great number of branching and straight looped1 tubuli 7little tubes7 converge on an open space1 the pelvis4 ,towards the outer layers 7cortex7 of the kidney1 these tubuli terminate in little dilatations into which tangled knots of blood-vessels project3 the dilatations are called ,bowman's capsules 7,b4c471 and each coil of bloodvessel a glomerulus 7gl474 ,in the capsules1 water is drained from the blood2 in the tubuli1 urea and other salts in the urine are secreted from a branching network of vessels4 ,section #fc4 ,in all the epithelial tissues that we have considered we have one feature in common3 they are cells1 each equivalent to the amoeba1 that have taken on special duties-- in a word1 they are specialists4 ,the amoeba is ,jack of all trades and a free lance2 the protective epidermal cell1 the current-making ciliated cell1 the bile or urea-making secretory cell1 is master of one trade1 and a soldier in a vast and wonderfully organized host4 ,we will now consider our second kind of cell in this organization1 the cell of which the especial aim is the building round it of a tissue4 ,section #fd4 ,the simplest variety in this group is hyaline 7i4e4 glassy7 cartilage 7gristle74 ,in this the formative cells 7the cartilage corpuscles7 are enjellied in a clear structureless matrix 7,figure ,,xii471 consisting entirely of organic compounds accumulated by their activity4 ,immediately round the cell lies a capsule of newer material4 ,some of the cells have recently divided 7#a72 others have done so less recently1 and there has been time for the interpolation of matrix1 as at #b4 ,in this way the tissue grows and is repaired4 ,a thin layer of connective tissue 7see below71 the perichondrium1 clothes the cartilaginous structure4 ,section #fe4 ,connective tissue 7,figure ,,xiii7 is a general name for a group of tissues of very variable character4 ,it is usually described as consisting typically in the mammals of three chief elements felted together2 of comparatively unmodified corpuscles 7c4c471 more or less amoeboid1 and of fibres which are elongated1 altered1 and distorted cells4 ,the fibres are of two kinds3 yellow1 branching1 and highly elastic 7y4e4f471 in consequence of which they fall into sinuous lines in a preparation1 and white and inelastic ones 7w4i4f471 lying in parallel bundles4 ,where the latter element is entirely dominant1 the connective tissue is tendon1 found especially at the point of attachment of muscles to the parts they work4 ,some elastic ligaments are almost purely yellow fibrous tissue4 ,a loose interweaving of the three elements is areolar tissue1 the chief fabric of mesentery1 membrane1 and the dermis 7beneath the epidermis74 ,with muscle it is the material of the walls of the alimentary canal and bloodvessels1 and generally it enters into1 binds together1 and holds in place other tissue4 ,the connective tissue of fishes displays the differentiation of fibres in a far less distinct manner4 ,section #ff4 ,through connective tissues wander the phagocytes1 cells that are difficult to distinguish1 if really distinct1 from the white blood corpuscles4 ,these cells possess a remarkable freedom2 they show an initiative of their own1 and seem endowed with a subordinate individuality4 ,they occur in great numbers in a tissue called1 botryoidal tissue 7,figure ,,xiv471 which occurs especially in masses and patches along the course of the alimentary canal1 in its walls4 ,the tonsils1 swellings on either side of the throat1 are such masses1 and aggregates occur as visible patches1 the ,peyer's patches1 on the ileum4 ,it also constitutes the mass of the vermiform appendix and the wall of the sacculus rotundus2 and in the young animal the thymus gland1 ventral to the heart1 and less entirely1 the thyroid gland1 ventral to the larynx1 are similar structures1 which are reduced or disappear as development proceeds4 ,it is evident that in these two latter cases the term gland is somewhat of a misnomer4 ,the matrix of botryoidal tissue is a network of stretched and hollowed connective tissue cells-- it is not a secretion1 as cartilage matrix appears to be4 ,during digestion1 the phagocytes prowl into the intestine1 and ingest and devour bacteria1 that might otherwise give rise to disease4 ,in inflammation1 we may note here1 they converge from all directions upon the point wounded or irritated4 ,they appear to be the active agents in all processes of absorption 7see osteoclasts under bone71 and for instance1 migrate into and devour the tissue of the tadpole's tail1 during its metamorphosis to the adult frog4 ,section #fg4 ,within the connective tissue cells fat drops may be formed1 as in ,figure ,,xv4 ,adipose tissue is simply connective tissue loaded with fat-distended cells4 ,the tissue is1 of course1 a store form of hydro-carbon 7,section #ag7 provided against the possible misadventure of starvation4 ,with the exception of some hybernating animals1 such store forms would seem to be of accidental importance only among animals1 whereas among plants they are of invariable and necessary occurrence4 ,section #fh4 ,we now come to ,bone1 a tissue confined to the vertebrata1 and typically shown only in the higher types4 ,as we descend in the scale from birds and mammals to lizards1 amphibia 7frogs and toads7 and fish1 we find cartilage continually more important1 and the bony constituent of the skeleton correspondingly less so4 ,in such a type as the dog-fish1 the skeleton is entirely cartilaginous1 bone only occurs in connection with the animal's scales2 it must have been in connection with scales that bone first appeared in the vertebrate sub-kingdom4 ,in the frog we have a cartilaginous skeleton overlaid by numerous bony scutes 7shield-like plates7 which1 when the student comes to study that type1 he will perceive are equivalent to the bony parts of such scales as occur in the dog-fish1 sunk inward1 and plating over the cartilage2 and in the frog the cartilage also is itself1 in a few places1 replaced by bony tissue4 ,in the adult rabbit these two kinds of bone1 the bone overlying what was originally cartilage 7membrane bone71 and the bone replacing the cartilage 7cartilage bone7 have1 between them1 practically superseded the cartilage altogether4 ,the structure of the most characteristic kind of bone will be understood by reference to ,figure ,,xvi4 ,it is a simplified diagram of the transverse section of such a bone as the thigh bone4 ,m4,c4 is the central marrow cavity1 ,h4v41 ,h4v4 are cross sections of small bloodvessels1 the ,haversian vessels running more or less longitudinally through1 the bone in canals1 the ,haversian canals4 ,arranged round these vessels are circles of the formative elements1 the bone corpuscles or osteoblasts 7b4c47 each embedded in bony matrix in a little bed1 the lacuna1 and communicating one with another by fine processes through canaliculi in the matrix1 which processes are only to be seen clearly in decalcified bone 7,see ,section #gj74 ,the osteoblasts are arranged in concentric series1 and the matrix is therefore in concentric layers1 or lamellae 7c4l474 ,without and within the zone of ,haversian systems are 7o4l4 and i4l471 the outer and inner lamellae4 ,the bone is surrounded by connective tissue1 the periosteum4 ,in addition to this compact bone1 there is a lighter and looser variety in which spicules and bars of bony tissue are loosely interwoven4 ,many flat bones1 the bones of the skull1 for instance1 consist of this spongy bone1 plated 7as an electro spoon is plated7 with compact bone4 ,section #fi4 ,among the bony bars and spicules of spongy bone occurs the red marrow-- which must not be confused with the yellow marrow1 the fatty substance in the central cavity of long bones4 ,in this red marrow are numerous large colourless cells1 which appear to form within their substance and then liberate red blood corpuscles4 ,this occurs especially in the spongy bone within the ribs4 ,section #gj4 ,the matrix of bone differs from that of cartilage or of most other tissues in consisting chiefly of inorganic salts4 ,the chief of these is calcium phosphate1 with which much smaller quantities of calcium carbonate1 and magnesium phosphate and carbonate occur4 ,these inorganic salts can be removed by immersion of the bone in weak hydrochloric acid1 and a flexible network of connecting tissue1 ,haversian vessels1 bone corpuscles1 and their processes remains4 ,this is decalcified bone alluded to above4 ,section #ga4 ,in the very young rabbit1 the limb bones1 vertebral column1 and many of the skull bones are simply plates and bars of cartilage2 the future membrane bones1 however are planned out in connective tissue4 ,the development of the latter is simple1 the connective tissue corpuscles functioning by a simple change of product as osteoblast4 ,the development of the cartilage bones1 however1 is more complicated4 ,figure ,,xvii41 represents1 in a diagrammatic way1 the stages in the conversion of a cartilaginous bar to bone4 ,to begin with1 the previously sporadically-arranged 7scattered anyhow7 corpuscles 7u4c4c47 are gathered into groups in single file1 or in other words1 into columnar groups 7as at c4c474 ,the matrix becomes clouded with inorganic salts of lime1 and it is then said to be calcified4 ,this calcified cartilage then undergoes absorption-- it must not be imagined for a moment that bone is calcified cartilage4 ,simultaneous with the formation of the cavities 7s47 due to this absorption1 connective tissue 7p4c4i47 from the surrounding perichondrium 7p4c47 grows into the ossifying99 bar4 ,it is from this connective tissue that the osteoblasts 7o4b47 arise1 and bone is built up4 ,throughout life a bone is continually being absorbed and reformed by the activity of the osteoblasts4 ,an osteoblast engaged in the absorption instead of the formation of bone is called an osteoclast4 99 ,the formation of bone is called ossification4 ,to ossify is to become bony4 ,section #gb4 ,the great thing to notice about this is that cartilage does not become bone1 but is eaten into and ousted by it2 the osteoblasts and osteoclasts replace entirely the cartilage corpuscles1 and are not derived from them4 ,section #gc4 ,we may mention here the structure of the spleen 7,figure #a1 ,sheet #a74 ,it consists of a connective tissue and muscular coating1 with an internal soft matrix much resembling botryoidal tissue1 traversed by fibrous trabeculne 7 beams1 planks7 containing blood-vessels1 and the whole organ is gorged with blood1 particularly after meals4 ,the consideration of its function the student may conveniently defer for the present4 ,section #gd4 ,here also1 we may notice the lymphatics1 a series of small vessels which return the overflow of the blood serum from the capillaries1 in the nutrition of the tissues in all parts of the body1 to the thoracic duct 7see ,section #cf71 and the general circulation4 ,at intervals their course is interrupted by gland-like dilatations1 the lymphatic glands1 in which masses of rapidly dividing and growing 7proliferating7 cells occur1 of which1 doubtless1 many are detached and become1 first lymph corpuscles1 and1 when they reach the veins1 white blood corpuscles4 #e4 ,the ,skeleton ,section #ge4 ,we are now in a position to study the rabbit's skeleton4 ,we strongly recommend the student to do this with the actual bones at hand-- they may be cleared very easily in a well-boiled rabbit4 ,this recommendation may appear superfluous to some readers1 but1 as a matter of fact1 the marked proclivity of the average schoolmaster for mere book-work has put such a stamp on study1 that1 in nine cases out of ten1 a student1 unless he is expressly instructed to the contrary1 will go to the tortuous1 and possibly inexact1 descriptions of a book for a knowledge of things that lie at his very finger-tips4 ,we have not written1 this chapter to give a complete knowledge of the skeleton1 but simply as an aid in the actual examination of the bones4 ,section #gf4 ,we may take the skeleton under five headings4 ,there is the central axis1 the chain of little bones1 the vertebrae1 threaded on the spinal cord 7see ,figure #a and ,section #a72 the thorax1 the box enclosed by ribs and sternum2 the fore-limb and bones connected with it 7pectoral girdle and limb71 and the hind-limb and bones connected with it 7pelvic girdle74 ,finally there is the skull1 but following the ,london ,university syllabus1 we shall substitute the skull of the dog for of that of the rabbit1 as more typically mammalian 7,section #d74 ,section #gg4 ,in ,section #c 7which the student should refer to7 we have a division of the vertebrae into four varieties4 ,of these most representative is the thoracic4 ,a thoracic vertebra 7,figure #d1 ,sheet #e1 ,t4,v47 consists of a central bony mass1 the body or centrum 7b471 from which there arises dorsally an arch1 the neural arch 7n4a471 completed by a keystone1 the neural spine 7n4s472 and coming off laterally from the arch is the transverse process 7tr4p474 ,looking at the vertebra sideways1 we see that the arch is notched1 for the exit of nerves4 ,jointed to the thoracic vertebrae on either side are the ribs 7r474 ,each rib has a process1 the tuberculum1 going up to articulate with the transverse process1 and one1 the capitulum articulating between the bodies of two contiguous vertebrae4 ,the facets for the articulation of the capitulum are indicated in the side view by shading4 ,at either end of the body of a vertebra of a young rabbit are bony caps1 the epiphyses 7ep471 separated from the body by a plane of unossified cartilage 7indicated1 by the dots74 ,these epiphyses to the vertebral bodies occur only among mammals1 and are even absent in some cases within the class4 ,in the adult rabbit they have ossified continuously with the rest of the body4 ,section #gh4 ,a cervical vertebra 7,c4,v47 seems1 upon cursory inspection1 to have no rib4 ,the transverse processes differ from those of thoracic series in having a perforation1 the vertebrarterial canal1 through which the vertebral artery runs up the neck4 ,a study of the development of these bones shows that the part marked f4r4 ossifies separately from the rest of the transverse process2 and the form of the equivalent structures in certain peculiar lower mammals and in reptiles leaves no doubt that f4r4 is really an abbreviated rib2 fused up with the transverse process and body4 ,the two anterior cervical vertebrae are peculiar4 ,the first 7at47 is called the ,atlas-- the figure shows the anterior view-- and has great articular faces for the condyles 7,section #hf7 of the skull1 and a deficient centrum4 ,the next is the axis1 and it is distinguished by an odontoid peg 7od4p471 which fits into the space where the body of the atlas is deficient4 ,in development the centrum of the axis ossifies from one centre1 and the odontoid1 peg from another1 which at that time occupies the position of centrum of the atlas4 ,so that it would seem that the atlas is a vertebra minus a centrum1 and the axis is a vertebra plus a centrum1 added at the expense of the atlas4 ,section #gi4 ,the lumbar vertebrae 7l4v47 are larger1 and have cleft transverse processes1 each giving rise to an ascending limb1 the metapophyses1 and a descending one4 ,the latter 7generally spoken of as the transverse processes7 point steeply downward1 and are considerably longer than those of thoracic series4 ,the sacral vertebrae 7s4v47 have great flattened transverse propcesses for articulation with the ilia4 ,the caudal vertebrae 7c4v47 are gradually reduced to the mere elongated centra1 as we proceed1 towards the tip of the tail4 ,section #hj4 ,all the vertebrae join with their adjacent fellows through the intermediation of certain intervertebral pads1 and also articulate by small processes at either end at the upper side of the arch1 the zygapophyses4 ,the normals to the polished facets of these point1 in the case of the anterior zygapophyses1 up and in 7mnemonic3 ant-up-in71 and in the case of the posterior1 down and out4 ,the student should make this1 and the other features of vertebrae1 out upon actual specimens4 ,section #ha4 ,the thorax is bounded dorsally by the vertebral column1 and ventrally by the sternum4 ,the sternum consists of segments1 the sternebrae 7st472 anteriorly there is a bony manubrium 7mb471 posteriorly a thin cartilaginous plate1 the xiphisternum 7xi474 ,seven pairs of ribs articulate by cartilaginous ends 7sternal ribs7 with the sternum directly1 as indicated in the figure2 five 7false7 ribs are joined1 to each other and to the seventh1 and not to the sternum directly4 ,the last four ribs have no tuberculum 7,section #gg74 ,section #hb4 ,the fore-limb 7pectoral limb7 consists of an upper arm bone1 the humerus 7hum47 the distal end of which is deeply excavated by the olecranon fossa 7o4f47 as indicated by the dotted lines2 of two bones1 the ulna 7u47 and radius 7r47 which are firmly bound by ligament in the position of the figure 7i4e41 with the palm of the hand downward1 prone72 of a number of small bones 7carpalia71 the carpus 7c472 of a series of metacarpals 7mc472 and of three digits 7 fingers7 each1 except the first1 or pollex1 of three small bones-- the phalanges1 only the proximal of which appear in the figure4 ,the ulna has a hook-like head1 the olecranon 7o47 which distinguishes it easily from the distally thickened radius4 ,the limb is attached to the body through the intermediation of the shoulder-blade 7scapula1 sc47 a flattened bone with a median external ridge with a hook-like termination1 the acromion 7acr474 ,there is also a process overhanging the glenoid cavity 7g47 wherein the humerus articulates1 which process is called coracoid 7co472 it is ossified from two separate centres1 and represents a very considerable bone in the bird1 reptile1 and frog4 ,along the dorsal edge of the scapula of the rabbit is unossified cartilage1 which is called the supra-scapula 7s4sc474 ,in man there runs from the acromion to the manubrium of the sternum a bone1 the collar-bone or clavicle4 ,this is represented by a very imperfectly ossified rudiment in the rabbit4 ,the scapula and clavicle1 the bones of the body connected with the fore-limb1 are frequently styled the pectoral girdle1 or shoulder-girdle2 this name of girdle will appear less of a misnomer when lower vertebrate types are studied4 ,section #hc4 ,the hind limb and its body bones-- pelvic limb and girdle-- are shown in ,figure #b4 ,the limb skeleton corresponds closely with that of the fore-limb4 ,the femur 7fe47 answers to the humerus1 and is to be distinguished from it by the greater distinctness of its proximal head 7hd47 and by the absence of an olecranon fossa from its distal end4 ,the tibia 7ti the radius7 is fused for the distal half of its length with the fibula 7fb4 ulna74 ,a tarsus 7tarsalia7 equals the carpus499 ,two of the proximal tarsalia may be noted3 one working like a pulley under the tibia1 is the astragalus 7as472 one forming the bony support of the heel1 is the calcaneum 7ca474 ,there is a series of metatarsals1 and then come four digits of three phalanges each4 99 ,such a resemblance as exists between one vertebra and another in the rabbit1 or between the humerus and the femur1 is called serial homology2 the two things correspond with each other to the extent of imperfect reduplication4 ,homology simply is commonly used to indicate the resemblance between any two structures in different animals1 in origin and position as regards other parts4 ,thus the heart of the rabbit and of the frog are homologous structures1 corresponding in position1 and resembling each other much as two memory sketches of one picture might do4 ,section #hd4 ,the pelvic girdle differs from the pectoral in most land vertebrata in being articulated with the vertebral column4 ,this difference does not exist in fishes4 ,it consist in the rabbit of four bones2 the ilium 7i471 the ischium 7is471 the pubis 7pb471 and the small cotyloid bone-- the first two and the latter one meeting in the acetabular fossa 7ac47 in which the head of the femur works4 ,the pubes and ischia are fused along the mid-ventral line4 ,many morphologists regard1 the ilium as equivalent to1 that is1 strictly corresponding in its relation1 to the scapula1 the pubis to the cartilaginous substratum of the clavicle1 and the ischium to the coracoid4 ,section #he4 ,these bones will be studied at the greatest advantage when dissected out from a boiled rabbit4 ,prepared and wired skeletons1 disarticulated skeletons1 plates of figures1 and written descriptions are in succession more tedious and less satisfactory ways to a real comprehension1 of this matter4 ,this chapter directs the student's attention to the most important points in the study of the skeleton1 but it is in no way intended to mitigate the necessity of practical work4 ,it is a guide simply4 ,section #hf4 ,the mammalian skull will be better understood after the study of that of some lower vertebrate4 ,we shall describe its main features now1 but their meaning will be much clearer after the lower type is read4 ,our figures are of ,canis4 ,in section 7,figure ,,vi41 ,sheet #f71 we perceive a brain case 7cranium7 opening behind by a large aperture1 the foramen magnum 7,f4,m474 ,in front of this is an extensive passage1 the nasal passage 7,e4,n4 to ,p4,n47 which is divided from the mouth by a bony floor1 the palate1 and which opens into the pharynx behind at the posterior nares 7,p4,n47 and to the exterior by the anterior or external nares 7,e4,n474 ,it is divided into right and left passages by a middle partition1 the nasal septum4 ,outside the skull1 on its wings1 is a flask-like bone1 the bulla tympani 7b4 in ,figures #b and #c71 protecting the middle ear1 and from above this there passes an arch1 the cheek bone 7ju4 in ,figures #a1 #b1 and #c71 to the upper jaw1 forming in front the bony lower protection of the cavity containing the eye1 the orbit4 ,the cheek arch1 nasal passage1 and jaws1 form collectively the facial apparatus1 as distinguished from the cranium1 and the whole skull is sometimes referred to as1 the cranio-facial apparatus4 ,two eminences for articulation with the atlas vertebra1 the condyles 7c471 lie one on each side of the lower boundary of the foramen magnum4 ,section #hg4 ,the floor of the cranium consists of a series of cartilage bones1 the basi-occipital 7b4o471 basi-sphenoid 7b4sp471 pre-sphenoid 7p4sp471 and in front1 the ethmoid 7eth471 which sends down a median plate1 not shown1 in the figure1 to form the nasal septum between right and left nasal passages4 ,like extended wings on either side of the basi-occipital are the ex-occipital 7e4o47 7the bone is marked in ,figure #d1 but the letters are a little obscured by shading74 ,similarly the ali-sphenoids 7a4s471 are wings to the basi-1 and the orbito-sphenoids 7o4s471 to the pre-sphenoid bone 7p4sp474 ,between the ex-occipital and ali-sphenoid there is wedged in a bone1 the periotic 7p4o47 containing the internal ear 7,section #aae74 ,above the foramen magnum the median supra-occipital bone completes what is called the occipital arch4 ,a pair of parietals 7pa47 come above the ali-sphenoids1 and a pair of frontals 7f47 above the orbito-sphenoids4 ,at the side the brain case is still incomplete1 and here the aquamosal 7sq47 enters into its wall4 ,in the external view 7,figure #c7 the bulla hides the periotic bone from without4 ,the student should examine all four figures for these bones before proceeding4 ,section #hh4 ,the outer edge of the upper jaw and the cheek arch are made up of three paired bones4 ,first comes the premaxilla 7p4m47 7not p4m4#a or p4m4#d71 containing in the dog1 the three incisors of either side4 ,then comes the maxilla1 bearing the rest of the teeth499 ,the jugal or malar 7ju47 reaches over from the maxilla to meet a zygomatic process 7 connecting outgrowth7 7z4p47 of the squamosal bone4 99 ,in the dog a sabre-like canine 7c471 four premolars 7p4m4#a and p4m4#d7 and two molars 7m4#a and m4#b74 ,section #hi4 ,in the under view of the skull 7,figure #b7 it will be seen that the maxilla sends in a plate to form the front part of the hard palate4 ,behind1 the hard palate is completed by the pair of palatine bones 7pal471 which conceal much of the pre- and orbito-sphenoid in the ventral view1 and which run back as ridges to terminate in two small angular bones1 the pterygoids 7pt47 which we shall find represent much more important structures in the lower vertebrata4 ,section #ij4 ,the pre-maxillae and maxillae bound the sides of the nasal passage1 and it is completed above by a pair of splints1 the nasals4 ,along the floor of the nasal passage1 on the middle line1 lies a splint of bone formed by the coalescence of two halves4 ,it embraces in a ,v-like groove the mesethmoid 7nasal septum7 above1 and lies on the palate4 ,lines from ,first ,edition only4 -,its position is indicated by a heavy black line in #d1 and it is called1 the vomer bone 7vo474- ,lines from ,second ,edition only4 ,in the frog it is represented by two laterally situated bones4 ,this is the vomer bone 7vo474 ,the nasal passages are partially blocked by foliated bony outgrowths1 from the inner aspect of their walls1 which in life are covered with mucous membrane1 and increase the surface sensitive to smell4 ,the ethmoid ends in the ethmo-turbinal 7e4t472 the nasal1 the naso-turbinal 7n4t472 and the maxilla1 the maxillo-turbinal 7m4t474 ,in the anterior corner of the orbit there is a bone1 the lachrymal 7lc4 ,figure #a71 which is hidden by the maxilla in the side view of the skull4 ,section #ia4 ,the lower jaw 7mandible7 is one continuous bone in the mammal4 ,three incisors bite against the three of the upper jaw4 ,then comes a canine1 four premolars1 and three molars1 the first of which is blade-like 7sectorial tooth71 and bites against the similar sectorial tooth 7last premolar7 of the upper jaw4 ,the third molar is small4 ,the arrangement of tooth is indicated in the following dental formula3-- ,i4 #c4c_/c4c1 ,c4 #a4a_/a4a1 ,p4,m4 #d4d_/d4d1 ,m4 #b4b_/c4c ,section #ib4 ,attached just behind the bulla above1 and passing round on either side of the throat to meet at the base of the tongue1 is the hyoid apparatus 7,figure #f74 ,the stylohyal 7s4h471 epihyal 7e4h471 and ceratohyal 7c4h47 form the anterior cornu of the hyoid4 ,the body of the hyoid 7b4h47 forms a basis for the tongue4 ,the posterior coruna 7t4h47 of the hyoid are also called the thyrohyals4 ,section #ic4 ,the following table presents these bones in something like their relative positions4 ,a closer approximation to the state of the case will be reached if the student will imagine the maxilla raised up so as to overlie and hide the palatine and presphenoid1 the squamosal similarly overlying the periotic bone1 and the jugal reaching between them4 ,membrane bones are distinguished by capital letters4 -,cranium -,nasal 7paired71 ,ethmoid ,bone 7median71 -,vomer -,frontal 7paired71 -,lachrymal 7paired71 ,orbito-sphenoid 7paired71 ,pre-sphenoid 7median71 ,eye -,parietal 7,paired71 ,ali-sphenoid 7paired71 ,basi-sphenoid 7median7991 ,periotic ,bone 7paired7 -,bulla 7paired7 ,supra-occipital 7median71 ,ex-occipital 7paired71 ,basi-occipital 7median7 -,upper ,jaw -,pre-,maxilla 7paired7 ,palatine 7paired7 ,pterygoid 7paired7 -,lower ,jaw -,maxilla 7paired7 -,jugal 7paired7 -,squamosal 7paired7 99,in this table the small bones of the ear are simply indicated by an asterisk4 ,section #id4 ,hidden by the bulla1 and just external to the periotic bone1 are the auditory ossicles1 the incus1 malleus1 os orbiculare1 and stapes4 ,these will be more explicitly treated when we discuss the ear4 ,section #ie4 ,when we come to the study of the nerves1 we shall revert to the skull1 and treat of its perforations4 ,the student should not fail1 before proceeding1 to copy and recopy our figures1 and to make himself quite familiar with them1 and he should also obtain and handle an actual skull4 ,for all practical purposes the skull of a sheep or cat will be almost as useful as that of the dog4 #f4 ,muscle and ,nerve ,section #if4 ,we have1 in the skeleton1 a complicated apparatus of parts hinged and movable upon one another2 the agent moving these parts is the same agent that we find in the heart walls propelling the blood through the circulation1 in the alimentary canal squeezing the food along its course1 and universally in the body where motion occurs1 except in the case of the creeping phagocytes1 and the ciliary waving of ciliated epithelium4 ,this agent is muscle4 ,we have1 in muscular tissue1 a very wide departure from the structure of the primordial cell2 to use a common biological expression1 a very great amount of modification 7 differentiation74 ,sheet #g represents the simpler kind of muscular tissue1 unstriated muscle1 in which the cell character is still fairly obvious4 ,the cells are fusiform 7spindle-shaped71 have a distinct nucleus and faint longitudinal striations 7striations along their length71 but no transverse striations4 ,section #ig4 ,in striated muscle extensive modifications mask the cell character4 ,under a #a_/d inch objective1 transverse striations of the fibres are also distinctly visible1 and under a much higher power we discern in a fibre 7,sheet #g7 transverse columns of rod-like sarcous elements 7s4e471 the columns separated by lines of dots1 the membranes of ,krause 7k4m471 and nuclei 7n471 flattened and separated into portions1 and lying1 in some cases1 close to the sarcolemma 7sc47 the connective tissue enclosing the fibre1 in others scattered throughout the substance of the fibre4 ,the figure shows the fibre ruptured1 in order to display the sarcolemma2 e4p4 is the end plate of a nerve 7n4v471 and fb4 are the fibrillae into which a fibre may be teased4 ,section #ih4 ,in the heart we have an intermediate kind of muscle cardiac muscle 7,figure #b71 in which the muscle fibres branch2 there is apparently no sarcolemma1 and the undivided nuclei lie in the centre of the cell4 ,section #ii4 ,unstriated muscle is sometimes called involuntary1 and striated1 voluntary muscle2 but there is really not the connexion with the will that these terms suggest4 ,we have just mentioned that the heart-muscle is striated1 but who can alter the beating of the heart by force of will8 ,and the striated muscles of the limbs perform1 endless involuntary acts4 ,it would seem that unstriated muscle contracts slowly1 and we find it especially among the viscera2 in the intestine for instance1 where it controls that peristaltic movement which pushes the food forward4 ,voluntary muscle1 on the other hand1 has a sharp contraction4 ,the muscle of the slow-moving snails1 slugs1 and mussels is unstriated2 all the muscle of the active insects and crustacea 7crabs1 lobsters1 and crayfish7 is striated4 ,still if the student bears the exception of the heart in mind1 and considers muscles as voluntary that his will can reach1 the terms voluntary and involuntary will serve to give him an idea of the distribution of these two types of muscle in his own body1 and in that of the rabbit4 ,section #ajj4 ,muscular contraction1 and generally all activity in the body is accompanied by kataboly4 ,the medium by which these katabolic changes are set going and controlled is the nervous system4 ,the nervous system holds the whole body together in one harmonious whole2 it is the governing organization of the multicellular community 7,section #ee71 and the supreme head of the government resides in the brain1 and is called the mind4 ,but just as in a political state only the most important and most exceptional duties are performed by the imperial body1 and minor matters and questions of routine are referred to boards and local authorities1 so the mind takes cognisance only of a few of the higher concerns of the animal1 and a large amount of the work of the nervous system goes on insensibly1 in a perfectly automatic way-- even much that occurs in the brain4 ,section #aja4 ,the primary elements in the tissue of the nervous system are three2 nerve fibres1 which are simply conducting threads1 telegraph wires2 ganglion cells1 which are the officials of the system2 and neuroglia1 a fine variety of connective tissue which holds these other elements together1 and may also possibly exercise a function in affecting impressions4 ,a message along a nerve to a ganglion cell is an afferent impression1 from a cell to a muscle or other external end is an efferent impression4 ,the passage of an impression may be defined as a flash of kataboly along the nerve1 and so every feeling1 thought1 and determination involves the formation of a certain quantity of katastases1 and the necessity for air and nutrition4 ,section #ajb4 ,unlike telegraph wires1 to which they are often compared1 nervous fibres usually convey impressions only in one direction1 either centrally 7afferent or sensory nerve fibres71 or outwardly 7efferent or motor nerve fibres74 ,but the so-called motor nerve fibres include not only those that set muscles in motion1 but those that excite secretion1 check impulsive movements1 and govern nutrition4 ,section #ajc4 ,figure #g1 ,sheet #h1 shows the typical structure of nervous tissues4 ,the nerve fibres there figured are bound together by endoneurium into small ropes1 the nerves1 encased in perineurium4 ,there is always a grey axis cylinder 7a4c471 which may 7in medullated nerves71 or may not 7in non-medullated or grey nerves7 have a medullary sheath 7s4,s47 interrupted at intervals by the nodes of ,ranvier 7n4,r474 ,nuclei 7n47 at intervals under the sheath indicate the cells from which nerve fibres are derived by a process of elongation4 ,the nerves of invertebrata1 where they possess nerves1 are mostly grey1 and so are those of the sympathetic system of vertebrata1 to be presently described1 g4c41 g4c4 are ganglion cells2 they may have many hair-like processes1 usually running into continuity with the axis cylinders of nerve fibres1 in which case they are called multi-polar cells1 or they may be uni- or bi-polar4 ,section #ajd4 ,the simplest example of the action of the nervous system is reflex action4 ,for instance1 when the foot of a frog1 or the hand of a soundly sleeping person1 is tickled very gently1 the limb is moved away from the irritation1 without any mental action1 and entirely without will being exercised4 ,and when we go from light into darkness1 the pupil of the eye enlarges1 without any direct consciousness of the change of its shape on our part4 ,similarly1 the presence or food in the pharynx initiates a series of movements-- swallowing1 the digestive movements1 and so on-- which in health are entirely beyond our mental scope4 ,section #aje4 ,a vast amount of our activities are reflex1 and in such action an efferent stimulus follows an afferent promptly and quite mechanically4 ,it is only where efferent stimuli do not immediately become entirely transmuted into outwardly moving impulses that mental action comes in and an animal feels4 ,there appears to be a direct relation between sensation and motion4 ,for instance1 the shrieks and other instinctive violent motions produced by pain1 shunt off a certain amount of nervous impression that would otherwise register itself as additional painful sensation4 ,similarly most women and children understand the comfort of a good cry1 and its benefit in shifting off a disagreeable mental state4 ,section #ajf4 ,the mind receives and stores impressions1 and these accumulated experiences are the basis of memory1 comparison1 imagination1 thought1 and apparently spontaneous will4 ,voluntary actions differ from reflex by the interposition of this previously stored factor4 ,for instance1 when a frog sees a small object in front of him1 that may or may not be an edible insect1 the direct visual impression does not directly determine his subsequent action4 ,it revives a number of previous experiences1 an image already stored of similar insects and associated with painful or pleasurable gustatory experiences4 ,with these arise an emotional effect of desire or repulsion which1 passes into action of capture or the reverse4 ,section #ajg4 ,voluntary actions may1 by constant repetition1 become quasi-reflex in character4 ,the intellectual phase is abbreviated away4 ,habits are once voluntary and deliberated actions becoming mechanical in this way1 and slipping out of the sphere of mind4 ,for instance1 many of the detailed movements of writing and walking are performed without any attention to the details4 ,an excessive concentration of the attention upon one thing leads to absent-mindedness1 and to its consequent absurdities of inappropriate1 because imperfectly acquired1 reflexes4 ,section #ajh4 ,this fluctuating scope of mind should be remembered1 more especially when we are considering the probable mental states of the lower animals4 ,an habitual or reflex action may have all the outward appearance of deliberate adjustment4 ,we cannot tell in any particular case how far the mental comes in1 or whether it comes in at all4 ,seeing that in our own case consciousness does not enter into our commonest and most necessary actions1 into breathing and digestion1 for instance1 and scarcely at all in the details of such acts as walking and talking we might infer that nature was economical in its use1 and that in the case of such an animal as the ,rabbit1 which follows a very limited routine1 and in which scarcely any versatility in emergencies is evident1 it must be relatively inconsiderable4 ,perhaps after all1 pain is not scattered so needlessly and lavishly throughout the world as the enemies of the vivisectionist would have us believe4 #g4 ,the ,nervous ,system ,section #aji4 ,a little more attention must now be given to the detailed anatomy of the peripheral and central nerve ends4 ,a nerve1 as we have pointed out1 terminates centrally in some ganglion cell1 either in a ganglion or in the spinal cord or brain2 peripherally there is a much greater variety of ending4 ,we may have tactile 7touch7 ends of various kinds1 and the similar olfactory and gustatory end organs2 or the nerve may conduct efferent impressions1 and terminate in a gland which it excites to secretion1 in a muscle end-plate1 or in fact1 anywhere1 where kataboly can be set going and energy disengaged4 ,we may now briefly advert to the receptive nerve ends4 ,section #aaj4 ,many sensory nerves1 doubtless1 terminate in fine ends among the tissues4 ,there are also special touch corpuscles1 ovoid bodies1 around which a nerve twines1 or within which it terminates4 ,section #aaa4 ,the eye 7,figure #h7 has a tough1 dense1 outer coat1 the sclerotic 7sc471 within which is a highly vascular and internally pigmented layer1 the choroid1 upon which the percipient nervous layer1 the retina 7r47 rests4 ,the chief chamber of the eye is filled with a transparent jelly1 the vitreous humour 7v4h474 ,in front of the eye1 the white sclerotic passes into the transparent cornea 7c474 ,the epidermis is continued over the outer face of this as a thin1 transparent epithelium4 ,the choroid coat is continued in front by a ring-shaped muscle1 the iris 7ir47 the coloured portion of the eyes4 ,this iris enlarges or contracts its central aperture 7the black pupil7 by reflex action1 as the amount of light diminishes or increases4 ,immediately behind this curtain is the crystalline lens 7l471 the curvature of the anterior face or which is controlled by the ciliary muscle 7c4m474 ,in front of the lens is the aqueous humour 7a4h474 ,the description of the action of this apparatus involves the explanation of several of the elementary principles of optics1 and will be found by the student in any text-book of that subject4 ,here it would have no very instructive bearing1 either on general physiological considerations or upon anatomical fact4 ,section #aab4 ,the structure of the retina demands fuller notice4 ,figure #i shows an enlarged1 diagram of a small portion of this1 the percipient part of the eye4 ,the optic nerve 7o4n4 in ,figure #h7 enters the eye at a spot called the blind spot 7,b4,s471 and the nerve fibres spread thence over the inner retinal surface4 ,from this layer of nerve fibres 7o4n4 in ,figure #i7 threads run outward1 through certain clear and granular layers1 to an outermost stratum of little rods 7r47 and fusiform bodies called cones 7c471 lying side by side4 ,the whole of the retina consists of quite transparent matter1 and it is this outermost layer of rods and cones 7r4 and c47 that receives and records the visual impression4 ,this turning of the recipient ends away from the light is hardly what one would at first expect-- it seems such a roundabout arrangement-- but it obtains in all vertebrata1 and it is a striking point of comparison with the ordinary invertebrate eye4 ,section #aac4 ,we may pause to call the student's attention to a little point in the physiology of nerves1 very happily illustrated here4 ,the function of a nerve fibre is the conduction of impressions pure and simple2 the light radiates through the fibrous layer of the retina without producing the slightest impression1 and at the blind spot1 where the rods and cones are absent1 and the nerve fibres are gathered together1 no visual impressions are recorded4 ,if there is any doubt as to the existence of a blind spot in the retinal picture1 the proof is easy4 ,let the reader shut his left eye1 and regard these two asterisks1 fixing his gaze intently upon the left-hand one of them4 99 99 ,at a distance of three or four inches from the paper1 both spots will be focussed on his retina1 the left one in the centre of vision1 and the right one at some spot internal to this1 and he will see them both distinctly4 ,now1 if he withdraws his head slowly1 the right spot will of course appear to approach the left1 and at a distance of ten or twelve inches it will1 in its approach1 pass over the blind spot and vanish1 to reappear as he continues to move his head away from the paper4 ,the function of nerve fibres is simply conduction1 and the nature of the impressions they convey is entirely determined by the nature of their distal and proximal terminations4 ,section #aad4 ,certain small muscles in the orbit 7eye-socket7 move the eye1 and by their action contribute to our perception of the relative position of objects4 ,there is a leash of four muscles rising from a spot behind the exit of the optic nerve from the cranium to the upper1 under1 anterior1 and posterior sides of the eyeball4 ,these are the superior1 inferior1 anterior1 and posterior recti4 ,running from the front of the orbit obliquely to the underside of the eyeball is the inferior oblique muscle4 ,corresponding to it above is a superior oblique4 ,a lachrymal gland lies in the postero-inferior angle of the orbit1 and a ,handerian gland in the corresponding position in front4 ,in addition to the upper and lower eyelids of the human subject1 the rabbit has a third1 the nictitating lid1 in the anterior corner of the eye4 ,section #aae4 ,the ear 7,sheet ,,vii47 consists of an essential organ of hearing1 and of certain superadded parts4 ,the essential part is called the internal ear1 and is represented in all the true vertebrata 7i4e41 excluding the lancelet and its allies74 ,in the lower forms it is a hollow membranous structure1 embedded in a mass of cartilage1 the otic capsule2 in the mammal the latter is entirely ossified1 to form the periotic bone4 ,the internal ear consists of a central sac1 from which three semicircular canals spring4 ,the planes of the three canals are mutually at right angles2 two are vertical1 the anterior and posterior 7p4v4c47 vertical canals1 and one is horizontal1 the horizontal canal 7h4c474 ,there are dilatations1 called ampullae1 at the anterior base of the anterior1 and at the posterior base of the posterior and horizontal canals4 ,indirectly connected with the main sac is a spirally-twisted portion1 resembling a snail shell in form1 the cochlea4 ,this last part is distinctive of the mammalia1 but the rest of the internal ear is represented in all vertebrata1 with one or two exceptions4 ,the whole of the labyrinth is membranous1 and contains a fluid1 the endolymph2 between the membranous wall of the labyrinth and the enclosing bone is a space containing the perilymph4 ,strange as it may appear at first1 the entire lining of the internal ear is1 at an early stage1 continuous with the general epidermis of the animal4 ,it grows in just as a gland might grow in1 and is finally cut off from the exterior2 but a considerable relic of this former communication remains as a thin1 vertical blind tube 7not shown in the figure71 the ductus endolymphaticus4 ,section #aaf4 ,the eighth nerve runs from the brain case 7,cr471 into the periotic bone1 and is distributed to the several portions of this labyrinth4 ,in an ordinary fish this internal ear is the sole auditory organ we should find2 the sound-waves would travel through the water to the elastic cranium and so reach and affect the nerves4 ,but in all air-frequenting animals this original plan of an ear has to be added to1 to fit it to the much fainter sound vibrations of the compressible and far less elastic air4 ,a receiving apparatus is needed1 and is supplied by the ear-drum1 middle ear1 or tympanic cavity 7,t474 ,in the mammal there is also a collecting ear trumpet 7the ear commonly so-called71 the external ear1 and external auditory meatus 7e4a4m474 ,a tightly stretched membrane1 the tympanic membrane1 separates this from the drum4 ,a chain of small bones1 the malleus 7m471 the incus 7i471 the os orbiculare 7o4or471 a very small bone1 and a stirrup-shaped stapes1 swing across the tympanum1 from the tympanic membrane to the internal ear4 ,at two points the bony investment of this last is incomplete-- at the fenestra rotunda 7f4r471 and at the fenestra ovalis1 7f4o471 into which latter the end of the stapes fits1 and so communicates the sound vibrations of the tympanic membrane to the endolymph4 ,a passage1 the ,eustachian tube1 communicates between the tympanic cavity and the pharynx 7,ph471 and serves to equalize the pressure on either side of the drum-head4 ,a comparative study of the ears of the vertebrata brings to light the fact that1 as we descend in the animal scale1 the four ear ossicles are replaced by large bones and cartilages connected with the jaw1 and the drum and ,eustachian tube by a gill slit4 ,we have1 in fact1 in the ear1 as the student will perceive in the sequel1 an essentially aquatic auditory organ1 added to and patched up to fit the new needs of a life out of water4 ,section #aag4 ,the impressions of smell are conducted through the first nerve to the brain1 and are first received by special hair-bearing cells in the olfactory mucous membrane of the upper part of the nasal passage4 ,the sense of taste has a special nerve in the ninth1 the fibres of which terminate in special cells and cell aggregates in the little papillae 7velvet pile-like processes7 that cover the tongue4 ,section #aah4 ,at an early stage in development1 the brain of a mammal consists of a linear arrangement of three hollow vesicles 7,figure #e1 ,sheet ,,viii41 #a1 #b1 and #c71 which are the fore-1 mid-1 and hind-brain respectively4 ,the cavities in these in these vesicles are continuous with a hollow running through the spinal cord4 ,on the dorsal side of the fore-brain is a structure to be dealt with more fully later1 the pineal gland 7p4g471 while on its under surface is the pituitary body 7pt474 ,section #aai4 ,the lower figure of 7#e7 shows1 in a diagrammatic manner1 the derivation of the adult brain from this primitive state4 ,from the fore-brain vesicle1 a hollow outgrowth on either side gives rises to the 7paired7 cerebral hemisphere 7c4h471 which is prolonged forward as the olfactory lobe 7o4l474 ,from the fore-brain the retina of the eye and the optic nerve also originate as an1 at first1 hollow outgrowth 7op474 ,the roof of the mid-brain is also thickened1 and bulges up to form two pairs of thickenings1 the corpora quadrigemina1 7c4q474 ,the hind-brain sends up in front a median outgrowth1 which develops lateral wings1 the cerebellum 7cbm471 behind which the remainder of the hind-brain is called the medulla oblongata1 and passes without any very definite demarcation into the spinal cord4 ,section #abj4 ,figure #a is a corresponding figure of the actual state of affairs in the adult4 ,the brain is seen in median vertical section4 7ch47 is the right cerebral hemisphere1 an inflated vesicle1 which1 in the mammal-- but not in our lower types-- reaches back over the rest of the fore-brain1 and also over the mid-brain1 and hides these and the pineal gland in the dorsal view of the brain 7,figure #b74 ,the hollow of the hemisphere on either side communicates with the third ventricle1 the original cavity of the fore-brain 7#a in ,figure #e71 by an aperture 7the foramen of ,monro71 indicated by a black arrow 7f4,m474 ,besides their original communication through the intermediation of the fore-brain1 the hemispheres are also united above its roof by a broad bridge of fibre1 the corpus callosum 7c4c471 which is distinctive of the mammalian animals4 ,the original fore-brain vesicle has its lateral walls thickened to form the optic thalami 7o4th471 between which a middle commissure1 7m4c471 absent in lower types1 stretches like a great beam across the third ventricle4 ,the original fore-brain is often called the thalamencephalon1 the hemisphere1 the prosencephalon1 the olfactory lobes1 the rhinencephalon4 ,section #aba4 ,the parts of mid-brain 7mesencephalon7 will be easily recognised4 ,its cavity is in the adult mammal called the iter2 its floor is differentiated into bundles of fibres1 the crura cerebri 7c4cb471 figured also in ,figure #d4 ,section #abb4 ,the cerebellum 7metencephalon7 consists of a central mass1 the vermis 7v4cbm471 and it also has lateral lobes 7l4l471 prolonged into flocculi 7f4cbm471 which lastare -em-bedded in pits1 in the periotic bone1 and on that account render the extraction of the brain from the cranium far more difficult than it would otherwise be4 ,the roof of the hind-brain1 before and behind the cerebellum1 consists of extremely thin plates of nervous matter4 ,its floor is greatly thickened to form the mass of the medulla1 and in front a great transverse track of fibres is specialized1 the pons ,varolii 7p4,v474 ,its cavity is called1 the fourth ventricle4 ,section #abc4 ,figure #b gives a dorsal view of the rabbit's brain2 a horizontal slice has been taken at the level of the corpus callosum4 ,the lateral ventricle 7i4e41 the hollows of the hemisphere7 is not yet opened4 ,a lower cut 7,figure #c7 exposes this 7,v4,l474 ,the level of these slices is approximately indicated in ,figure #a by the lines ,a and ,b4 ,this latter figure will repay careful examination4 ,the arrow1 ar41 plunges into the third ventricle1 behind the great middle commissure 7m4c471 and the barb is supposed to lie under the roof of the mid-brain1 the corpora quadrigemina 7c4q474 ,the position of ar4 is also indicated in ,figure #a4 ,before reading on1 the beginner should stop a while here2 he should carefully copy or trace our figures and1 putting the book aside1 name the parts1 and he should then recopy1 on an enlarged scale1 and finally draw from memory1 correct1 and again draw4 ,by doing this before the brain is dissected a considerable saving of time is possible4 ,section #abd4 ,proceeding from the brain are twelve pairs of cranial nerves4 ,from the fore-brain spring two pairs1 which differ from the rest of the cranial nerves in being1 first of all1 hollow outgrowths of the brain-- the others are from the beginning solid4 ,the first nerve is the olfactory lobe1 which sends numerous filaments through the ethmoid bone to the olfactory organ4 ,the second is the optic nerve1 the visual sensory nerve4 ,section #abe4 ,the mid-brain gives rise to only one nerve1 the third1 which supplies all the small muscles of the eye 7see ,section #aad71 except the superior oblique and external rectus4 ,section #abf4 ,the remainder of the nerves spring from the hind-brain4 ,the fourth pair supply the superior obliques1 and the sixth the external recti2 so that ,,iii41 ,,iv41 and ,,vi4 are alike purely motor nerves1 small and distributed1 to the orbit4 ,the fifth nerve1 the trigeminal1 is a much larger and more important one2 it is a mixed nerve1 having three main branches1 of which the first two are chiefly sensory1 the third almost entirely motor2 it lies deeply in the orbit4 ,v#a 7see ,sheet #i7 runs up over the recti behind the eyeball1 it is the ophthalmic branch2 ,v#b1 the maxillary branch1 runs deeply under the eyeball and emerges in front of the malar1 and ,v#c1 the mandibular branch1 runs down on the inner side of the jaw-bone to the jaw muscles and tongue4 ,section #abg4 ,if the student will now recur to the figures of the dog's skull 7,sheet #f71 he will see certain apertures indicated in the cranial wall4 ,of these1 o4f4 is the optic foramen for the exit of nerve ,,ii41 perforating the orbito-sphenoid4 ,behind this there comes an irregular aperture1 7f4l4a471 the foramen lacerum anterius1 giving exit to ,,iii41 ,,iv41 ,,vi41 and ,v#a4 ,v#b emerges from the foramen rotundum1 and ,v#c from the foramen ovale1 two apertures uniting behind a bony screen499 ,just in front of the bulla is a foramen lacerum medium 7f4l4,m471 through which no nerve passes4 99 ,in the rabbit's skull f4l4 anterius1 the foramen rotundum1 and foramen ovale are not distinct1 and there are two condylar foramina instead of one1 through each of which1 a moiety of ,,xii4 passes4 ,section #abh4 ,the eighth nerve 7auditory7 is purely sensory1 the nerve of the special sense of hearing2 it runs into the periotic bone1 and breaks up on the labyrinth4 ,the seventh nerve 7facial7 is almost entirely motor2 it passes through the periotic anterior to ,,viii41 and emerges by the stylo-mastoid foramen 7s4m4f47 behind the bulla1 to run outside the great jaw muscle across the cheek immediately under the skin 7,figure #a74 ,section #abi4 ,the ninth 7glossopharyngeal7 nerve is chiefly sensory2 it is the special nerve of taste1 and is distributed to the tongue4 ,the tenth nerve 7vagus7 arises by a number of roots1 and passes out of the skull1 together with ,,ix and ,,xi1 by the foramen lacerum -posterium- posterius 7f4l4p474 ,it is a conspicuous white nerve1 and runs down the neck by the side of the common carotid artery4 ,it sends a superior laryngeal branch 7,xa7 to the larynx4 ,the left vagus passes ventral to the aortic arch1 and sends a branch 7l4x4b47 under this along the trachea to the larynx-- the recurrent laryngeal nerve4 ,the corresponding nerve on the right 7r4x4b47 loops under the subclavian artery4 ,the main vagus1 after this branching1 passes behind the heart to the oesophagus and along it to the stomach4 ,,xi41 the spinal accessory1 supplies certain of the neck nerves4 ,,xii41 the hypoglossal1 runs out of the skull by the condylar foramen 7c4f471 is motor1 crosses the roots of ,,xi41 ,x41 and ,,ix41 passes ventral to the carotid1 and breaks up among the muscles of the tongue and neck4 ,section #acj4 ,of the functions of the several parts of the brain there is still very considerable doubt4 ,with disease or willful destruction of the cerebral tissue the personal initiative is affected-- the animal becomes more distinctly a mechanism2 the cerebellum is probably concerned in the coordination of muscular movements2 and the medulla is a centre for the higher and more complicated respiratory reflexes1 yawning1 coughing1 and so on4 ,the great majority of reflex actions centre1 however1 in the spinal cord1 and do not affect the brain4 ,section #aca4 ,a cross section of the spinal cord is shown in ,figure #f1 ,sheet #h4 ,it is a cylinder1 almost bisected by a dorsal 7d4f47 and a ventral 7v4f47 fissure4 ,through its centre runs a central canal 7c4c471 continuous with the brain ventricles1 and lined by ciliated epithelium4 ,the spinal cord consists of an outer portion1 mainly of nervous fibres1 the white matter1 and of inner1 ganglionated1 and more highly vascular grey matter4 7,in the cerebrum the grey matter is external1 and the white internal47 ,the cord1 like the brain1 is surrounded by a vascular fibrous investment1 and protected from concussion by a serous fluid4 ,the nerves which emerge from the vertebral column between the vertebrae1 arise1 unlike the cranial nerves1 by two roots4 ,the dorsal of these1 the sensory root 7d4n471 has a swelling upon it1 the dorsal ganglion1 and-- by experiments upon living animals-- has been shown to contain only afferent fibres2 the ventral1 the motor root1 is without a ganglion1 and entirely or mainly motor4 ,the two unite outside the cord1 and thereafter the spinal nerves are both sensory and motor4 ,section #acb4 ,besides the great mass of brain and spinal cord 7cerebro-spinal axis71 there is1 on either side of the dorsal wall of the body cavity1 a sympathetic nervous chain4 ,the nerve fibres of this system1 like the nerve fibres of invertebrates1 are non-medullated4 ,it may be seen as a greyish thread running close by the common carotid in the neck 7sym41 ,figure #a72 it then runs over the heads of the ribs in the thorax and close beside the dorsal aorta in the abdominal region4 ,in the anterior region of the neck it dilates to form a superior cervical ganglion1 and opposite the first rib it forms an inferior cervical ganglion4 ,thence1 backwards1 there is a ganglion on each sympathetic chain opposite each spinal nerve1 and the two exchange fibres through a thread1 the ramus communicans4 ,to the sympathetic chain is delegated much of the routine work of reflex control of the bloodvessels and other viscera1 which would otherwise fall upon the spinal cord4 ,section #acc4 ,there are eight cervical 7spinal7 nerves1 one in front of the atlas1 and one behind each of the cervical vertebrae4 ,the last four and the first thoracic 7spinal7 contribute to a leash of nerves running out to the fore limb1 the brachial plexus 7plexus1 literally network1 but here meaning a plaited cord74 ,the fourth cervical also sends down a phrenic nerve 7p4n41 ,figure #a71 along by the external jugular vein and the superior caval vein to the diaphragm4 ,the last three lumbar and the sacral nerves form a sacral plexus1 supplying the hind limb4 ,section #acd4 ,from the sympathetic in the hinder region of the thorax a nerve1 the great splanchnic nerve1 arises1 and runs1 back to a ganglionated nervous network1 just behind the coeliac artery1 into which the vagus also enters2 this is the coeliac ganglion1 and together with a similar superior mesenteric ganglion around the corresponding artery1 makes up a subsidiary visceral nervous network1 the solar plexus4 ,a similar and smaller nervous tangle1 bearing an inferior mesenteric ganglion1 lies near the inferior mesenteric artery4 ,section #ace4 ,finally1 we may note the pineal gland and the pituitary body1 as remarkable appendages above and below the thalamencephalon4 ,their function1 if they have a function1 is altogether unknown4 ,probably1 they are inherited from ancestors to whom they were of value4 ,such structures are called reduced or vestigial structures1 and among other instances are the clavicles of the rabbit1 the hair on human limbs1 the little pulpy nodule in the corner of the human eye1 representing the rabbit's third eyelid1 and the caudal vertebrae at the end of the human spinal column4 ,in certain lowly reptiles1 in the lampreys1 and especially in a peculiar ,new ,zealand lizard1 the pineal gland has the most convincing resemblance to an eye1 both in its general build and in the microscopic structure of its elements2 and it seems now more than probable that this little vascular pimple in our brains is a relic of a third and median eye possessed by ancestral vertebrata4 ,the pituitary body is probably equivalent to a ciliated pit we shall describe in the lacelet 7,amphioxus74 #h4 ,renal and ,reproductive ,organs ,section #acf4 ,we have now really completed our survey of the individual animal's mechanism4 ,but no animal that was merely complete in itself would be long sanctioned by nature4 ,for an animal species to survive1 there must evidently1 also1 be proper provision for the production of young1 and the preservation of the species as well as of the individual4 ,hence in an animal's physiology and psychology we meet with a vast amount of unselfish provision1 and its structure and happiness are more essentially dependent on the good of its kind than on its narrow personal advantage4 ,the mammalia probably owe their present dominant position in the animal kingdom to the exceptional sacrifices made by them for their young4 ,instead of laying eggs and abandoning them before or soon after hatching1 the females retain the eggs within their bodies until the development of the young is complete1 and thereafter associate with them for the purposes of nourishment1 protection1 and education4 ,in the matter of the tail1 for instance1 already noted1 the individual rabbit incurs the disadvantage of conspicuousness for the rear1 in order to further the safety of the young4 ,section #acg4 ,the female organs of reproduction are shown in ,sheet #aj4 ,the essential organ is the ovary 7ov471 in which the ova 7eggs7 are formed4 ,figure #c gives an enlarged and still more diagrammatic rendering of the ovary4 ,there is a supporting ground mass1 or stroma1 into which numerous bloodvessels and nerves enter and break up4 ,the ova appear first as small cells in the external substance of the ovary 7as at #a71 and move inward 7#b and #c71 surrounded by a number of sister cells1 which afford them nourishment4 ,at 7#d7 an ovum with its surrounding group of cells is more distinct and near the centre of the ovary2 a fluid is appearing within the ovisac as the development proceeds4 7#e7 is a much more mature ovisac or ,graafian follicle4 ,section #ach4 ,the ovum 7ov471 is now large1 and its nucleus and nucleolus 7the germinal vesicle and spot7 are very distinct4 ,the wall of the follicle consists1 in the mammal1 of several layers of cells1 the membrana granulosa 7or granulosa simply72 the ovum lies on its outer side embedded in a mass of cells1 discus proligerus1 separated from actual contact with the ovum by a zona pellucida4 ,the ripening follicle moves to the surface of the ovary and bursts1 the ovum falls into the body cavity4 ,in ,figure #b1 a ripe ,graafian follicle 7,g4,f471 projects upon the ovary4 ,section #aci4 ,the liberated ovum is caught up by the funnel-shaped opening of the ,fallopian tube1 which passes without any very conspicuous demarcation into the cornu uteri 7c4ut47 of its side2 the two uterine cornua meeting together in the middle line form the vagina 7,v471 which runs out into a vestibule 7vb47 opening between tumid lips to the exterior4 ,the urinary bladder 7ur4b47 also opens into the vestibule1 and receives the two ureters from the kidney4 ,section #adj4 ,in the male we find1 in the position of the female uterus1 a uterus masculinus 7u4m474 ,the essential sexual organ is the testis 7,t471 a compact mass of coiling tubuli1 which opens by a number of ducts1 the vasa efferentia1 into a looser and softer epididymis 7ep471 which sends the sexual product onward through a vas deferens 7v4d471 to open at the base of the uterus masculinus4 ,the urinary bladder and ureters correspond with those of the female1 and the common urogenital duct 7 vestibule71 the urethra1 is prolonged into an erectile penis 7,p47 surrounded by a fold of skin1 the prepuce4 ,a prostate gland 7pr471 contributes to the male sexual fluid4 ,the character of the essential male element1 the spermatozoon1 the general nature of the reproductive process1 will be conveniently deferred until the chapters upon development are reached4 #i4 ,classificatory ,points ,section #ada4 ,the following facts of classificatory importance may now be considered1 but their full force will be better appreciated after the study of other vertebrate types4 ,they are such as come prominently forward in the comparison of the rabbit with other organisms4 ,section #adb4 ,in the first place1 the rabbit is a metazoon1 one of the metazoa1 i4e41 a multicellular organism1 as compared with the amoeba1 which belongs to the protozoa or one-cell animals 7,section #ee74 ,in the next place1 it is externally bilaterally symmetrical1 its parts balance1 and where1 in its internal anatomy1 it departs from this symmetry 7as in the case of the aorta1 the stomach and intestines1 and the kidneys71 the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan4 ,and the facts of development strengthen this idea2 in the very earliest stages we have paired aortic arches1 of which1 the left only remains1 a straight alimentary canal1 and less asymmetrical kidneys4 ,in the vast majority of animals the same bilateral symmetry is to be seen1 but in the star-fish and sea-urchins1 and in the jelly-fish1 corals1 sea anemones1 and hydra1 the general form of the animal is1 instead1 arranged round a centre1 like a star and its rays1 and the symmetry is called radial4 ,section #adc4 ,we also see in various organs of the rabbit1 and especially in the case of the limbs and vertebral column1 what is called metameric segmentation1 that is1 a repetition of parts1 one behind the other1 along the axis of the body4 ,thus the bodies and arches of the vertebrae repeat each other1 and so do the spinal nerves4 ,the renal organ of the rabbit1 some time before birth1 displays a metameric arrangement of its parts2 but this disappears1 as development proceeds1 into the compact kidney of the adult4 ,but the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm1 for instance1 which is visibly a chain of rings4 ,if the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages1 or a string of beads4 ,one bead1 one carriage1 one vertebra1 would be a metamere4 ,section #add4 ,in contrast to metameric segmentation is the antimeric repetition of radial symmetry 7,section #adb71 in which each ray of the star is called an antimere4 ,it is possible to have bilateral symmetry without a metameric arrangement of parts1 as in the mussel and the cuttle-fish2 but metameric segmentation without complete or reduced bilateral symmetry does not occur4 ,section #ade4 ,we are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case1 that the primary division should be into protozoa and metazoa1 and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation4 ,the rabbit is one of the vertebrata1 and1 in common with all the other animals collected under this head1 it has-- 7a7 ,a skeletal axis 7the vertebral column7 between its central nervous system and its body cavity4 ,in the adult rabbit this consists of a chain of vertebrae1 but in the embryo 7i4e41 the young rabbit before birth7 it is represented by a continuous chord1 the notochord1 and it remains as such in some of the lowest vertebrata throughout life4 ,in other words1 in these lower vertebrata1 the vertebral axis is not metameric4 7b7 ,a dorsal and -,tubular nervous axis4 7,section #aca1 the central canal7 7c7 ,it has1 though in the embryo only1 certain slits between the throat and the exterior1 like the gill slits of a fish4 ,such slits are-- with one or two remarkable exceptions outside the sub-kingdom-- distinctly vertebrate features1 and remain1 of course1 in fishes throughout life4 ,the presence of true cartilage and bone mark a vertebrate1 but vertebrata occur in which -these tissues- bone -are- is absent4 ,section #adf4 ,the rabbit shares the following features with all the vertebrata1 except the true fishes1 which do not possess any of them-- 7a7 ,lungs 7but many fish have a swimming bladder which answers to the lungs in its anatomical relations47 7b7 ,limbs which consist of a proximal joint of one bone an intermediate part of two1 and a distal portion which has five digits1 or is evidently a reduced form of the five-digit limb499 7c7 ,the absence of a median fin supported by fin rays49999 99 ,the frog shows indications of a sixth digit4 9999 ,the frog's tadpole has a median fin1 but no fin rays4 ,section #adg4 ,the rabbit shares the following features with all the vertebrata above the fishes and amphibia 7 frogs1 toads1 newts1 and etc47-- 7a7 ,absence of gills 7not gill slits1 note7 at any stage in development4 7b7 ,an amnion1 and 7c7 ,an allantois in development4 ,the meaning of 7b7 and 7c7 we shall explain to the student in the chapters on embryology4 ,we simply mention them here to render our table complete4 ,section #adh4 ,the rabbit shares with all mammals1 and differs from all other vertebrata 7i4e41 birds1 reptiles1 amphibia1 and fishes71 in having-- 7a7 ,hair4 7b7 ,a diaphragm4 7c7 ,only one aortic arch1 and that on the left side of the body4 7d7 ,its young born alive4 7,but two very reptile-like mammals of ,australia1 the duck-billed platypus and the echidna1 lay eggs1 and certain fish and reptiles bear living young47 7e7 ,epiphyses to its vertebral -centre- centra499 7f7 ,the cerebral hemispheres covering the mid-brain4 7g7 ,corpora quadrigemina instead of bigemina4 7h7 ,a corpus callosum4 7i7 ,a spirally coiled cochlea to the internal ear4 7,in respect to h and i also1 the echidna and platypus are scarcely mammalinan47 99 ,but certain mammals have no such epiphyses4 ,section #adi4 ,the rabbit1 together with the hares and conies1 rats and mice1 voles1 squirrels1 beavers1 cavies1 guineapigs is included in that order of the class of mammals which is called the rodentia1 and is distinguished by the character of the incisor teeth from other orders of the class4 #aj4 ,questions and ,exercises #a4 ,describe the venous circulation of the rabbit 7with diagrams74 ,compare a vein and artery4 ,compare the distribution of the great venous trunks with that of the arterial system4 #b4 ,construct a general diagram of the circulation of the rabbit1 to show especially the relation of the portal system1 the lymphatics and lacteals1 and the renal circulation to the main blood current4 #c4 ,draw the alimentary canal of the rabbit from memory4 #d4 ,what is a villus8 ,describe its epithelium1 and the vessels within it4 ,write as explicit an account as you can of the absorbent action of a villus4 #e4 ,tabulate the alimentary secretions1 and their action on the food4 #f4 ,what is botryoidal tissue8 ,where does it occur8 ,what is known of its functions8 #g4 ,copy ,diagram ,i4 7enlarged71 and insert upon it the visceral nerves as far as you can4 #h4 ,what are the most characteristic points in the mammalian vertebral column8 #i4 ,describe cartilage and bone1 and compare them with one another4 #aj4 ,give an account of the amoeba1 and compare it with a typical tissue cell in a metazoon 7e4g41 the rabbit74 #aa4 ,give a general account of connective tissue4 ,what is tendon8 #ab4 ,trace1 briefly1 the increased modification of tissues in the vertebrata4 #ac4 ,describe1 with diagrams1 the structure of blood4 ,state the function of each factor you describe4 #ad4 ,compare the pectoral with the pelvic limb and girdle4 ,what other structures of the adult rabbit display a similar repetition of similar parts8 #ae4 ,draw from memory typical vertebrae from each region of the vertebral column4 #af4 ,what are bilateral symmetry and metameric segmentation8 #ag4 ,give a schedule of distinctive mammalian features4 #ah4 ,describe the rabbit's brain 7with diagrams74 #ai4 ,give a list of the cranial nerves of the rabbit1 and note their origin in the brain4 #bj4 ,give a list of the nerve apertures of the dog's skull4 #ba4 ,what are the chief anatomical differences between a typical cranial1 a spinal1 and a sympathetic nerve8 #bb4 ,describe and figure the distribution of nerves ,v41 ,,vii41 ,,ix41 and ,x4 #bc4 ,describe the muscles1 glands1 and nerves of the orbit of the rabbit4 #bd4 ,describe1 with figures1 the eye of the rabbit4 #be4 ,give a diagram of the rabbit's internal ear4 #bf4 ,draw and describe the ear ossicles4 ,what is their function8 #bg4 ,draw and state the precise position of the hyoid bone1 the clavicle1 the calcaneum1 and the olecranon process4 #bh4 ,describe1 as accurately as possible1 the position of palatine bones1 pterygoids1 the ethmoid bone1 the pre- and basi-sphenoids1 in the dog's skull4 #bi4 ,what is membrane bone8 ,what is cartilage bone8 ,discuss their mutual relationship4 #cj4 ,what is an excretion8 ,what are the chief excretory products of an animal8 ,how are they removed8 #ca4 ,describe the minute anatomy of the liver4 ,give a general account of its functions4 #cb4 ,describe the minute anatomy of the kidney1 and the functions of the several parts4 #cc4 ,what is ciliated epithelium8 ,where does it occur in the rabbit8 #cd4 ,describe the mechanism of respiration4 ,what is the relation of respiration to the general life of the animal8 #ce4 ,what are the functions of the skin8 ,describe its structure4 #cf4 ,what is a secretion8 ,tabulate and classify secretary organs4 ,what is a goblet cell8 #cg4 ,draw1 from memory1 the dorsal and ventral aspects of1 and a median section through1 a dog's skull4 #ch4 ,name any structures that appear to you to be vestiges or rudiments1 i4e41 structures without adequate physiological reason1 in the rabbit's anatomy4 #ci4 ,how are such structures interpreted8 #dj4 ,describe the structure of striated muscular fibre4 ,describe its functions1 and the various means by which they may be called into activity4 #da4 ,describe the characters and structure of the blood of the rabbit4 ,what is the lymphatic system8 ,describe its relation to the blood system in a mammal4 #db4 ,describe the structure of 7a7 blood1 7b7 hyaline cartilage1 7c7 bone1 in the rabbit2 7d7 point out the most important resemblances and differences between these tissues2 7e7 state what you know of the development of the same tissues4 #dc4 ,draw diagrams1 with the parts named1 of the male and female generative organs of the rabbit4 #dd4 ,in the rabbit provided dissect on one side and demonstrate by means of flag-labels the main trunk of the vagus nerve1 the phrenic nerve1 and the recurrent laryngeal nerve4 #de4 ,dissect the rabbit provided so as to expose the abdominal viscera4 ,mark with flag-labels the duct of the pancreas1 the ureters1 and the oviducts or the sperm ducts 7as the case may be74 ,many of the above questions were actually set at ,london ,university ,examinations in ,biology4 ,in ,both ,editions4 -,the ,frog #a4 ,general ,anatomy ,section #a4 ,we will now study the adult anatomy of the frog1 and throughout we shall make constant comparisons with that of the rabbit4 ,in the rabbit we have a distinctly land-loving1 burrowing animal2 it eats purely vegetable food1 and drinks but little4 ,in the frog we have a mainly insectivorous type1 living much in the water4 ,this involves the moister skin1 the shorter alimentary canal1 and the abbreviated neck 7,rabbit1 ,section #b7 of the frog2 the tail is absent-- in a fish it would do the work the frog accomplishes with his hind legs-- and the apertures which are posterior in the rabbit1 run together into one dorsal opening1 the cloaca4 ,there is1 of course 7,rabbit1 ,section #d71 no hair the skin is smooth1 and an external ear is also absent4 ,the remarkable looseness of the frog's skin is due to great lymph spaces between it and the body wall4 ,section #b4 ,if we now compare the general anatomy of the frog 7vide ,sheet #aa7 with that of the rabbit1 we notice that the diaphragm is absent 7,rabbit1 ,section #d71 and the body cavity1 or coelom1 is1 with the exception of the small bag of the pericardium round the heart1 one continuous space4 ,the forked tongue is attached in front of the lower jaw1 and can be flicked out and back with great rapidity in the capture of the small insects upon which the frog lives4 ,the posterior nares open into the front of the mouth-- there is no long nasal chamber1 and no palate1 and there is no long trachea between the epiglottis and the lungs4 ,the oesophagus is less distinct1 and passes gradually1 so far as external appearances go1 into the bag-like stomach1 which is much less inflated and transverse than that of the rabbit4 ,the duodenum is not a ,u-shaped loop1 but makes one together with the stomach2 the pancreas lies between it and the stomach1 and is more compact than the rabbit's4 ,there is no separate pancreatic duct1 but the bile duct runs through the pancreas1 and receives a series of ducts from that gland as it does so4 ,the ileum is shorter1 there is no sacculus rotundus1 and the large intestine has no caecum1 none of the characteristic sacculations of the rabbit's colon1 and does not loop back to the stomach before the rectum section commences4 ,the anus opens not upon the exterior1 but into a cloacal chamber4 ,the urinary and genital ducts open separately into this cloaca1 and dorsally and posteriorly to the anus4 ,the so-called urinary bladder is ventral to the intestine1 in a position answering to that of the rabbit1 but it has no connection with the ureters1 and it is two-horned4 ,section #c4 ,the spleen is a small1 round body1 not so intimately bound to the stomach as in the rabbit1 but in essentially the same position4 ,section #d4 ,much that we knew of the physiology of the frog is arrived at mainly by inferences from our mammalian knowledge4 ,its histology is essentially similar4 ,ciliated epithelium is commoner and occurs more abundantly than in the rabbit1 in the roof of the mouth for instance1 and its red blood corpuscles are much larger1 oval1 and nucleated4 ,section #e4 ,the lungs of the frog are bag-like2 shelves and spongy partitions project into their cavities1 but this structure is much simpler than that of the rabbit's lung1 in which the branching bronchi1 the imperfect cartilaginous rings supporting them1 alveoli1 arteries and veins1 form together a quasi-solid mass4 ,section #f4 ,the mechanism of respiration is fundamentally different from that of the mammal4 ,the method is as follows3-- ,the frog opens its anterior nares1 and depresses the floor of the mouth1 which therefore fills with air4 ,the anterior nares are then closed1 and the floor mouth rises and forces the air into the lungs-- the frog1 therefore1 swallows its air rather than inhales it4 ,the respiratory instrument of the rabbit is a suction pump1 while that of the frog is a buccal force pump4 ,section #g4 ,the heart is not quadrilocular 7i4e41 of four chambers71 but trilocular 7of three71 and two structures1 not seen in ,lepus1 the truncus arteriosus and the sinus venosus1 into the latter of which the venous blood runs before entering the right auricle1 are to be noted4 ,the single ventricle is blocked with bars of tissue that render its interior1 not an open cavity1 but a spongy mass4 ,figure #b1 ,sheet #aa1 shows the heart opened2 l4au4 and r4au4 are the left and right auricles respectively2 the truncus arteriosus is seen to be imperfectly divided by a great longitudino-spiral valve 7l4s4v472 p4c4 is the pulmo-cutaneous artery -going to the lungs- supplying skin and lungs2 d4ao41 the dorsal aorta furnishing the supply of the body and limbs2 and c4a4 the carotid artery going to the head2 all of which vessels 7compare ,figure #a7 are paired4 ,section #h4 ,it might be inferred from this that pure and impure blood mix in the ventricle1 and that a blood of uniform quality flows to lungs1 head1 and extremities2 but this is not so4 ,the spongy nature of the ventricle sufficiently retards this mixing4 ,it will be noted that the opening of pulmonary arteries lies nearest to the heart1 next come the aortic and carotid arches1 which have a common opening at ,a4 ,furthermore1 at c4g4l4 the carotid artery1 repeatedly divides to form a close meshwork of arterioles1 the carotid gland1 forming a sponge-like plug in this vessel4 is a spongy mass of matter1 the carotid gland inserted upon the carotid4 ,hence the pulmonary arteries yawn nearest for the blood1 and1 being short1 wide vessels1 present the least resistance to the first rush of blood-- mainly venous blood for the right auricle4 ,as they fill up1 the back resistance in them becomes equal1 and then greater1 than the resistance at ,a1 and the rush of blood1 now of a mixed quality passes through that aperture4 ,it selects the dorsal aorta1 because the carotid arch1 plugged by the carotid gland1 offers the greater resistance4 ,presently1 however1 the back resistance of the filled dorsal aorta rises above this1 and the last flow of blood1 from the ventricular systole-- almost purely oxygenated blood for the left auricle-- goes on towards the head4 ,section #i4 ,at the carotid gland the carotid artery splits into -an- a -external carotid- lingual 7e4c471 and a deeper internal carotid4 ,the dorsal aorta passes round on each side of the oesophagus1 as indicated by the dotted lines in ,figure #b1 ,sheet #aa1 and meets its fellow dorsal to the liver4 ,each arch gives off subclavian arteries to the limbs1 and the left1 immediately before meeting the right1 gives off the coeliaco-mesenteric artery to the alimentary canal4 ,this origin of the coeliaco-mesenteric artery a little to the left1 is the only asymmetry 7want of balance7 in the arterial system of the frog1 as contrasted with the very extensive asymmetry of the great vessels near the heart of the rabbit4 ,posteriorly the dorsal aorta forks into two common iliac arteries 7right and left7 supplying the hind limbs4 ,section #aj4 ,figure #c gives a side view of the frog1 to display the circulation4 ,lines from ,second ,edition only4 ,the venous return to the heart1 as in the rabbit1 is by paired venae cavae anteriores and by a single vena cava inferior4 ,the factors of the anterior cava on either side are an external jugular 7ex4j47 an innominate vein 7in4v47 and subclavian 7scl4v474 ,the last receives not only the brachial vein 7b4v47 from the fore limb1 but also a large vein bringing blood for the skin1 the cutaneous 7p4v474 ,the innominate vein has also two chief factors1 the internal jugular 7l4i4j4v47 and the subscapular 7s4s4v474 ,the blood returns from each hind limb by a sciatic 7l4sc47 or femoral 7f4m47 vein1 and either passes to a renal portal vein 7l4r4p471 which breaks into capillaries in the kidney1 or by a paired pelvic vein 7l4p4v4 in ,figures #a and #c7 which meets its fellow in the middle line to form the anterior abdominal vein 7a4ab4v47 going forward and uniting with the 7median7 portal vein 7p4v47 to enter the liver4 -,the vessels are named in the references to the figure1 which should be carefully copied and mastered4 ,here we need only- ,comparing with the rabbit1 we would especially call attention to the fact that the vena cava inferior extends posteriorly only to the kidney1 and that there is a renal portal system4 ,the blood from the hind limbs either flows by the anterior abdominal vein to the portal vein and liver1 or it passes by the renal portal vein to the kidney4 ,there the vein breaks up1 and we find in the frog's kidney1 just as we find in the frog's and rabbit's liver1 a triple system of 7a7 nutritive arterial1 7b7 afferent99 venous and 7c7 efferent9999 venous vessels4 99 a1 ad to2 9999 e1 ex out of4 ,this ,section missing from ,second ,edition4 -,section #aa4 ,it is not very improbable that the kidney of the frog shares1 or performs1 some of the functions of the rabbit's liver1 or parallel duties1 in addition to the simply excretory function4 ,since specialization of cells must be mainly the relatively excessive exaggeration of some one of the general properties of the undifferentiated cell1 it is not a difficult thing to imagine a gradual transition1 as we move from one organism to another1 of the functions of glands and other cellular organs4 ,it is probable that the mammalian kidney is1 physiologically1 a much less important 7though still quite essential7 organ than the structures which correspond to it in position and development in the lower vertebrate types4- ,section #ab4 ,the lymphatic system is extensively developed in the frog1 but1 in the place of a complete system of distinctly organized vessels1 there are great lymph sinuses 7compare ,section #a74 ,in ,figure #e1 ,sheet #ab1 the position of two lymph hearts 7l4h41 l4h47 which pump lymph into the adjacent veins1 is shown4 ,section #ac4 ,the skull of the frog will repay a full treatment1 and will be dealt with by itself later4 ,the vertebral column 7,sheet #ab7 consists of nine vertebrae1 the centra of which have faces1 not flat1 but hollow in front 7pro-coelous71 and evidently without epiphyses 7compare the ,rabbit74 ,the anterior is sometimes called the atlas1 but it is evidently not the homologue of the atlas of the rabbit1 since the first spinal nerve has a corresponding distribution to the twelfth cranial of the mammal1 and since1 therefore1 it is probable that the mammalian skull the frog's skull one 7or more7 vertebrae incorporated with it4 ,posteriorly the vertebral column terminates in the urostyle1 a calcified unsegmented rod4 ,the vertebrae have transverse processes1 but no ribs4 ,section #ad4 ,the fore-limb 7,figure #f1 ,sheet #ab7 consists of an upper segment of one bone1 the humerus1 as in the rabbit2 a middle section1 the radius and ulna1 fused here into one bone1 and not1 as in the mammalian type1 separable2 of a carpus1 and of five digits1 of which the fourth is the longest4 ,the shoulder girdle is more important and complete than that of the higher type4 ,there is a scapula 7sc47 with an unossified cartilaginous supra-scapula 7s4sc472 the anterior border of the scapula answers to the acromion4 ,on the ventral side a cartilaginous rod1 embraced by the clavicle 7cl47 7a membrane bone in this type71 runs to the sternum1 and answers to the clavicle of the rabbit4 ,in the place of the rabbit's coracoid process1 is a coracoid bone 7co471 which reaches from the glenoid cavity to the sternum2 it is hidden on the right side of ,figure #f1 which is a dorsal view of the shoulder girdle4 ,there is a pre-omosternum 7o4st47 and a post-omosternum1 sometimes termed a xiphisternum 7x474 ,section #ae4 ,figure #g shows the pelvic girdle and limb of the frog4 ,there is a femur 7f472 tibia and fibula 7t4 and f47 are completely fused2 the proximal bones of the tarsus1 the astragalus 7as471 and calcaneum 7cal47 are elongated1 there are five long digits1 and in the calcar 7c47 an indication of a sixth4 ,with considerable modifications of form1 the three leading constituents of the rabbit's pelvic girdle occur in relatively identical positions4 ,the greatly elongated ilium 7il47 articulates with the single 7compare ,rabbit7 sacral vertebra 7s4v4 in ,figure #e74 ,the ischium 7is47 is relatively smaller than in the rabbit1 and the pubis 7pu47 is a ventral wedge of unossified cartilage4 ,the shape of the pelvic girdle of the frog is a wide departure from that found among related forms4 ,in connection with the leaping habit1 the ilia are greatly elongated1 and the pubes and ischia much reduced4 ,generally throughout the air-frequenting vertebrata1 we find the same arrangement of these three bones1 usually in the form of an inverted4 ,y-- the ilium above1 the ischium and pubis below1 and the acetabulum at the junction of the three4 ,section #af4 ,the uro-genital organs of the frog1 and especially those of the male1 correspond with embryonic stages of the rabbit4 ,in this sex the testes 7,t41 ,sheet #ac7 lie in the body cavity1 and are white bodies usually dappled with black pigment4 ,vasa efferentia 7v4e47 run to the internal border of the anterior part of the kidney1 which answers1 therefore1 to the rabbit's epididymis4 ,the hinder part of the kidney is the predominant renal organ4 ,there is a common uro-genital duct1 into which a seminal vesicle1 which is especially large in early spring1 opens4 ,this is the permanent condition of the frog4 ,in the rabbit1 for urogenital duct1 we have ureter and vas deferens2 the testes and that anterior part of the primitive kidney1 the epididymis1 shift back into the scrotal sacs1 and the ureters shift round the rectum and establish a direct connection with the bladder1 carrying the genital ducts looped over them4 ,the oviducts of the female do not fuse distally to form a median vagina as they do in the rabbit4 ,in front of the genital organ in both sexes is a corpus adiposum 7c4ad471 which acts as a fat store1 and is peculiar to the frogs and toads4 ,the distal end of the oviduct of the female is in the breeding season 7early ,march7 enormously distended with ova1 and the ovaries become then the mere vestiges of their former selves4 ,the distal end of the oviduct is1 therefore1 not unfrequently styled the uterus4 ,there is no penis in the male1 fertilisation of the ova occurring as they are squeezed out of the female by the embracing fore limbs of the male4 ,the male has a pad1 black in winter1 shown in ,figure #a1 which is closely pressed against the ventral surface of the female in copulation1 and which serves as a ready means of distinguishing the sex4 ,section #ag4 ,the spinal cord has a general similarity to that of the rabbit2 the ratio of its size to that of the brain is larger1 and the nerves number ten pairs altogether4 ,the first of these 7sp4 #a1 in ,figure #b1 ,sheet -#ab- 7 ,first ,edition error4 #ac corresponds in distribution with the rabbit's hypoglossal nerve1 a point we shall refer to again when we speak of the skull4 ,the second and third constitute the brachial plexus4 ,the last three form the sciatic plexus going to the hind limb4 ,section #ah4 ,the same essential parts are to be found in the brain of both frog and rabbit1 but in the former the adult is not so widely modified from the primitive condition as in the latter4 ,the fore-brain consists of a thalamencephalon 7th4c4 and #a71 which is exposed in the dorsal view of the brain1 and which has no middle commissure4 ,the cerebral hemispheres 7c4h47 are not convoluted1 do not extend back to cover parts behind them1 as they do in the rabbit1 and are not connected above the roof of the thalamencephalon by a corpus callosum4 ,moreover1 the parts usually regarded1 as the olfactory lobes 7rh47 fuse in the middle line4 ,the mid-brain gives rise to the third nerve1 and has the optic lobes on its dorsal side1 but these are hollow1 and they are not subdivided by a transverse groove into corpora quadrigemina1 as in the rabbit4 ,in the hind-brain the cerebellum is a mere band of tissue without lateral lobes or flocculi1 and the medulla gives origin only to nerves four to ten2 there is no eleventh nerve1 and the hypoglossal is the first spinal-- from which it has been assumed that the rabbit's medulla equals that of the frog1 plus a portion of the spinal cord incorporated with it4 ,the hypoglossal is very distinctly seen on opening the skin beneath the hyoid plate4 ,section #ai4 ,the first1 second1 third1 and fourth cranial nerves of the frog correspond with those of the rabbit in origin and distribution4 ,so do five1 six and eight4 ,the seventh nerve forks over the ear-drum-- the larger branch emerging behind it and running superficially1 as shown in ,figure #d4 ,there is also a deeper palatine branch of ,,vii4 7,p47 running under ,v#b and ,v#c below the orbit1 and to be seen together with ,v#a and ,v#b after removal of the eyeball4 ,the ninth nerve similarly forks over the first branchial slit of the tadpole1 and evidence of the fork remains in the frog4 ,it is seen curving round anterior to the hypoglossal nerve1 and lying rather deeper in dissection4 ,the vagus 7tenth7 nerve is distributed to heart1 lungs1 and viscera-- in the tadpole it also sends for forking branches over the second1 third1 and fourth branchial slits4 ,it lies deeper than ,,ix41 and internal to the veins1 and runs close beside the cutaneous artery4 ,most of these nerves are easily dissected and no student should rest satisfied until he has actually seen them4 ,section #bj4 ,the sympathetic chain is closely connected with the aorta4 ,it is1 of course1 paired1 and is easily found in dissection by lifting the dorsal aorta and looking at its mesentery4 ,in the presence of ganglia corresponding to the spinal nerves1 and of rami communicantes1 it resembles that of the rabbit4 ,section #ba4 ,the whole of this chapter is simply a concise comparison1 of frog and rabbit4 ,in addition to reading it1 the student should very carefully follow the annotations to the figures1 and should copy and recopy these side by side with the corresponding diagrams of the other types4 #b4 ,the ,skull of the ,frog 7and the vertebrate skull generally7 ,section #bb4 ,we have already given a description of the mammalian skull1 and we have stated where the origin of the several bones was in membrane1 and where in cartilage2 but a more complete comprehension of the mammalian skull becomes possible with the handling of a lower type4 ,we propose now1 first to give some short account of the development and structure of the skull of the frog1 and then to show briefly how its development and adult arrangement demonstrate the mammalian skull to be a fundamentally similar structure1 complicated and disguised by further development and re-adjustment4 ,section #bc4 ,figure #a1;,i4 ,sheet #ad1 shows a dorsal view of a young tadpole cranium2 the brain has been removed1 and it is seen that it was supported simply upon two cartilaginous rods1 the trabeculae cranii 7tr4c474 ,behind these trabeculae comes the notochord 7n4c471 and around its anterior extremity is a paired tract of cartilage1 the parachordals 7p4c474 ,these structures1 underlying the skull1 are all that appears at first of the brain box4 ,in front1 and separate from the cranium1 are the nasal organs 7n4c472 the eyes lie laterally to the trabeculae1 and laterally to the parachordals are two tracts of cartilage enclosing the internal ear1 the otic capsules4 ,section #bd4 ,figure #a1 ,,ii41 is a more advanced1 phase of the same structures4 ,the trabeculae have met in front and sent forward a median 7c4t47 and lateral parts 7a4o47 to support the nasal organs4 ,they have also flattened1 out very considerably1 and have sent up walls on either side of the brain to meet above it and form an incomplete roof 7t47 over it4 ,the parachordals have similarly grown up round1 the hind-brain and formed a complete ring1 the roof of which is indicated1 by b4 ,further1 the otic capsules are fusing with the brain-case4 ,with certain differences of form these elements-- the trabeculae1 the parachordals1 and the otic capsules1 are also the first formed structures of the mammalian cranium4 ,section #be4 ,in ,figures #a1;,i4 and ,,ii41 there appears beneath the eye a bar of cartilage 7p4p471 the palato-pterygoid cartilage1 which is also to be seen from the side in ,figures #h1;,i4 and ,,iii4 ,it will be learnt from these latter that this bar is joined in front to the cranium behind the nasal organ1 and behind to the otic capsule4 ,the cartilaginous bar from the palato-pterygoid to the otic capsule is called the quadrate1 and at the point of junction1 at the postero-ventral angle of the palato-pterygoid1 articulates with the cartilaginous bar which is destined to form the substratum of the lower jaw-- ,meckel's cartilage 7,m4c4 in ,figure #h1;,i474 ,section #bf4 ,figure #b shows a dorsal view of these structures in a young frog4 ,the parts corresponding to these in #a1;,,ii4 will be easily made out1 but now ossification has set in at various points of this cartilaginous cranium4 ,in front of the otic capsule is the paired pro-otic bone 7p4o472 behind it at the sides of the parachordal ring is the paired ex-occipital 7e4o472 in front of the cranium box1 and behind the nasal capsules1 is a ring of bone1 the 7median1 but originally paired7 sphenethmoid 7s4e474 -,a paired ossification appears in the palato-pterygoid cartilage the pterygoid bone 7pt471 while- ,a splint of bone1 the quadrato-jugal1 appears at the angle of articulation with the lower jaw4 ,these are all the cartilage bones that appear in the cranium and upper jaw of the frog4 ,section #bg4 ,but another series of bones1 developed first chiefly in dermal connective tissue1 and coming to plate over the cranium of cartilage1 are not shown in ,figure #b4 ,they are1 however1 in ,figure #c4 ,these membrane bones are3 along the dorsal middle line1 the parieto-frontals 7p4f471 originally two pairs of bones which fuse in development1 and the nasals 7na474 ,round the edge of the jaw1 and bearing the teeth1 are pre-maxillae 7p4m471 and maxillae 7mx471 and overlying the quadrate cartilage and lateral to the otic capsules are the ,t-shaped squamosal bones 7sq474 ,in the ventral view of the skull 7,figure #d7 we see a pair of vomers 7vo47 bearing teeth1 a pair of palatines 7pal471 and a pair of pterygoids 7pt47 7which palatines and pterygoids1 we may note1 unlike those of the rabbit1 are -stated to be- membrane bones71 and a great median dagger-shaped para-sphenoid 7p4sp474 ,these two ,figures1 and #e1 which shows the same bones in side view1 should be carefully mastered before the student proceeds with this chapter4 ,the cartilage bones are distinguished from membrane bones by cross-shading4 ,section #bh4 ,turning now to ,figure #h1;,i41 we have a side view of a tadpole's skull4 ,on the ventral side of the head is a series of vertical cartilaginous bars1 the visceral arches supporting the walls of the tadpole's gill slits4 ,the first of these is called the hyoid arch 7c4h471 and the four following this1 the first 7br4#a71 second1 third1 and fourth 7br4#d71 branchial arches4 ,altogether there are four gill slits and between the hyoid arch and the jaw arch1 as it is called 7 ,meckel's cartilage the palato-pterygoid71 is an imperforate slit1 which becomes the ear-drum499 ,the frog no longer breathes by gills1 but by lungs1 and the gills are lost1 the gill slits closed1 and the branchial arches consequently much reduced4 ,figures #h1 ,,ii41 and #h1 ,,iii41 show stages in this reduction4 ,the hyoid arch becomes attached1 to the otic capsule1 and its median ventral plate1 including also the vestiges of the first1 second1 and fourth branchial arches1 is called the hyoid apparatus4 ,in ,figure #e1 the apparatus is seen from the side2 c4h4 is called the 7right7 anterior cornu9999 of the hyoid4 ,the function of the hyoid apparatus in the frog is to furnish1 a basis of attachment to the tongue muscles2 it remains cartilaginous1 with the exception of the relic of one branchial arch1 which ossifies as the thyro-hyal 7,figure #g th4h474 ,it will be noted that1 as development proceeds1 the angle of the jaw swings backward1 and the hyoid apparatus1 shifts relatively forward4 ,these changes of position are indicated in ,figure #h1 ,,iii41 by little arrow-heads4 99 ,we may note here that1 comparing the ear of the frog with that of the rabbit1 there is no external ear4 ,there is1 moreover1 no bulla supporting the middle ear1 and the tympanic membrane stretches between the squamosal in front and the anterior cornu of the hyoid behind4 ,a rod-like columella auris replaces the chain of ear ossicles1 and may1 or may not1 answer to the stapes alone1 or even possibly to the entire series4 ,in the internal ear there is no cochlea1 and the otic mass is largely cartilaginous instead of entirely bony4 9999 ,plural cornua4 ,section #bi4 ,before proceeding to the comparison of the mammalian skull with this1 we would strongly recommend the student thoroughly to master this portion of the work1 and in no way can he do this more thoroughly and quickly than by taking a parboiled frog1 picking off the skin1 muscle1 and connective tissue from its skull1 and making out the various bones with the help of our diagrams4 ,section #cj4 ,figure #i represents1 in the most diagrammatic way1 the main changes in form of the essential constituents of the cranio-facial apparatus1 as we pass from the amphibian to the mammalian skull4 ,f4 is the frog from the side and behind2 b4c4 is the brain-case1 o4c4 the otic capsule1 e4 the eye1 n4c4 the nasal capsule1 p4p4 the palato-pterygoid cartilage1 mx4 the maxillary membrane bones1 sq4 the squamosal1 and mb4 the mandible4 ,the student should compare with ,figure #e1 and convince himself that he appreciates the diagrammatic rendering of these parts4 ,now all the distinctive differences in form1 from this of the dog's skull 7,d471 are reducible to two primary causes-- 7#a7 ,the brain is enormously larger1 and the brain-case is vastly inflated1 so that-- 7a7 the otic capsule becomes embedded in the brain-case wall2 7b7 the palato-pterygoid rod lies completely underneath the brain-case instead of laterally to it2 7c7 the squamosal tilts down and in1 instead of down and out1 and the lower jaw articulates with its outer surface instead of below its inner1 and1 moreover1 with the enormous distention of the brain-case it comes about that the squamosal is incorporated with its wall4 7#b7 ,the maxilla anteriorly and the palatine posteriorly send down palatine plates that grow in to form the bony palate1 cutting off a nasal passage 7n4p47 from the mouth cavity 7m4p471 and carrying the posterior nares from the front part of the mouth1 as they are in the frog1 to the pharynx4 ,hence the vomers of the dog lie1 not in the ceiling of the mouth1 but in the floor of this nasal passage4 ,section #ca4 ,the quadrate cartilage of the frog is superseded by the squamosal as the suspensorium of the lower jaw4 ,it is greatly reduced1 therefore2 but it is not entirely absent4 ,in the young mammal1 a quadrate cartilage can be traced1 connected with the palato-pterygoid cartilage1 and articulating with ,meckel's cartilage4 ,its position is1 of course1 beneath the squamosal1 and just outside the otic capsule4 ,as development proceeds1 the increase in size of the quadrate1 does not keep pace with that of the skull structures4 ,it loses its connection with the palato-pterygoid1 and apparently ossifies as a small ossicle-- the incus of the middle ear4 ,a small nodule of cartilage1 cut off from the proximal end of ,meckel's cartilage1 becomes the malleus4 ,the stapes would appear to be derived from the hyoid arch4 ,hence these small bones seem to be the relics of the discarded jaw suspensorium of the frog utilized in a new function4 ,considerable doubt1 however1 attaches to this interpretation-- doubt that1 if anything1 is gaining ground4 ,section #cb4 ,the tympanic bulla of the dog is not indicated in ,diagram #i1 and it would appear to be a new structure 7neomorph71 not represented in the frog4 ,section #cc4 ,besides these great differences in form1 there are important differences in the amount and distribution of centres of ossification of the skull of frog and mammal4 ,there is no parasphenoid in the mammal992 and1 instead1 a complete series of ossifications1 the median-1 basi-1 and pre-sphenoids1 and the lateral ali- and orbito-sphenoids occur4 ,the points can be rendered much more luminously in a diagram than in the text1 and we would counsel the student to compare this very carefully with that of the ,rabbit4 99 ,faint vestigeal indications occur in the developing skulls of some insectivora4 ,section #cd4 -,cranium -,nasal 7paired71 -,vomer 7paired7 -,fronto-,parietal1 ,sphenethmoid ,bone 7median71 ,eye1 ,pro-otic ,bone1 ,otic ,cartilage1 ,ex-occipital 7paired7 -,para-sphenoid ,bone -,upper ,jaw -,pre-,maxilla 7paired71 -,palatine 7paired71 ,pterygoid 7paired71 -,squamosal1 ,quadrate ,cartilage ,to #a4 -,maxilla #a4 ,quadrato-,jugal -,lower ,jaw ,mento-meckelian1 -,dentary1 -,articulare- -,angulo ,splenial ,section #ce4 -,points especially- ,additional points to be noticed are3 7#a7 ,the otic capsule 7 periotic bone7 of the dog ossifies from a number of centres1 one of which is equivalent to the frog's prootic4 7#b7 ,the several constituents of the lower jaw are not to be distinguished in the adult mammal4 7#c7 ,the frog has no lachrymal bone4 ,section #cf4 ,we are now in a position to notice1 without any danger of misconception1 what is called the segmental theory of the skull4 ,older anatomists1 working from adult structure only1 conceived the idea that the brain-case of the mammal represented three inflated vertebrae4 ,the most anterior had the pre-sphenoid for its body1 the orbito-sphenoids for its neural processes1 and the arch was completed above by the frontals 7frontal segment74 ,similarly1 the basi-sphenoids1 ali-sphenoids1 and parietals formed a second arch 7parietal segment71 and the ex-1 basi-1 and supra-occipitals a third 7occipital segment74 ,if this were correct1 in the frog1 which is a more primitive rendering of the vertebrate plan1 we should find the vertebral characters more distinct4 ,but1 as a matter of fact1 as the student will perceive1 frontal segment1 parietal segment1 and occipital segment1 can no longer be traced2 and the mode of origin from trabeculae and para-chordals show very clearly the falsity of this view4 ,the vertebrate cranium is entirely different in nature from vertebrae4 ,the origin of the parietals and frontals as paired bones in membrane reinforces this conclusion4 ,section #cg4 ,but as certainly as we have no such metameric segmentation1 as this older view implies1 in the brain-case of the frog1 so quite as certainly is metameric segmentation evident in its branchial arches4 ,we have the four gill slits of the tadpole and their bars repeating one another2 the hyoid bar in front of these is evidently of a similar nature2 and that the ear drum is derived from an imperforate gill slit is enforced by the presence of an open slit 7the spiracle7 in the rays and dog-fish in an entirely equivalent position4 ,does the mouth answer to a further pair of gill slits1 and is the jaw arch 7palato-pterygoid ,meckel's cartilage7 equivalent to the arches that come behind it8 ,this question has been asked1 and answered in the affirmative1 by many morphologists1 but not by any means by all4 ,the cranial nerves have a curious similarity of arrangement with regard to the gill slits and the mouth2 the fifth nerve forks over the mouth1 the seventh forks over the ear drum1 the ninth1 in the tadpole and fish1 forks over the first branchial slit1 and the tenth is1 as it were1 a leash of nerves1 each forking over one of the remaining gill slits4 ,but this matter will be more intelligible when the student has worked over a fish type1 and need not detain us any further now4 ,section #ch4 ,see also ,section #ac again1 in which is the suggestion that the occipital part of the skull is possibly a fusion of vertebrae1 a new view with much in its favour1 and obviously an entirely different one from the old segmental view of the entire skull1 discussed in ,section #cf4 ,questions on the ,frog ,all these questions were actually set at ,london ,university ,examinations4 ,in ,both ,editions4 #a4 ,give an account1 with illustrative sketches1 of the digestive organs of the common frog1 specifying particularly the different forms of epithelium met with in the several regions thereof4 #b4 ,describe the heart of a frog1 and compare it with that of a fish and of a mammal1 mentioning in each case the great vessels which open into each cavity4 #c4 ,compare with one another the breathing organs and the mechanism of respiration in a frog and in a rabbit4 ,give figures showing the condition of the heart and great arteries in these animals1 and indicate in each case the nature of the blood in the several cavities of the heart4 #d4 ,draw diagrams1 with the parts named1 illustrating the arrangement of the chief arteries of 7a7 the frog1 7b7 the rabbit4 7c7 ,compare briefly the arrangements thus described4 7d7 ,in what important respects does the vascular mechanism of the frog differ from that of the fish1 in correlation with the presence of lungs8 #e4 ,in the frog provided1 free the heart1 both aortic arches1 dorsal aorta as far as its terminal bifurcation1 and both chains of sympathetic ganglia from surrounding structures2 and remove them1 in their natural connection1 from the animal into a watch-glass4 #f4 ,describe the male and female reproductive organs of the common frog1 and give some account of their development4 #g4 ,describe1 with figures1 the bones of the limbs and limb-girdles of a frog4 #h4 ,remove the brain from the frog provided1 and place it in spirit4 ,make a lettered drawing of its ventral and dorsal surfaces4 #i4 ,point out the corresponding regions in the brain of a frog and a mammal1 and state what are the relations of the three primary brain-vesicles to these regions4 #aj4 7a7 ,give an account1 with diagrams1 of the brain of the frog2 7b7 point out the most important differences between it and the brain of the rabbit4 7c7 ,describe the superficial origin and the distribution of the third1 7d7 of the fifth1 7e7 of the seventh41 7f7 of the ninth1 and 7g7 of the tenth cranial nerves of the frog4 #aa4 ,describe1 with figures1 the brain of a frog1 and compare it with that of a rabbit4 ,what do you know concerning the functions of the several parts of the brain in the frog8 #ab4 ,describe briefly the fundamental properties of the spinal cord in the frog4 ,by what means would you determine whether a given nerve is motor or sensory8 #ac4 ,prepare the skull of the frog provided4 ,remove from it and place in glycerine on a glass slip the fronto-parietal and parasphenoid bones4 ,label them4 ,mark on the skull with long needles and flag-labels the sphenethmoid and the pro-otic bones4 #ad4 ,compare the skull of the rabbit and the frog2 especially in regard to the attachment of the jaw apparatus to the cranium1 and other points which distinctly characterize the higher as contrasted with the lower vertebrata4 #ae4 ,describe the skeleton of the upper and lower jaw 7a7 in the frog1 7b7 in the rabbit4 ,point out exactly what parts correspond with one another in the two animals compared4 7c7 ,what bone in the rabbit is generally regarded as corresponding to the quadrate cartilage of the frog8 -,the ,dog-,fish #a4 ,general ,anatomy ,section #a4 ,in the dog-fish we have a far more antique type of structure than in any of the forms we have hitherto considered4 ,forms closely related to it occur among the earliest remains of vertebrata that are to be found in the geological record4 ,since the immeasurably remote ,silurian period1 sharks and dog-fish have probably remained without any essential changes of condition1 and consequently without any essential changes of structure1 down to the present day4 ,then1 as now1 they dominated the seas4 ,they probably branched off from the other vertebrata before bone had become abundant in the inner skeleton1 which is consequently in their case cartilaginous1 with occasional calcification and no distinct bones at all4 ,unlike the majority of fish1 they possess no swimming bladder-- the precursor of the lungs2 but in many other respects1 notably in the uro-genital organs1 they have1 in common with the higher vertebrata1 preserved features which may have been disguised or lost in the perfecting of such modern and specialized fish as1 for instance1 the cod1 salmon1 or herring4 ,section #b4 ,comparing the general build1 of a dog-fish with that of a rabbit1 we notice the absence of a distinct neck1 and the general conical form2 the presence of a large tail1 as considerable1 at first1 in diameter as the hind portion of the body1 and of the first importance in progression1 in which function the four paddle-shaped limbs1 the lateral fins1 simply co-operate with the median fin along the back for the purpose of steering2 and1 as a consequence of the size of the tail1 we note also the ventral position of the apertures of the body4 ,the anus1 and urinary and genital ducts unite in one common chamber1 the cloaca4 ,behind the head1 and in front of the fore fin 7pectoral fin71 are five gill slits 7g4s47 leading from the pharynx to the exterior4 ,just behind the eye is a smaller and more dorsal opening of the same kind1 the spiracle 7sp474 ,on the under side of the head1 in front of the mouth1 is the nasal aperture 7olf471 the opening of the nasal sac1 which1 unlike the corresponding organ of the air-frequenting vertebrata1 has no internal narial opening4 ,there is1 however1 a groove running from olf4 to the corner of the mouth1 and this1 closing1 in the vertebrate types that live in air and are exposed to incessant evaporation of their lubricating secretions1 constitutes the primitive nasal passage4 ,the limbs are undifferentiated into upper1 lower1 and digital portions1 and are simply jointed1 flattened expansions4 ,section #c4 ,the skin of the dog-fish is closely set with pointed tooth-like scales1 the placoid scales1 and these are continued over the lips into the mouth as teeth4 ,each scale consists of a base of true bone1 with a little tubercle of a harder substance1 dentine1 capped by a still denser covering1 the enamel4 ,the enamel is derived from the outer layer of the embryonic dog-fish1 the epiblast1 which also gives rise to the epidermis2 while the dentine and bony base arise in the underlying mesoblast1 the dermis4 ,a mammalian tooth has essentially the same structure3 an outer coat of enamel1 derived from epiblast1 overlies a mass of dentine1 resting on bone1 but the dentine is excavated internally1 to form a pulpcavity containing blood-vessels and nerves4 ,most land animals1 however1 have teeth only in their mouths1 and have lost altogether the external teeth which constitute the armour of the dog-fish4 ,besides the teeth there perhaps remain relics of the placoid scales in the anatomy of the higher vertebrata1 in the membrane bones4 ,how placoid scales may have given rise to these structures will be understood by considering such a bone as the vomer of the frog4 ,this bone lies on the roof of the frog's mouth1 and bears a number of denticles1 and altogether there is a very strong resemblance in it to a number of placoid scales the bony bases of which have become confluent4 ,in the salamander1 behind the teeth-bearing vomers comes a similar toothed parasphenoid bone4 ,the same bone occurs in a corresponding position in the frog1 but without teeth4 ,in some tailed amphibians the vomers and splenials are known to arise by the fusion of small denticles4 ,these facts seem to point to stages in the fusion of placoid bases1 and their withdrawal from the surface to become incorporated with the cranial apparatus as membrane bones1 a process entirely completed in the mammalian type4 ,section #d4 ,the alimentary canal of the dog-fish1 is a simple tube thrown into a ,z shape4 ,the mouth is rough with denticles1 and has a fleshy immovable tongue on its floor4 ,in the position of the ,eustachian tube there is a passage1 the spiracle 7sp471 running out to the exterior just external to the cartilage containing the ear4 ,the pharynx communicates with the exterior through five gill slits 7g4s471 and has1 of course1 no glottis or other lung opening4 ,there is a wide oesophagus passing into a ,u-shaped stomach 7st471 having1 like the rabbit's1 the spleen 7sp47 on its outer curvature4 ,there is no coiling small intestine1 but the short portion1 receiving the bile duct 7b4d47 and duct of the pancreas 7pan471 is called the duodenum 7d'dum474 ,the liver has large left 7,l4lv47 and right lobes1 and a median lobe 7,m4lv471 in which the gall bladder 7g4bl47 is embedded4 ,the next segment of the intestine is fusiform1 containing a spiral valve 7,figure #d71 the shelf of which points steeply forward2 it is sometimes called the colon 7co474 ,it is absorptive in function and probably represents morphologically1 as it does physiologically1 the greater portion of the small intestine4 ,a rectal gland 7r4g47 opens from the dorsal side into the final portion of the canal 7rectum74 ,section #e4 ,the circulation presents1 in many respects1 an approximation to the state of affairs in the developing embryos of the higher types4 ,the heart 7,figure #c1 ,sheet -#ad- ,error in ,first ,edition #af7 is roughly1 ,z shape1 and transmits only venous blood4 ,it lies in a cavity1 the pericardial cavity 7,p4c4c471 cut off by a partition from the general coelome4 ,at one point this partition is imperfect1 and the two spaces communicate through a pericardio-peritoneal canal 7p4p4c471 which is also indicated by an arrow 7p4p47 in the position and direction in which the student1 when dissecting1 should thrust his seeker1 in ,figure #a ,sheet #ae4 ,a sinus venosus 7s4v4 in ,figure #c1 ,sheet #af7 receives the venous trunks1 and carries the blood through a valve into the baggy and transversely extended -auricle- atrium 7au471 whence it passes into the muscular ventricle 7,vn471 and thence into the truncus arteriosus4 ,this truncus consists of two parts3 the first1 the conus or pylangium 7c4a471 muscular1 contractile1 and containing a series of valves2 the second1 the bulbus or synangium 7b4a471 without valves and pulsatile4 ,in the rabbit both sinus and truncus are absent1 or merged in the adjacent parts of the heart4 ,section #f4 ,from the bulbus there branch1 on either side1 four arterial trunks1 the first of which forks1 so that altogether there are five afferent branchials 7a4br47 taking blood to be aerated in the gills1 here highly vascular filamentary outgrowths of the internal walls of the gill slits4 ,lines from ,second ,edition only4 ,there are altogether nine vascular outgrowths 7demi-branchs71 one on each wall of each gill slit except the last1 on the hind wall of which there is none4 7,in the spiracle is a miniature demibranch1 the pseudo-branch4 ,this suggests that the spiracle is really a somewhat modified gill slit47 ,four efferent branchials 7e4br47 carry the aerated blood on to the dorsal aorta 7d4ao474 ,a carotid artery runs forward to the head1 and a hypo-branchial artery supplies the ventral side of the pharyngeal region4 ,there are sub-clavian1 coeliac1 mesenteric1 and pelvic arteries1 and the dorsal aorta is continued through the length of the tail as the caudal artery 7,cd4,a474 ,section #g4 ,a caudal vein 7,cd4,v471 bringing blood back from the tail1 splits behind the kidneys 7,k471 and forms the paired renal portal veins 7r4p4v471 breaking up into a capillary system in the renal organ4 ,a portal vein brings blood from the intestines to the liver4 ,section #h4 ,instead of being tubular vessels1 the chief veins of the dog-fish are1 in many cases1 irregular baggy sinuses4 ,three main venous trunks flow into the sinus venosus4 ,in the median line from behind comes the hepatic sinus 7,h4,s472 and laterally1 from a dorsal direction1 the ,cuvierian sinuses 7,c4,s47 enter it4 ,these1 as the student will presently perceive1 are the equivalents of the rabbit's superior cavae4 ,they receive1 near their confluence with the sinus venosus1 the inferior jugular vein 7,i4,j4,v474 ,at their dorsal origin1 they are formed by the meeting of the anterior 7,a4,c4,s47 and posterior 7,p4,c4,s47 cardinal sinuses4 ,the anterior cardinal sinus -is1 roughly1 the equivalent of the internal jugular vein-1 lies along dorsal to the gill slits 7g4s471 and receives an orbital sinus from the eye4 ,the posterior cardinal sinus receives a sub-clavian vein 7s4c4v47 and a lateral vein 7,l4,v471 and fuses posteriorly with its fellow in the middle line4 ,this median fusion is a departure from the normal fish type4 ,it must not be confused with the inferior cava1 which is not found in the dog-fish1 the right posterior cardinals representing the rabbit's azygos vein4 ,a simplified diagram of the circulation of a fish is given in ,figure #b1 ,sheet #af1 and this should be carefully compared with the corresponding small figure given of the vascular system of our other types4 ,lines from ,second ,edition only4 ,the blood of the dog-fish resembles that of the frog4 ,section #i4 ,the internal skeleton1 as we have said1 is entirely cartilaginous1 and only those parts which are pre-formed in cartilage in the skeletons of the higher types are represented here4 ,the spinal column consists of two types of vertebrae1 the trunk1 bearing short1 distinct1 horizontally-projecting ribs 7r471 and the caudal4 ,the diagrams of ,figure #e 7,sheet #ah7 are to illustrate the structure of the centrum of a dog-fish vertebra2 ,c is a side view1 ,d a horizontal median section1 ,a and ,b are transverse sections at the points indicated by -,b and ,a- ,a and ,b respectively in ,figure ,c4 -7,by an unfortunate slip of the pen in the figure1 ,a was substituted for ,b2 section ,a corresponds to line ,b1 and vice versa47- ,the vertebrae are hollowed out both anteriorly and posteriorly 7amphi-coelous71 and a jelly-like notochord runs through the entire length of the vertebral column1 being constricted at the centres of the centra1 and dilated between them4 ,the neural arch above the centrum1 and containing the spinal cord1 is made up of neural plates 7n4p471 and interneural plates 7i4n4p471 completed above by a median neural spine 7n4s474 ,in the caudal region1 instead of ribs projecting outwardly1 there are haemal processes1 inclined downwards and meeting below1 forming an arch1 the haemal arch1 containing the caudal artery and vein-- the vein ventral to the artery-- and resembling the neural arch1 which contains the spinal cord above1 in shape and size4 ,section #aj4 ,the pectoral limb and girdle 7,figure #d1 ,sheet #af7 have only a very vague resemblance to the corresponding structures in the rabbit4 ,the girdle 7g47 is a transverse bar lying ventral to the pericardial wall1 and sending up a portion 7sc471 dorsal to the attachment of the limb1 which answers to the scapula and supra-scapula of the forms above the fish4 ,three main cartilages1 named respectively the pro- 7p4p471 meso- 7m4p471 and meta-pterygium1 form the base of the limb4 ,with these1 smaller cartilaginous plates1 rods1 and nodules articulate1 and form a flattened skeletal support for the fin4 ,section #aa4 ,the pelvic girdle and limb 7,figure #b1 ,sheet #ae7 are similar in structure1 but the pro-pterygium and meso-pterygium are absent1 and the cartilage answering to the meta-pterygium goes by the name of the basi-pterygium4 ,in the male1 but not in the female1 the pelvic fins are united behind the cloaca1 and there are two stiff grooved copulatory organs1 the claspers 7cl4 in ,figure #a71 which have a cartilaginous support 7cl4c474 ,these claspers form the readiest means of determining the sex of a specimen before dissection4 ,section #ab4 ,the skull consists of a cartilaginous cranium1 and of jaw and visceral arches4 ,the cranium persists throughout life1 in what closely resembles a transitory embryonic condition of the higher types4 ,there is a nasal capsule 7na4c471 a brain case proper1 and lateral otic 7auditory7 capsules 7ot4c47 containing the internal ear4 7,this should be compared with the frog's embryonic skull47 ,the upper jaw has a great bar of cartilage1 the palato-pterygoid1 as its sole support2 the arch of premaxilla1 maxilla1 jugal1 and squamosal-- all membrane bones-- is1 of course1 not represented4 ,in the frog this bar of cartilage is joined directly to the otic capsule by a quadrate portion1 but this is only doubtfully represented in the dog-fish by a nodule of cartilage in the pre-spiracular ligament 7p4s474 ,the lower jaw is supported1 by ,meckel's cartilage 7,m4,c474 ,the hyoid arch consists of two main masses of cartilage1 the hyomandibular 7h4m471 and the ceratohyal 7c4h472 the former of these is tilted slightly forward1 so that the gill slit between it and the jaw arch is obliterated below1 and the cartilage comes to serve as the intermediary in the suspension of the jaw from the otic mass4 ,there are five branchial arches1 made up pharyngo-1 epi- and cerato-branchials1 and the ventral elements fuse in the middle line to form a common plate of cartilage4 ,outside these arches are certain small cartilages1 the extra branchials 7ex4b47 which1 together with certain small labials by the nostrils and at the sides of the gape1 probably represent structures of considerably greater importance in that still more primitive fish1 the lamprey4 ,the deep groove figured lateral to the otic capsule is the connecting line of the orbital and anterior cardinal sinuses2 the outline of the anterior cardinal sinus in this figure and in ,figure #a is roughly indicated by a dotted line4 ,section #ac4 ,figure #c;a is a rough diagram of the internal ear-- the only auditory structure of our type 7compare ,rabbit1 ,sheet #g74 ,to dissect out the auditory labyrinth without injury is a difficult performance1 but its structure may be made out very satisfactorily by paring away successive slices of the otic mass4 ,such a section is shown by ,figure #c;b2 through the translucent hyaline cartilage the utriculus and horizontal canal can be darkly seen4 ,the ductus endolymphaticus 7vide ,rabbit7 is indicated by a dotted line in our figure4 ,it is situated internal to the right-angle between the two vertical canals1 and reaches to the surface of the otic capsule4 ,section #ad4 ,the brain shows the three primary vesicles much more distinctly than do our higher types4 ,the fore-brain has large laterally separated olfactory lobes 7rh471 there are relatively small hemispheres 7pr4c471 the stalk of the pineal gland tilts forward1 and the gland itself is much nearer the surface1 being embedded in the cartilage of the brain case1 and the pituitary body is relatively very large1 and has lateral vascular lobes on either side4 ,following the usual interpretation of the parts1 we find optic lobes 7op4l47 as the roof of the mid-brain1 and behind a very large1 median1 hollow1 tongue-shaped cerebellum 7c4b474 ,the medulla is large1 and certain lateral restiform tracts 7r4t47 therein1 which also occur in the higher types1 are here exceptionally conspicuous4 ,section #ae4 ,the dog-fish has ten pairs of cranial nerves1 corresponding to the anterior ten of the rabbit very closely1 when we allow for the modification the latter has suffered through the conversion of some part of the spiracular cleft to an eardrum1 and the obliteration of the post-hyoid branchial slits4 ,the first and second nerves are really brain lobes1 and nerves of the special senses of smell and sight respectively4 ,the third 7oculomotor71 the fourth 7patheticus71 and the sixth 7abducens7 are distributed to exactly the same muscles of the eyeball as they are in the rabbit4 ,the fifth nerve1 has1 in the dog-fish1 as in the rabbit1 three chief branches4 ,v4#b and ,v4#c fork over the mouth just as they do in the mammal2 ,v4#a passes out of the cranium by a separate and more dorsal opening1 and runs along a groove along the dorsal internal wall of the orbit1 immediately beneath a similar branch of ,,vii41 which is not distinct in the rabbit4 ,the grooves are shown in the figure of the cranium1 ,sheet #ah2 the joint nerve thus compounded of ,v4 and ,,vii4 is called the ophthalmic 7oph474 ,it is distributed to the skin above the nose and orbit4 ,when the student commences to dissect the head of a dog-fish he notices over the dorsal surface of the snout an exudation of a yellowish jelly-like substance1 and on removing the tough skin over this region and over the centre of the skull he finds1 lying beneath it1 a quantity of coiling simple tubuli full of such yellowish matter4 ,these tubuli open on the surface by small pores1 and the nerves terminate in hair-like extremities in their lining4 ,these sense tubes are peculiar to aquatic forms2 allied structures are found over the head and along a lateral line 7see below7 in the tadpole1 but when the frog emerges from the water they are lost4 ,they1 doubtless1 indicate some unknown sense entirely beyond our experience1 and either only possible or only necessary when the animal is submerged4 ,in addition to the ophthalmic moiety mentioned above1 the seventh nerve has a vidian branch 7vid47 running over the roof of the mouth1 and besides this its main branches fork over the spiracle1 just as ,v4 forks over the mouth1 and as ,,ix4 and ,x4 fork over gill clefts4 ,this nerve in the rabbit is evidently considerably modified from this more primitive condition4 ,the eighth is the auditory nerve1 as in the rabbit4 ,the ninth nerve forks over the first branchial cleft4 ,the tenth nerve is easily exposed by cutting down through the body wall muscles over the gill clefts1 into the anterior cardinal sinus 7,a4,c4,s474 ,it gives off 7a7 branches forking over the posterior four gill slits1 7b7 a great lateral nerve running inward1 and back through the body-wall muscle1 and connected with a line of sense organs similar to those in the head1 the lateral line1 and 7c7 a visceral nerve curving round to the oesophagus and stomach4 ,in dissection it becomes very evident that the tenth nerve is really a leash of nerves1 each one equivalent to the ninth4 ,we may here call the attention of the reader to the fact of the singular resemblance of ,v41 ,,vii41 ,,ix41 and the factors of ,x4 ,that each has a ventral fork1 we have already noticed4 ,each also 78,,ix47 has a dorsal constituent connected with the sense organs of the skin4 ,the vidian branch of ,,vii41 however1 is not evidently represented in the others4 ,section #af4 ,the coelom of the dog-fish is peculiar-- among the types we treat of-- in the possession of two direct communications with the exterior1 in addition to the customary indirect way through the oviduct4 ,these are the abdominal pores 7a4p47 on either side of the cloaca in either sex4 ,they can always be readily demonstrated by probing out from the body cavity1 in the direction indicated by the arrow 7a4p47 in ,figure #a1 ,sheet #ae4 ,they probably serve to equalize the internal and external pressure of the fish as it changes its depth in the water1 just as the ,eustachian tubes equalize the pressure on either side of the mammal's tympanic membrane4 ,section #ag4 ,the musculature of the dog-fish body is cut into ,v-shaped segments1 the point of the ,v being directed forward4 ,the segments alternate with the vertebrae1 and are called myomeres4 ,such a segmentation is evident1 though less marked1 in the body wall muscles of the frog1 and in the abdominal musculature of the rabbit and other mammals it is still to be traced4 ,section #ah4 ,the uro-genital organs of the female dog-fish 7,figure #a1 ,sheet #ag7 consist of an unpaired ovary 7ov471 paired oviducts 7o4d471 enlarged at one point to form an oviducal gland 7o4d4g471 kidneys 7k471 with ureters 7ur47 uniting to form a urinary sinus 7u4s47 opening into the cloaca by a median urinary papilla separate from the oviducal openings4 ,the eggs contain much yolk1 and1 like those of the fowl1 are very large2 like the fowl1 too1 one of the ovaries is suppressed1 and it is the right ovary that alone remains4 ,the two oviducts meet in front of the liver ventral to the oesophagus1 and have there a common opening by which the ova are received after being shed into the body cavity4 ,the eggs receive an oblong horny case in the oviduct2 in the figure such a case is figured as distending the duct at e4 ,the testes of the male 7,t4 in ,figure #b7 are partially confluent in the middle line4 ,they communicate through vasa efferentia 7v4e47 with the modified anterior part of the kidney1 the epididymis 7ep471 from which the vas deferens 7v47 runs to the median uro-genital sinus 7u4g4s471 into which the ureters 7ur47 also open4 ,the silvery peritoneum 7lining of the body cavity7 covers over the reddish kidneys1 and hides them in dissection4 ,section #ai4 ,figure #c1 ,sheet #ag1 is a generalized diagram of the uro-genital organs in the vertebrata2 ,m4,l4 is the middle line of the body1 ,g4 is the genital organ1 ,pr4 is the pronephros1 or fore kidney1 a structure which is never developed in the dog-fish1 but which has functional importance in the tadpole and cod1 and appears as a transitory rudiment in the chick4 ,a duct1 which is often spoken of as the pronephric duct 7p4d471 and which we have figured under that name1 is always developed4 ,anteriorly it opens into the body cavity4 ,it is also called the ,mullerian duct1 and in the great majority of vertebrata it becomes the oviduct1 uniting with its fellow1 in the case of the dog-fish1 ventral to the oesophagus4 ,in the male it usually disappears2 the uterus masculinus of the rabbit is still very generally regarded as a vestige of it4 ,kolliker has shown1 however1 that this interpretation is improbable4 ,ms4 is the mesonephros1 some or all of which becomes the epididymis in the male of types possessing that organ1 and is connected with ,g4 by the vasa efferentia4 ,mt41 the metanephros1 is1 in -actual fact- the frog1 indistinguishably continuous with ,ms41 and is the functional kidney1 its duct 7metanephric duct7 being either undifferentiated from the mesonephric 7as is the case with the frog7 or largely split off from it1 as in the dog-fish1 to form the ureter4 ,section #bj4 ,the correspondence of the male organs of the dog-fish with those of the rabbit1 will be more evident if the student imagine-- 7a7 the testes1 vasa efferentia1 and epididymis of each side to shift posteriorly until they reach a position on either side of the cloaca2 and 7b7 ,the uro-genital apertures1 instead of meeting dorsally and posteriorly to the anus1 to shift round that opening and meet anteriorly and ventrically to it4 ,section #ba4 ,this completes our survey of this type4 ,except where we have specified differences1 the general plan of its anatomy follows the lines of the other vertebrate types described4 ,questions on the ,dog-,fish #a4 ,describe the alimentary canal of the dog-fish1 and compare it with that of the rabbit in detail4 #b4 ,compare the coelom of the dog-fish and rabbit4 #c4 ,draw diagrams to illustrate the course of the circulation in the dog-fish4 #d4 7a7 ,describe fully the heart of a dog-fish4 7b7 ,compare it with that of a rabbit4 #e4 ,give an account of the respiratory apparatus of the dog-fish4 #f4 ,draw diagrams of a dog-fish vertebra1 and compare the centrum with that of a rabbit4 #g4 ,compare the vertebral column of the dog-fish and rabbit4 #h4 ,draw diagrams of the limbs and limb-girdles of the dog-fish4 ,compare the pectoral with the pelvic fin4 #i4 ,draw diagrams of 7a7 the male and 7b7 the female urogenital organs of the dog-fish4 7c7 ,compare them carefully with those of the rabbit4 #aj4 ,compare the circulation in the kidney of dog-fish and rabbit4 #aa4 ,give an account of the cranio-facial apparatus of the dog-fish4 ,state clearly what representation of this occurs in the frog and in the rabbit4 #ab4 ,give drawing 7a7 from above1 7b7 from the side1 of the dog-fish brain4 #ac4 ,state the origin and the distribution of the fifth1 seventh1 ninth1 and tenth cranial nerves in the dog-fish4 #ad4 ,compare1 one by one1 the cranial nerves of the dog-fish with those of any higher vertebrate1 as regards their origin and their distribution4 #ae4 ,describe the auditory organ of the dog-fish4 ,what parts are added to this in the higher type8 #af4 ,draw the cloaca 7a7 of a male1 7b7 a female dog-fish4 #ag4 7,practical47 ,demonstrate in a dog-fish the pathetic nerve1 the opening between pericardium and coelom4 the abdominal pores1 and the ureter4 -,amphioxus #a4 ,anatomy ,section #a4 ,we find in ,amphioxus the essential vertebrate features reduced to their simplest expression and1 in addition1 somewhat distorted4 ,there are wide differences from that vertebrate plan with which the reader may now be considered familiar4 ,there are no limbs4 ,there is an unbroken fin along the median dorsal line and coming round along the ventral middle line for about half the animal's length4 ,but two lowly vertebrates1 the hag-fish and lamprey1 have no limbs and a continuous fin4 ,there is1 as we shall see more clearly1 a structure1 the respiratory atrium1 not apparently represented in the true vertebrate types1 at least in their adult stages4 ,there is no distinct heart1 only a debateable brain1 quite without the typical division into three primary vesicles1 no skull1 no structures whatever of cartilage or bone1 no genital ducts1 no kidneys at all resembling those of the vertebrata1 no pancreas1 no spleen2 apparently no sympathetic chain1 no paired sense organs1 eyes1 ears1 or nasal sacs1 in all of which points we have striking differences from all true vertebrata2 and such a characteristic vertebrate peculiarity as the pineal gland we can only say is represented very doubtfully by the eye spot4 ,section #b4 ,the vertebral column is devoid of vertebrae2 it is throughout life a rod of gelatinous tissue1 the notochord 7,figure #a1 n4c471 surrounded by a cellular sheath4 ,such a rod is precursor to the vertebral column in the true vertebrates1 but1 except in such lowly forms as the lamprey1 is usually replaced1 partially 7e4g41 dog-fish7 or wholly 7as in the rabbit7 by at first cartilaginous vertebrae whose bodies are derived from its sheath4 ,further1 while in all true vertebrata the notochord of the developing young reaches anteriorly at most to the mid-brain1 and is there at its termination enclosed by the middle portion of the skull1 in ,amphioxus it reaches far in front of the anterior extremity of the nervous system1 to the end of the animal's body499 ,on this account the following classification is sometimes made of those animals which have a notochord3-- -,chordata 7 ,vertebrata1 as used by ,lankester74 #a4 ,having the notochord reaching in front of the brain4 ,cephalochorda ,amphioxus4 #b4 ,having the notochord reaching anteriorly to the mid-brain1 a brain of three primary vesicles and a skull4 ,craniata all true vertebrata3 fishes1 amphibia1 reptiles1 birds1 and mammals 7,vertebrata of ,balfour74 #c4 ,having the notochord confined to the tail4 ,urochorda the ascidians1 or sea-squirts1 certain forms of life only recently recognised as relatives of the vertebrata4 99 ,the anterior end of the notochord in the developing rabbit or dog lies where the middle of the basisphenoid bone is destined to be4 ,section #c4 ,figure #a1 ,sheet #ai1 shows the general anatomy of ,amphioxus4 ,we recognise four important points of resemblance to the earlier phases of the higher and the permanent structure of the lower members of the vertebrata1 and it is these that justify the inclusion of amphioxus in this volume4 ,in the first place there is the-- -,notochord4 ,in the next1 just above it 7at s4c47 we find-- -,a ,dorsal ,tubular ,nervous ,axis1 the spinal cord4 ,thirdly1 the pharynx 7ph47 is perforated by-- -,respiratory ,slits1 though these1 instead of being straight slashes1 are modified from a ,u-shape slant very much forward and are much more numerous than in any true vertebrate44 -,and-1 ,fourthly1 there is1 as we shall see1 a-- -,vertebrate ,type of ,circulation4 ,and finally the body-wall muscles are divided into-- -,myomers4 ,section #d4 ,the alimentary canal of ,amphioxus commences with an oral cavity1 not represented in our vertebrata1 surrounded by a number of cirri1 or tentacles1 supported by a horny substance which seems to be chitin1 a common skeletal material among invertebrates4 ,a velum 7v47 forms a curtain1 perforated by the mouth and by two smaller hyoidean apertures1 between the oral cavity and the pharynx 7ph474 ,pharyux is here used in a wider sense than in the true vertebrata2 it reaches back close to the liver1 and is therefore equivalent to pharynx oesophagus a portion or all of the stomach4 ,the so-called hyoidean apertures are not equivalent to the similarly-named parts of the vertebrata4 ,behind the pharynx the intestine 7int47 runs straight out to the anus 7an471 which opens not in the middle line1 as one might expect1 but in the left side6 ,the liver lies usually on the creature's right1 and instead of being a compact gland1 is simply bag-like4 ,section #e4 ,the circulation is peculiarly reduced 7,figure #b74 ,the cardiac aorta 7c4ao47 lies along the ventral side of the pharynx1 and sends branches up along the complete bars between the gill slits4 ,there is no -distinct- heart1 but the whole of the cardiac aorta is contractile1 and at the bases of the aortic arches that run up the bars there are contractile dilatations that assist in the propulsion of the blood4 ,dorsal to the pharynx1 as in fishes1 there is a pair of dorsal aorta 7d4ao47 that unite above the liver 7compare the frog1 for instance71 and thence run backward as a median dorsal aorta 7d4ao4'74 ,a portal vein 7p4v47 bring blood back from the intestine 7and apparently from the whole posterior portion of the animal7 to the liver4 ,thence hepatic veins 7hep47 take it to the cardiac aorta4 ,lines from ,first ,edition only4 -,when we remember that in the embryonic vertebrate the heart is at first a straight tube1 this circulation appears even more strikingly vertebrate in its character than before4- ,section #f4 ,the coelom1 or body cavity1 of ,amphioxus lies1 of course1 as in the vertebrata1 between the intestinal wall and the body walls1 and1 just as in the vertebrata1 it is largely reduced where gill slits occur4 ,but matters are rather complicated by the presence of an atrial cavity round the pharynx1 which is not certainly represented in the vertebrata1 and which the student is at first apt to call the body cavity1 although it is entirely distinct and different from that space4 ,the mutual relation of the two will become apparent after a study of ,figures #aj1 #aa1 #ab 7,sheet #ba74 ,figure #aj gives diagrammatically a section of a very young stage of ,amphioxus2 ,p is the pharynx portion of the alimentary canal1 coe4 is the coelom surrounding it at this stage here as elsewhere2 mt4c4 are certain lymph spaces1 the metapleural canals1 between which a small invagination 7i4e41 a pushing-in71 at41 of the outer epidermis occurs2 n4c4 is the notochord1 and s4c4 the spinal cord4 ,the gill slits1 by which ,p4 communicates with the exterior1 are not shown4 ,next ,figure #aa shows the invagination 7at47 pushing its way in1 and cut off from the exterior by a meeting of the body wall below4 ,note that at4 is a portion of the animal's exterior thus embraced by its body1 and that its lining is therefore of the same material as the external integument4 ,in ,figure #ab1 at4 is developing upward1 so that the true body hangs into it4 ,now imagine the gill slits perforated1 as shown by the double-headed arrow in ,figure #ab4 ,figure #c1 on ,sheet #bj1 is a less diagrammatic representation of a cross-section of the pharyngeal region 7vide ,figure #a1 ,sheet #ai74 ,the student should compare ,figure #c1 ,sheet #bj1 and ,figure #ab1 ,sheet #ba4 ,the atrium and metapleural canals are easily recognised in both4 ,in ,figure #c the coelom is much cut up by the gill slits1 and we have remaining of it 7a7 the dorsal coelomic canals 7d4c4c47 and 7b7 the branchial canals 7br4c47 in the bars between the slits4 ,the atrial cavity remains open to the exterior at one point1 the atrial pore 7at4p474 ,section #g4 ,the method of examining cross-sections is an extremely convenient one in the study of such a type as ,amphioxus4 ,the student should very carefully go over and copy the six sections on ,sheet #bj1 comparing ,figure #a as he goes4 ,he should do this before reading what follows4 ,one little matter must be borne in mind4 ,these figures are merely intended to convey the great structural ideas1 and they are considerably simplified2 they must not be regarded as a substitute for the examination of microscopic sections4 ,he will notice a number of rounded masses from the body wall4 ,the -,for instance1 the body-wall- muscles of ,amphioxus are arranged in bundles bent sharply in an arrow shape1 the point forward4 -,a number of these bundles are cut in any one section1 and so the even shading of our diagrams1 if they professed to be anything more than diagrams1 should be broken up into masses4- ,these -bundles1 we may mention-1 are called myomeres1 and they are indicated in ,figure #a by lines pointing acutely forward4 ,several are consequently cut in any transverse section 7,sheet #bj71 and these are the rounded masses he sees4 ,similar myomeres1 similarly situated1 are found in fish1 behind the head1 and1 less obviously1 they occur with diminishing importance as the scale of the vertebrata is ascended4 ,section #h4 ,if we compare the nervous system of amphioxus with that of any vertebrate1 we find at once a number of striking differences4 ,in the first place1 the skeletal covering of it1 the cranium and the neural arches of vertebrae1 are represented only by a greatly simplified connective tissue4 ,in the next1 a simple and slight anterior dilatation alone represents the brain4 ,a patch of black pigment anterior to this 7e4s47 may or may not be what its name implies an eye-spot4 ,there is a ciliated funnel1 c4f4 7,figure #a1 ,sheet #ai71 opening on the left side1 which has been assumed to be olfactory in its functions1 and in the mouth chamber a ciliated pit 7c4p471 which may1 or may not1 be an organ of taste4 ,the ventral fissure of the spinal cord is absent4 ,the dorsal nerves are without ganglia1 and do not come off in pairs1 but alternately1 one to the left1 then one to the right1 one to the left1 one to the right1 and so on4 ,the ventral nerves are very short1 more numerous than the dorsal1 and never unite with these latter to form mixed nerves4 ,the student will observe that here1 just as in the case of the ciliated funnel and anus1 the ,amphioxus is not strictly symmetrical1 but twisted1 as it were1 and so departs from the general rule of at least external bilateral symmetry obtaining among the vertebrates4 ,it habitually lies on one side in the mud of the sea bottom1 and it is probable that this external asymmetry is due to this habit1 so that too much classificatory importance must not be attached to it4 ,the soles and other related fish1 for instance1 are twisted and asymmetrical1 through a similar specific habit1 to such an extent that both eyes lie on one side of the animal4 ,section #i4 ,no kidney on the vertebrate pattern is found1 but the following structures have1 among others1 been suggested as renal organs3-- 7a7 ,certain canals1 the brown tubes of ,lankester 7b4t4,l41 ,figure #b1 ,sheet #ai71 a pair of pigmented tubes opening into the atrium at the hind end of the pharynx1 lying forward along by the dorsal coelomic canals1 and having an internal opening also4 7b7 ,certain tubuli described by ,weiss as situated in a series along the upper corners of the atrial cavity1 and communicating1 after the fashion1 of the nephridia of the earthworm1 with the coelom and with the exterior 7or1 rather1 with that portion of the animal's exterior enclosed in by the atrial wall2 compare ,section #f74 7c7 ,the general epithelial lining of the atrium4 ,the reproductive organs 7,figure #d1 ,sheet #bj1 g47 are masses of cells situated in an isolated part of the coelom in the atrial folds1 and1 having no ducts1 their contents must escape into the atrium by rupture of the body-wall4 ,thence they escape either by gill-slits1 pharynx and mouth1 or1 more generally1 through the atrial pore4 ,the animals1 like all the vertebrata1 are dioecious1 i4e41 male or female4 ,section #aj4 ,the endostyle 7end471 in ,figures #c and #d1 is a ciliated path or groove on the under side of the pharynx1 which is generally supposed to represent the thyroid gland of vertebrates4 ,the vertebrate thyroid1 early in development1 is certainly an open and long narrow groove in the ventral side of the pharynx4 ,the hyper-pharyngeal groove 7h4p47 has been in the past compared to the pituitary body1 but there is little doubt now that this structure is represented by the ciliated pit4 ,section #aa4 ,the student is advised to revise this chapter before proceeding1 and to schedule carefully the anatomical features under the headings of 7#a47 distinctly vertebrate characters1 7#b47 characters contrasting with the normal vertebrate structure1 7#c47 facts of doubtful import1 with the suggestions given in the text written against them4 #b4 ,the ,development of ,amphioxus ,section #ab4 ,the development of amphioxus1 studied completely1 is at once one of the most alluring and difficult tasks in the way of the zoologist2 but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now4 ,section #ac4 ,the phenomena of the extrusion of polar bodies and fertilization are treated of later1 and will1 therefore1 not be considered now4 ,we will start our description with an egg-cell1 which has escaped1 of course1 since there are no genital ducts1 by rupture of the parent1 has been fertilized by the male element1 and is about to develop into a young amphioxus4 ,it is simply a single cell1 with some power of amoeboid motion1 a single nucleus and nucleolus2 and in amphioxus its protoplasm is clear and transparent4 ,frequently ova are loaded with granules of food store 7yolk71 which enable the young animal to go far with its development before it is hatched and has to begin fending for itself4 ,such an ovum as that of our present type1 however being devoid of such yolk 7alecithal without yolk71 necessitates a very early start in life1 and1 for reasons too complicated to state fully here1 the development in such a case is considered particularly instructive and primitive by zoologists4 ,section #ad4 ,the first thing to be seen in the developing cell is a deepening circular groove 7,figure #a1 ,sheet #ba71 which divides the ovum into two parts4 ,another groove then cuts at right angles to this subdividing the two into four 7,figure #b74 ,another groove1 at right angles to both the former1 follows1 making the four eight 7,figure #c74 ,and so subdivision goes on4 ,the whole process is called segmentation or cleavage4 ,section #ae4 ,at the end of segmentation we get a hollow sphere of small cells1 the cells separating from one another centrally and enclosing a cavity as the process proceeds4 ,this is the blastosphere1 shown diagrammatically in ,figure #d1 and of which an internal view1 rather truer to the facts of the case as regards shape1 is given as ,figure #e4 ,the central cavity is the segmentation cavity 7s4c474 ,section #af4 ,invagination follows 7,figure #f74 ,in this process a portion of the blastosphere wall is the tucked into the rest1 as indicated by the arrow1 so that a two-layered sack is formed4 ,the space ar4 is the archenteron1 the primordial intestine1 and its mouth is called1 the blastopore 7bp474 ,the outer layer of this double-walled sac is called the epiblast4 ,for the present we will give the inner lining no special term4 ,the young amphioxus has1 at this stage1 which is called the gastrula stage1 a curious parallelism with such a lowly form as the ,hydra of our ditches4 ,this latter creature1 like the gastrula1 consists essentially of two layers of cells1 an outer protective and sensory layer1 and an inner digestive one2 it has a primordial intestine1 or archenteron1 and its mouth is sometimes regarded as being a blastopore4 ,all animals that have little yolk1 and start early in life for themselves1 pass through a gastrula stage1 substantially the same as this of amphioxus4 ,section #ag4 ,the anus is perforated later near the region occupied at this stage by the blastopore4 ,hence the anterior end of the future amphioxus1 the head end1 is pointing towards the ,figure #f1 and the letters ep4 are marked on the side which will be dorsal4 ,section #ah4 ,figure #g i4 is a dorsal view of the gastrula at a somewhat later stage1 and here indications of distinctly vertebrate relationships already appear4 ,figure #g ii4 is a cross-section1 its position1 being shown by cross-lines in #g i4 and #f4 ,note first that the epiblast along the mid-dorsal line is sinking in to form what is called the neural plate 7n4p471 and simultaneously on either side of it rise the neural folds 7n4f474 ,now1 at ,figure #h1 a slightly later stage is represented1 and at #i i4 the inturned part is separated from the general external epiblast as the spinal cord4 ,the remainder of the epiblast constitutes the epidermis4 ,section #ai4 ,reverting to ,figure #g ii41 along the dorsal side of the archenteron a thickening of its wall appears1 and is gradually pinched off from it to form a cellular rod1 lying along under the nervous axis and above the intestine4 ,this is the notochord 7compare ,figures #h and #i74 ,section #bj4 ,finally1 we note two series of buds of cells1 one on either side of the archenteron in ,figure #g ii4 ,in #h these buds have become hollow vesicles1 growing out from it1 the coelomic pouches4 ,they are further developed in #i2 and in #i ii41 which is a diagrammatic figure1 they are indicated by dotted lines4 ,they finally appear to 78 entirely7 obliterate the segmentation cavity-- they certainly do so throughout the body2 and their cavities are in time cut off from the mesenteron1 by the gradual constriction of their openings4 ,in this way the coelom 7body cavity7 arises as a series of hollow archenteric outgrowths1 and ms4 becomes the alimentary canal4 mt4c41 the metapleural canals1 probably arise subsequently to1 and independently of1 the general coelomic space1 by a splitting in the body-wall substance4 ,section #ba4 ,hence1 in considering the structure of amphioxus1 we have three series of cells from which its tissues are developed3-- #a4 ,the epiblast4 #b4 ,walls of the coelomic pouches1 which form 7a7 an inner lining to the epiblast1 7b7 an outer coating to the hypoblast1 and 7c7 the mesentery 7m471 by which the intestine is supported4 ,this is the mesoblast4 #c4 ,the lining of the mesenteron1 or hypoblast4 ,from the epiblast the epidermis 7not the dermis71 the nervous system 7including the nerves71 and the sensory part of all sense organs are derived4 ,from the mesoblast the muscles1 the dermis genital and excretory organs1 circulatory fluid and apparatus1 any skeletal structures2 and all connective tissue are derived4 ,the mass of the body is thus evidently made of mesoblast4 ,the hypoblast is the lining of the intestine and of the glands which open into it2 and the material of the notochord is also regarded1 as hypoblast4 ,section #bb4 ,figure #i ii4 shows all the essential points of the structure of amphioxus4 ,epiblast is indicated by a line of dashes1 mesoblast by dots1 and hypoblast1 dark or black4 ,the true mouth is formed late by a tucking-in of epiblast1 the stomodaeum 7s4d471 which meets and fuses with the hypoblast1 and is then perforated4 ,the position of this mouth is at the velum4 ,the formation of the atrium has been described4 ,the metapleural folds run forward in front of the velum1 as the epipleurs 7ep4 in ,sections #a and #b71 and form an oral hood 7b4c471 around which the tentacles appear1 and which is evidently not equivalent to the vertebrate mouth cavity1 but in front of and outside it4 ,the anus is formed by a tucking in1 the proctodaeum1 similar to the stomodaeum4 ,section #bc4 ,the formation of the respiratory slits is complicated1 and difficult to describe1 but1 since investigators have still to render its meaning apparent1 it need not detain the elementary student499 99 ,see ,balfour's ,embryology1 ,volume #b1 and ,quarterly ,journal of ,microscopical ,science ,march1 #ahia4 ,questions on ,amphioxus #a4 ,draw diagrams1 with the parts named1 of the alimentary canal of 7a7 amphioxus1 7b7 any craniate2 7c7 indicate very shortly the principal structural differences between the two4 #b4 ,describe1 with a diagram1 the circulation of amphioxus4 ,compare it with that of the craniata4 #c4 ,draw from memory transverse sections1 of amphioxus 7a7 in the oral region1 7b7 through the pharynx1 7c7 just anterior1 and 7d7 just posterior to atrial pore4 #d4 ,describe fully the coelom of amphioxus1 and compare it with that of the frog in regard to 7a7 development1 7b7 its relation to other organs in the adult4 #e4 ,compare the atrial cavity and coelom of amphioxus4 ,to what series of cavities in the frog are the metapleural canals to be compared8 #f4 ,describe the notochord of amphioxus1 and point out its differences from the vertebrate notochord4 #g4 ,describe1 with diagrams1 the nervous system of amphioxus1 and compare its nervous axis1 in detail1 with that of a vertebrate4 #h4 ,compare the genital organs of amphioxus with those of a higher vertebrate4 #i4 ,what structures have been regarded1 as renal organs in amphioxus8 #aj4 ,what is a gastrula8 ,with what lower type has the gastrula been compared8 ,discuss the comparison4 -,development ,the ,development of the ,frog ,section #a4 ,we have now to consider how the body of the frog is built up out of the egg cell1 but previously to doing so we must revert to the reproductive organs of our type4 ,section #b4 ,in the testes of the male is found an intricate network of tubuli1 the lining of which is1 of course1 an epithelium4 ,the cells of this epithelium have their internal borders differentiated into spermatozoa1 which1 at a subsequent stage1 are liberated4 ,a spermatozoon 7,figure #c1 ,sheet #ac1 sp47 is a rod-shaped cell containing a nucleus2 in fact1 consisting chiefly of nucleus1 with a tail1 the flagellum1 which is vibratile1 and forces the spermatozoon1 forward by its lashing4 ,the spermatozoa float in a fluid which is the joint product of the testes1 anterior part of the kidney1 and perhaps the prostate glands4 ,section #c4 ,in the ovary1 the ova are formed1 and grow to a considerable size4 ,they are nucleated cells1 the nucleus going by the special name of the germinal vesicle and the nucleolus the germinal spot4 ,the ova prey upon the adjacent cells as they develop4 ,the protoplasm of the ovum1 except at that part of the surface where the germinal vesicle lies1 is packed with a great amount of food material1 the yolk granules4 ,this yolk is non-living inert matter4 ,an ovum such as this1 in which the protoplasm is concentrated towards one pole1 is called telolecithal4 ,section #d4 ,after the ovum has finished its growth1 and elaborated the yolk within itself1 a peculiar change occurs in the small area free from yolk-- the animal pole1 in which the germinal vesicle lies4 ,this germinal vesicle divides1 and one moiety is budded off from the ovum4 ,the ovum has1 in fact1 undergone cell division into a very large cell containing most of its substance1 and a small protoplasmic pimple surrounding half of its nucleus4 ,the disproportion is so great between the two cells1 that the phenomenon does not at first suggest the idea of cell division1 and it is usually described as the extrusion of the first polar body4 ,there follows a second and similar small cell1 behind the first1 the second polar body4 ,since the nucleus of the ovum has divided twice1 it is evident that the nucleus remaining now in the ovum is a quarter of the original nucleus4 ,very little protoplasm is given off with the polar bodies2 they play no further part in development1 but simply drop off and disappear4 ,not only in the frog's ovum1 but in all vertebrata1 two polar bodies are given off in this way before the sexual process occurs4 ,their exact meaning has been widely discussed4 ,it is fairly evident that some material is removed from the nucleus1 which would be detrimental to further developments1 and the point debated is what is the precise nature of this excreted material4 ,this burning question we can scarcely deal with here4 ,section #e4 ,but here we may point out that in all cells the function of the nucleus appears to be to determine growth and division4 ,it is the centre of directive energy in the cell4 ,section #f4 ,fertilization is effected by a spermatozoon meeting with the ovum4 ,it fuses with it1 its nucleus becoming the male pro-nucleus4 ,this and the female pro-nucleus1 left after the extrusion of the polar cells1 move towards each other1 and unite to form the first segmentation nucleus4 ,section #g4 ,the ovum next begins to divide4 ,a furrow cutting deeper and deeper divides it into two2 another follows at right angles to this1 making the two four1 and another equatorial furrow cuts off the animal pole from the yolk or vegetative pole4 7,see ,sheet #bb1 ,figures #a1 #b1 and #c47 ,and so segmentation 7 cleavage7 proceeds1 and1 at last1 a hollow sphere1 the blastosphere 7,figure #d7 is formed1 with a segmentation cavity 7s4c474 ,but1 because of the presence of the yolk at the vegetative pole of ovum1 and of the mechanical resistance it offers to the force of segmentation1 the protoplasm there is not nearly so finely divided-- the cells1 that is to say1 are much larger than at the animal pole4 ,the blastosphere of the frog is like what the blastosphere of amphioxus would be1 if the future hypoblast cells were enormously larger through their protoplasm being diluted with yolk4 ,section #h4 ,the next phase of development has an equally curious resemblance to and difference from what occurs in the case of the ova of animals which do not contain yolk4 ,in such types 7e4g41 amphioxus7 a part of the blastosphere wall is tucked into the rest1 and a gastrula formed by this process of invagination4 ,in the frog 7,figure #e7 there is a tucking-in1 but the part that should lie within the gastrula1 the yolk-containing cells1 are far larger than the epiblast 7ep47 which should1 form the outer layer of cells4 ,hence the epiblast can only by continual growth accommodate what it must embrace1 and the process of tucking-in is accompanied by one of growth of the epiblast1 as shown by the unbarbed arrow1 over the yolk4 ,this stage is called the gastrula stage2 ar4 is the cavity of the gastrula1 the archenteron2 b4p4 is its opening or blastopore4 ,such a gastrula1 formed mainly by overgrowth of the epiblast1 is called an epibolic gastrula1 as distinguished from the invaginate gastrula of amphioxus4 ,the difference is evidently entirely due to the presence of yolk1 and the consequent modification of invagination in the former case4 ,section #i4 ,comparing the two gastrulas1 it is not difficult to see that if we imagine the ventral wall of the archenteron of amphioxus to have its cells enormously enlarged through the mixing of yolk with their protoplasm1 we should have a gastrula essentially like that of the frog4 ,section #aj4 ,figure #f shows a slightly later ovum than ,figure #e1 seen from the dorsal side4 b4p4 is the blastopore4 ,in front of that appears a groove1 the neural groove1 bordered on either side by a ridge1 the neural fold 7n4f474 ,this is seen in section in ,figure #g2 s4c4 is the neural groove2 n4f41 as before1 the neural fold4 ,the neural folds ultimately bend over and meet above1 so that s4c4 becomes a canal1 and is finally separated from the epiblast to form the spinal cord4 ,below the neural groove a thickening of the dorsal wall of the archenteron appears1 and is pinched off to form a longitudinal rod1 the precursor of the vertebral column1 the notochord1 shown in ,figure #g 7n4c471 as imperfectly pinched off4 ,section #aa4 ,simultaneously1 on either side of the notochord appear a series of solid masses of cells1 derived mainly by cell division from the cells of the wall of the archenteron1 and filling up and obliterating the segmentation cavity4 ,these masses increase in number by the addition of fresh ones behind1 during development1 and are visible in the dorsal view as brick-like masses1 the mesoblastic somites or proto-vertebrae 7,figure #f1 i41 ii41 iii474 ,in ,figure #g1 these masses are indicated by dotting4 ,in such a primitive type as amphioxus these mesoblastic -somites- masses contain a cavity1 destined to be the future body cavity1 from the first4 ,in the frog1 the cavity is not at first apparent2 the mesoblast at first seems quite solid1 but subsequently what is called the splitting of the mesoblast occurs1 and the body cavity 7b4c4 in ,figure #g7 appears4 ,the outer mesoblast1 lying immediately under the epiblast1 constitutes the substance of the somatopleur1 and from it will be formed the dermis1 the muscles of the body wall1 almost all the cartilage and bone of the skeleton1 the substance of the limbs1 the kidneys1 genital organs1 heart and bloodvessels1 and1 in short1 everything between the dermis and the coelom1 except the nervous system and nerves1 and the notochord4 ,the inner mesoblast1 the mass of the splanchnopleur1 will form the muscle and connective tissue of the wall of the alimentary canal1 and the binding substance of the liver and other glands that open into the canal4 ,section #ab4 ,figure #h is one which we reproduce1 with the necessary changes in each plate of embryological figures given in this book1 so that the student will find it a convenient1 one for the purpose of comparison4 ,the lines of dashes1 in all cases1 signify -epiblast- hypoblast 1 the unbroken black line is -hypoblast-1 epiblast dotting shows mesoblast1 and the shaded rod 7n4c47 is the notochord4 c4s4 is the spinal cord2 br4#a1 br4#b1 br4#c are the three primary vesicles which constitute the brain1 and which form fore1 mid1 and hind brain respectively4 ,i4 is the intestine and ,y4 the yolk cells that at this early stage constitute its ventral wall4 ,section #ac4 ,figure #i gives a similar diagram of a later stage1 but here the blastopore is closed4 ,an epiblastic tucking-in at st41 the stomodaeum pre-figures the mouth2 pr41 the proctodaeum1 is a similar posterior invagination which will become the anus4 ,y41 the yolk1 is evidently much absorbed4 ,figure #aj is a young tadpole1 seen from the side4 ,the still unabsorbed yolk in the ventral wall of the mesentery gives the creature a big belly4 ,its mouth is suctorial at this stage1 and behind it is a sucker 7s47 by which the larvae attach themselves to floating reeds and wood1 as shown in the three black figures below4 ,section #ad4 ,we may now consider the development of the different organs slightly more in detail1 though much of this has already been approached4 ,the nervous system1 before the closure of the neural groove1 has three anterior dilatations1 the fore-1 mid-1 and hind-brains1 the first of which gives rise by hollow outgrowths to two pairs of lateral structures1 the hemispheres and the optic vesicles4 ,the latter give rise to the retina and optic nerve as described in ,development ,section #dj4 ,section #ae4 ,the hypoblastic notochord is early embraced by a mesoblastic sheath derived from the protovertebrae4 ,this becomes truly cartilaginous1 and at regular intervals is alternately thicker and thinner1 compressing the notochord at the thicker parts4 ,hence the notochord has a beaded form within this1 at first1 continuous cartilaginous sheath4 ,this sheath is soon cut into a series of vertebral bodies by jointings appearing through the points where the cartilage is thickest and the notochord most constricted4 ,hence what remains of the notochord lies within the vertebral bodies in the frog2 while in a cartilaginous fish1 such as the dog-fish1 or in the embryonic rabbit1 the lines of separation appear where the notochord is thickest1 and it comes to lie between hollow-faced vertebrae4 ,cartilaginous neural arches and spines1 formed outside the notochordal sheath1 enclose the spinal cord in an arcade4 ,the final phase is ossification4 ,as the tadpole approaches the frog stage the vertebral column in the tail is rapidly absorbed1 and its vestiges appear in the adult as the urostyle4 ,section #af4 ,the development of the skull is entirely dissimilar to that of the vertebral column4 ,it is shown on ,figures #a and #h1 ,sheet #ad2 and in the section devoted to the frog's skull a very complete account of the process is given4 ,the process of ossification is described under the histology of the ,rabbit4 ,section #ag4 ,the origin of the circulatory and respiratory organs is of especial interest in the frog4 ,in the tadpole we have essentially the necessities and organization of the fish2 in the adult frog we have a clear exposition of the structure of pigeon and rabbit4 ,the tadpole has1 at first1 a straight tubular heart1 burrowed out in somatic mesoblast1 and produced forward into a truncus arteriosus4 ,from this arise four afferent branchial arteries1 running up along the sides of the four branchial arches1 and supplying gills4 ,they unite above on either side in paired hyper-branchial arteries1 which meet behind dorsal to the liver1 to form a median dorsal aorta4 ,internal and external carotid arteries supply the head4 ,these four afferent branchial arches are equivalent to the first four of the five vessels of the dog-fish4 ,at first1 the paired gills are three in number1 external1 and tree-like1 covered by epiblast 7,figures #aj and #aa1 e4g471 and not to be compared to fish gills in structure1 or in fact -with- to any other gills within the limits of the vertebrata4 ,subsequently 7hypoblastic7 internal gills 7int4g41 ,figure #ab71 strictly homologous with the gills of a fish1 appear4 ,then a flap of skin outside the hyoid arch grows back to cover over the gills2 this is the operculum 7op4 in ,figures #aa and #ab1 ,sheet #bb71 and it finally encloses them in a gill chamber1 open only by a pore on the left1 which resembles in structure and physiological meaning1 but differs evidently very widely in development1 from the amphioxus atrium4 ,at this time1 the lungs are developing as paired hollow outgrowths on the ventral side of the throat 7,figure #ab1 ,l474 ,as the limbs develop1 and the tail dwindles1 the gill chamber is obliterated4 ,the capillary interruptions of the gills on the branchial arches 7aortic arches7 are also obliterated4 ,the carotid gland occupies the position of the first of these in the adult4 ,the front branchial arch here1 as in all higher vertebrata1 becomes the carotid arch2 the lingual represents the base of a pre-branchial vessel2 the second branchial becomes the aortic arch4 ,the fourth loses its connection with the dorsal aorta1 and sends a branch to the developing lung1 which becomes the pulmonary artery4 ,the third disappears4 ,a somewhat different account to this is still found in some text-books of the fate of this third branchial arch4 ,balfour would appear to have been of opinion that it gave rise to the cutaneous artery1 and that the third and fourth vessels coalesced to form the pulmocutaneous1 the fourth arch moving forward so as to arise from the base of the third2 and most elementary works follow him4 ,this opinion was strengthened by the fact that in the higher types 7reptiles1 birds1 and mammals7 no fourth branchial arch was observed1 and the apparent third1 becomes the pulmonary4 ,but it has since been shown that a transitory third arch appears and disappears in these types4 ,section #ah4 ,the origin of the renal organ and duct has very considerable controversial interest499 ,in ,figure #ac1 ,sheet #bb1 a diagrammatic cross-section1 of an embryo is shown4 ,i4 is the intestine1 coe4 the coelom1 s4c4 the spinal cord2 n4c4 the notochord1 surrounded by n4s41 the notochordal sheath1 ao4 is the dorsal aorta4 ,in the masses of somatic mesoblast on either side1 a longitudinal canal appears1 which1 in the torpedo1 a fish related to the dog-fish1 and in the rabbit1 and possibly in all other cases1 is epiblastic in origin4 ,this is the segmental duct1 which persists1 apparently1 as the ,wolffian duct 7,w4,d474 ,ventral to this appears a parallel canal1 the ,mullerian duct 7,m4,d471 which is often described as being split off from the segmental duct1 but which is1 very probably1 an independent structure in the frog4 ,a number of tubuli1 at first metamerically arranged1 now appear1 each opening1 on the one hand1 into the coelom by a ciliated mouth1 the nephrostome 7n4s471 and on the other into the segmental duct4 ,these tubuli are the segmental tubes or nephridia4 ,there grows out from the aorta1 towards each1 a bunch1 of bloodvessels1 the glomerulus 7compare ,section #fb1 ,rabbit74 ,these tubuli ultimately become1 in part1 the renal tubuli1 so that the primitive kidney stretches1 at first1 along the length of the body cavity from the region1 of the gill-slits backward4 ,the anterior part of the kidney1 called the pronephros1 disappears in the later larval stages4 ,internal to the kidney on either side there has appeared a longitudinal ridge1 the genital ridge 7g4r471 which gives rise to testes or ovary1 as the case may be4 99 ,in the discussion whether the vertebrata have arisen from some ancestral type1 like the earthworm1 metamerically segmented1 and of fairly high organization1 or from a much lower form1 possibly even from a coelenterate4 ,such a discussion is entirely outside the scope of the book1 though its mention is necessary to explain the importance given to these organs4 ,section #ai4 ,the student should now compare the figures on ,sheet #ag4 ,in the male1 tubular connections are established between the testes and the middle part of the primitive kidney 7mesonephros74 ,these connections are the vasa efferentia 7v4e471 and the mesonephros is now equivalent to the epididymis of the rabbit4 ,the ,wolffian duct is the urogenital duct of the adult1 and the ,mullerian duct is entirely absorbed1 or remains1 more or less1 in exceptional cases4 ,in the female1 the ,mullerian duct increases greatly in length-- so that at sexual maturity its white coils appear thicker and longer than the intestine-- and becomes the oviduct2 the ,wolffian duct is the ureter1 and the mesonephros is not perverted in function from its primary renal duty4 ,section #bj4 ,tabulating these facts-- ,in the adult male3 ,pronephros disappears4 ,the ,mullerian duct 78 pronephric duct7 disappears4 ,mesonephros ,epididymis2 its duct1 the urogenital4 ,metanephros and duct1 not clearly marked off from ,mesonephros4 7,compare ,dog-fish1 ,section #ai47 ,in the adult female3 ,pronephros disappears4 ,the ,mullerian duct1 the oviduct4 ,mesonephros and ,metanephros1 the kidney1 and their unseparated ducts1 the ureters4 ,section #ba4 ,hermaphrodism 7i4e41 cases of common sex7 is occasionally found among frogs2 the testis produces ova in places1 and the ,mullerian duct is retained and functional4 ,the ciliated nephrostomata remain open to a late stage of development in the frog1 and in many amphibia throughout life4 ,their connection with the renal tubuli is1 however1 lost4 ,section #bb4 ,the alimentary canal is1 at first1 a straight tube4 ,its disproportionate increase in length throws it into a spiral in the tadpole 7int4 ,figure #aa71 and accounts for its coiling in the frog4 ,the liver and other digestive glands are first formed1 like the lungs1 as hollow outgrowths1 and their lining is therefore hypoblastic4 ,the greatest relative length of intestine is found in the tadpole1 which1 being a purely vegetable feeder1 must needs effect the maximum amount of preparatory change in its food4 ,the ,development of the ,fowl ,section #bc4 ,the frog has an ovum with a moderate allowance of yolk1 but the quantity is only sufficient to start the little animal a part of its way towards the adult state4 ,the fowl1 on the contrary1 has an enormous ovum1 gorged excessively1 with yolk1 and as a consequence the chick is almost perfected when it is hatched4 ,the so-called yolk1 the yellow of an egg1 is the ovum proper2 around that is a coating of white albumen1 in a shell membrane and a shell4 ,at either end of the yolk 7,figure #a1 y47 twisted strands of albuminous matter1 the chalazae 7ch47 keep the yolk in place4 ,the animal pole is a small grey protoplasmic area1 the germinal area 7g4a471 on the yolk4 ,section #bd4 ,we pointed out that the presence of the yolk in the frog's egg led to a difference in the size of the cells at the animal and vegetable poles4 ,the late ,f4,m4 ,balfour1 borrowing a mathematical technicality1 suggested that the rate of segmentation in any part of an ovum varies inversely with the amount of yolk4 ,in the fowl's egg1 except just at the germinal area1 the active protoplasm is at a minimum1 the inert yolk at a maximum2 the ratio of yolk to protoplasm is practically infinity1 and the yolk therefore does not segment at all4 ,the yolk has diluted the active protoplasm so much as to render its influence inappreciable4 ,the germinal area segments1 and lies upon the yolk which has defeated the efforts of its small mingling of protoplasm to divide4 ,such a type of segmentation in which only part of the ovum segments is called meroblastic4 ,if we compare this with the typical blastosphere of the lower type1 we see that it is1 as it were1 flattened out on the yolk4 ,this stage is shown in section in the lower figure of ,figure #a4 b4d41 the blastoderm1 is from this point of view1 a part of the ripped and flattened blastosphere1 spread out on the yolk2 s4c4 is the segmentation cavity1 and y4 the yolk4 ,section #be4 ,there is no open invagination of an archenteron in the fowl1 as in the frog--1 the gastrula1 like the blastosphere1 stage is also masked4 ,but1 in the hinder region of the germinal area1 a thick mass of cells1 grows inward and forward1 and1 appearing in the dorsal view of the egg as a white streak1 is called the primitive streak 7p4s474 ,by a comparison of the figures of frog and fowl the student will easily perceive the complete correspondence of the position of this with the blastopore of the frog4 ,the relation of the two will be easily understood if we compare the fowl's archenteron to a glove-finger under pressure-- its cavity is obliterated-- and the frog's to the glove-finger blown out4 ,the tension of the protoplasm1 straining over the enormous yolk1 answers to the pressure4 ,the gastrula in the fowl is solid4 ,the primitive streak is1 in fact1 the scar of a closed blastopore4 ,as we should expect from this view of its homology1 at the primitive streak1 the three embryonic layers are continuous and indistinguishable 7,figure #b74 ,elsewhere in the blastoderm they are distinctly separate4 ,just as the yolk cells of the frog form the ventral wall of the intestine1 so nuclei appear along the upper side of the yolk of the fowl1 where some protoplasm still exists1 and give rise to the ventral hypoblastic cells4 ,by conceiving a gradually increasing amount of yolk in the hypoblastic cells in the ventral side of the archenteron1 the substantial identity of the gastrula stage in the three types1 which at first appear so strikingly different1 will be perceived4 ,carry ,figures #d and #e of the frog one step further by increasing the size of the shaded yolk and leaving it unsegmented1 and instead of ar4 in #e show a solid mass of cells1 and the condition of things in the fowl would at once be rendered4 ,section #bf4 ,figure #c;a of the fowl will conveniently serve for comparison with ,figure #g of the frog4 ,the inturning of the medullary groove is entirely similar in the two cases4 ,the mesoblast appears as solid mesoblastic somites4 ,in the section above ,figure #d this layer is shown as having split into somatopleur 7so47 and splanchnopleur 7spch474 ,figure #c answers to ,figure #f of the frog1 and ,figure #d is a later stage1 in which the medullary groove is beginning to close at its middle part4 ,the clear club-shaped area around the embryo 7a4p47 is the area pellucida2 the larger area without this is the area opaca 7a4o471 in which the first bloodvessels arise by a running together and a specialization of cells4 ,the entire germinal area grows steadily at its edges to creep over and enclose the yolk4 ,section #bg4 ,so far1 the essential differences between the development of fowl and frog1 the meroblastic segmentation1 absence of a typical gastrula1 and the primitive streak1 seem comprehensible on the theory that such differences are due to the presence of an enormous amount of yolk4 ,another difference that appears later is that1 while the tadpole has an efficient pronephros1 the fowl1 which has no larval 7free imperfect7 stages in its life history1 has the merest indication of such a structure4 ,section #bh4 ,another striking contrast1 due to1 or connected with1 this plethora of yolk1 is the differentiation of a yolk sac 7 umbilical vesicle7 and the development of two new structures1 the amnion and allantois1 in the fowl4 ,if the student will compare ,figure #aj of the frog1 he will see that the developing tadpole encloses in its abdomen all the yolk provided for it4 ,this is a physical impossibility in the fowl4 ,in the fowl 7,figure #b1 ,sheet #bd7 the enormous yolk 7,y47 lies outside of the embryo1 and1 as the cells of the germinal area grow slowly over it1 umbilical bloodvessels are developed to absorb and carry the material to the embryo4 ,in the case of an embryo sinking in upon1 as it absorbs1 this mass of nutritive material1 a necessity for some respiratory structure is evident4 ,from the hinder end of the fowl's intestine1 in a position corresponding to the so-called1 urinary bladder of the frog1 a solid outgrowth1 the allantois1 which speedily becomes hollow1 appears4 ,early stages are shown in ,figures #a and #b1 ,sheet #bd 7al472 while the same thing is shown more diagrammatically on ,sheet #bc1 ,figure #f 7all474 ,this becomes at last a great hollow sac1 which is applied closely to the porous shell1 and the extent of which will be appreciated by looking at ,figure #e1 ,sheet #bd1 where the allantois is shaded4 ,allantoic bloodvessels ramify thickly over its walls1 and aeration occurs through the permeable shell4 ,section #bi4 ,the nature of the amnion will be understood by following ,figures #d;b1 #e1 and #f on ,sheet #bc4 ,the three embryonic layers are indicated by broken lines1 dots1 and black lines1 just as they are in the frog diagrams4 ,not only is the embryo slowly pinched off from the yolk sac 7y4s471 but1 as the yolk is absorbed beneath it1 and it grows in size1 it sinks into the space thus made1 the extra-embryonic somatopleur and epiblast rise up round it as two folds1 which are seen closing in #e1 and closed in #f1 over the dorsal side of the young chick4 ,in this way a cavity1 a41 lined by epiblast1 and called the amniotic cavity1 is formed4 ,dorsal to this1 in #f1 comes a space lined by somatic mesoblast1 and continuous with p4p41 the pleuro-peritoneal cavity1 or body cavity of the embryo4 ,outside this1 again1 is a layer1 of somatopleur internally and epiblast externally1 the false amnion 7f4a471 which is continuous with the serous membrane 7s4m47 enclosing the rest of the egg4 ,the student should1 carefully copy these diagrams1 with coloured pencils or inks for the different layers1 and should compare them with the more realistic renderings of ,figures #b1 #e1 and #h1 ,sheet #bd4 ,section #cj4 ,the heart in the fowl appears first as a pair of vessels1 which unite to form a straight trunk in the median line1 as the flattened-out embryo closes in from the yolk4 ,the way in which this straight trunk is thrown1 first of all1 into the ,s shape of the fish heart1 and then gradually assumes the adult form1 is indicated roughly by ,figure #c4 ,in one respect the development of the heart does not follow the lines one would expect4 ,since1 between the fish and the higher form comes the condition of such an animal as the frog1 in which the auricles are divided1 while there is only one ventricle1 we might expect a stage in which the developing chick's heart would have one ventricle1 and a septum between the auricles4 ,but1 as a matter of fact1 the ventricles in fowl and rabbit are separated first1 and the separation of the auricles follows1 and is barely complete at birth4 ,section #ca4 ,two vitelline veins from the yolk sac 7v4v47 flow into the heart from behind1 as shown in ,figure #a4 ,a later more complete and more diagrammatic figure of the circulation is seen in ,figure #g4 ,at first there are two anterior cardinal 7a4c471 and two posterior cardinal veins 7p4c47 uniting to form ,cuvierian sinuses 7c4s47 that open into the heart just as in the dog-fish4 ,but later the inferior cava is developed and extends backward1 the posterior cardinals atrophy1 the ,cuvierian sinuses become the superior cavae1 and the anterior cardinals the internal jugular veins4 ,the vitelline veins 7v4v47 flow1 at first1 uninterruptedly through the liver to the inferior cava1 but1 as development proceeds1 a capillary system is established in the liver1 and the through communication1 the ductus venosus1 is reduced-- at last-- completely4 ,bearing in mind that the yolk is outside the body in the fowl and inside it in the frog1 the vitelline veins of the former have a considerable resemblance in position1 and in their relation to the portal vein1 to a portion of the single anterior abdominal vein4 ,blood is taken out to the allantois1 however1 by the arteries of the latter type4 ,section #cb4 ,five aortic arches are generally stated to appear altogether in the fowl1 but not simultaneously4 ,the first two1 the mandibular and the hyoid vascular arches1 early disappear1 and are not comparable to any in the frog4 ,the third is the first branchial arch1 and1 like the corresponding arch in the frog1 forms the carotid artery2 the second branchial is the aortic arch2 and what has hitherto been regarded as the third 7the fifth arch1 i4e47 the pulmonary artery4 ,a transitory arch1 it is now known1 however1 appears between the second branchial and the last1 and it is therefore the fourth branchial arch which is the pulmonary1 just as it is in the frog4 ,section #cc4 ,blood1 it may be mentioned1 first appears in the area vasculosa1 the outer portion of the area opaca4 ,embryonic cells send out processes1 and so become multipolar2 the processes of adjacent cells coalesce4 ,the nucleus divides1 and empty spaces appear in the substance of each of the cells4 ,in this way1 the cavities of the smaller vessels and capillaries are formed1 and the products of the internal divisions of the cells become the corpuscles within the vessels4 ,the red blood corpuscles of the rabbit1 it may be added1 are nucleated for a considerable portion of embryonic life4 ,larger vessels and the heart are burrowed1 as it were1 out of masses of mesoblast cells4 ,the course of the blood in the embryo is by the veins to the right auricle1 thence through the imperfection of the auricular septum already alluded to1 into the left auricle4 ,then the left ventricle1 aortic arches 7for the future pulmonary artery is in communication by a part presently blocked1 the ductus arterious1 with the systemic aorta71 arteries1 capillaries1 veins4 ,the liver capillary system and the pulmonary system only become inserted upon the circulation at a comparatively late stage4 ,section #cd4 ,with the exception of the reduction of the pronephros1 what has been said of the development of the frog's nervous system1 renal and reproductive organs1 and skeleton1 applies sufficiently to the fowl for our present purposes4 ,the entire separation of ,wolffian and ,mullerian ducts from the very beginning of development is here beyond all question 7vide ,section #ah74 ,but the notochord in the fowl is not so distinctly connected with the hypoblast1 and so distinct from the mesoblast1 as it is in the lower type1 and no gills1 internal or external1 are ever developed4 ,the gill slits occur with a modification due to the slitting and flattening out of the embryo1 already insisted upon2 for1 whereas in the tadpole they may be described as perforations1 in the fowl they appear as four notches between ingrowing processes that are endeavouring to meet in the middle line4 ,the ,development of the ,rabbit ,section #ce4 ,the early development of the rabbit is apt to puzzle students a little at first4 ,we have an ovum practically free from yolk 7alecithal71 and1 therefore1 we find it dividing completely and almost equally4 ,we naturally assume1 from what we have learnt1 that the next stages will be the formation of a hollow blastosphere1 invagination1 a gastrula forming mesoblast by hollow outgrowths from the archenteron1 and so on4 ,there is no yolk here to substitute epiboly 7,section #i7 for invagination1 nor to obliterate the archenteron and the blastopore through its pressure4 ,yet none of these things we have anticipated occur6 ,we find solid mesoblastic somites1 we find primitive streak1 allantois and amnion1 features we have just been explaining as the consequence of an excess of yolk in the egg4 ,we even find a yolk sac with no yolk in it4 ,section #cf4 ,a solid mass of cells is formed at the beginning1 called a morula1 ,figure #a4 ,in this we are able to distinguish rather smaller outer layer cells 7o4l4c471 and rather larger inner layer cells 7i4l4c471 but these cells1 in their later development1 do not answer at all to the two primitive layers of the gastrula1 and the name of ,van ,beneden's blastopore 7,v4,b4b471 for a point where the outer layer of cells is incomplete over the inner1 only commemorates the authorship of a misnomer4 ,the uniformity1 or agreement1 in the development of our other vertebrate types is apparently departed from here4 ,illustration3 ,development ,section #cf4 ,section #cg4 ,as the egg develops1 however1 we are astonished to find an increasing resemblance to that of the fowl4 ,a split occurs at one point between outer layer and inner layer cells1 and the space resulting 7,y in ,figure #b7 is filled by an increasing amount of fluid1 and rapidly enlarges1 so that presently we have the state of affairs shown in #c1 in which the inner layer cells are gathered together at one point on the surface of the ovum1 and constitute the germinal area4 ,if1 with ,hubrecht1 we regard the outer layer cells as an egg membrane1 there is a curious parallelism between this egg and the fowl's the fluid ,y representing the yolk2 and the inner layer cells the cells of the fowl's germinal area4 ,at any rate1 the subsequent development goes far to justify such a view4 ,the inner cells split into epi-1 meso-1 and hypo-blast1 like the blastoderm in the fowl2 there is a primitive streak and no blastopore2 an amnion arises2 the yolk sac1 small and full of serous fluid1 is cut off just as the enormous yolk of the fowl is cut off2 and an allantois arises in the same way4 ,there is no need to give special diagrams-- ,figures #c1 #d;b1 #e1 and #f of the fowl will do in all respects1 except proportion1 for the development of the rabbit4 ,the differences are such as we may account for1 not on the supposition that the rabbit's ovum never had any yolk1 but that an abundant yolk has been withdrawn from it4 ,the nutrition of the embryo by yolk has been superseded by some better method4 ,the supposition that the rabbit is descended from ancestors which1 like the birds and reptiles1 laid eggs with huge quantities of yolk1 meets every circumstance of the case4 ,section #ch4 ,but the allantois and yolk sac of the rabbit1 though they correspond in development1 differ entirely in function from the similar organs of the fowl4 ,the yolk sac is of the very smallest nutritive value2 instead of being the sole source of food1 its contents scarcely avail the young rabbit at all as nourishment4 ,its presence in development is difficult to account for except on the supposition1 that it was once of far greater importance4 ,at an early stage1 the outgrowing allantois1 pushing in front of it the serous membrane1 is closely applied to the lining of the mother's uterus4 ,the maternal uterus and the embryonic allantois send out finger-like processes into each other which interlock1 and the tissue between the abundant bloodvessels in them thins down to such an extent that nutritive material1 peptones and carbohydrates1 and oxygen also1 diffuse freely through it from mother to foetus199 and carbon dioxide1 water1 and urea from the foetus to the mother4 ,the structure thus formed by the union of the wall of the maternal uterus1 allantois1 and the intermediate structures is called the placenta4 ,through its intermediation1 the young rabbit becomes1 as it were1 rooted and parasitic on the mother1 and utilizes her organs for its own alimentation1 respiration1 and excretion4 ,it gives off ,,co#b1 ,h#b,o1 and urea1 by the placenta1 and it receives ,o and elaborated food material through the same organ4 ,this is the better method that has superseded the yolk4 99 ,the embryo4 ,section #ci4 ,in its later development1 the general facts already enunciated with regard to the organs of frog and fowl hold1 and where frog and fowl are stated to differ1 the rabbit follows the fowl4 ,in the circulation the left fourth vascular arch 7second branchial7 gives rise to the aortic arch2 in the right the corresponding arch disappears1 except so much of it as remains as the innominate artery4 ,the azygos vein 7,chapter #c7 -is a vestige of- is derived from the right posterior cardinal sinus4 ,both pulmonary arteries in the rabbit are derived from the left sixth vascular arch 7 fourth branchial74 ,compare ,section #cb4 ,the allantois altogether disappears in the adult fowl2 in the adult mammal a portion of its hollow stalk remains as the urinary bladder1 and the point where it left the body is marked by the umbilicus or navel4 ,the umbilical arteries become the small hypogastric arteries on either side of the urinary bladder4 ,there is no trace of a pronephros at all in the rabbit4 ,section #dj4 ,we may note here the development of the eye4 ,this is shown in ,figure #d1 ,sheet #bd4 ,a hollow cup-shaped vesicle from the brain grows out towards an at first hollow cellular ingrowth from the epidermis4 ,the cavity within the wall of the cup derived from the brain is obliterated1 and the stalk withers1 the cup becomes the retina1 and -its stalk- thence fibres grow back to the brain to form the optic nerve4 ,the cellular ingrowth is the lens4 ,the remainder of the eye-structures are of mesoblastic origin1 except the superficial epithelium of the cornea4 ,the retinal cup is not complete at first along the ventral line1 so that the rim of the cup1 viewed as in ,figure #a1 r41 is horseshoe shaped4 -,hence the optic nerve differs from other nerves in being primitively hollow4- ,in all other sense organs1 as1 for instance1 the olfactory sacs and the ears1 the percipient epithelium is derived1 from the epiblast directly1 and not indirectly through the nervous system4 ,these remarks apply to all vertebrate types4 ,section #da4 ,the supposition1 that the general characters of the rabbit's ovum were stamped upon it as an heritage from a period when the ancestors of the mammals were egg-laying reptiles1 is strengthened by the fact that the two lowest and most reptile-like of all the mammalia1 the duck-billed platypus and the echidna1 have been shown to depart from the distinctive mammalian character1 and to lay eggs4 ,and1 in further confirmation of this supposition1 we find1 in tracing the mammals and reptiles back through the geological record1 that in the ,permian and ,triassic rocks there occur central forms which combine1 in a most remarkable way1 reptilian and mammalian characteristics4 ,section #db4 ,in conclusion1 we would earnestly recommend the student to see more of embryological fact than what is given him here4 ,it is seeing and thinking1 much more than reading1 which will enable him to clothe the bare terms and phrases of embryology with coherent knowledge4 ,in ,howes' ,atlas of ,biology there is a much fuller series of figures of the frog's development than can be given here1 and they are drawn by an abler hand than mine can pretend to be4 ,there is also an ,atlas d',embryologie1 by ,mathias ,duval1 that makes the study of the fowl's development entertaining and altogether delightful4 ,such complete series as these are1 from the nature of the case1 impossible with the rabbit4 ,many students who take up the subject of biology do so only as an accessory to more extended work in other departments of science4 ,to such1 practical work in embryology is either altogether impossible1 or only possibly to a very limited extent4 ,the time it will consume is much greater1 and the intellectual result is likely to be far less than the study of such plates as we have named4 ,the ,theory of ,evolution ,section #dc4 ,we have now considered our types1 both from the standpoint of adult anatomy and from embryological data2 and we have seen through the vertebrate series a common structure underlying wide diversity in external appearance and detailed anatomy4 ,we have seen a certain intermediateness of structure in the frog1 as compared with the rabbit and dog-fish1 notably in the skull and skeleton1 in the circulation1 in the ear1 and in the reduced myomeres2 and we have seen that the rabbit passes in these respects1 and in others1 through dog-fish- and frog-like stages in its development1 and this alone would be quite sufficient to suggest that the similarities of structure are due to other causes than a primordial adaptation to certain conditions of life4 ,section #dd4 ,it has been suggested by very excellent people that these resemblances are due to some unexplained necessity of adherence to type1 as though1 the power that they assume created these animals originally1 as they are now1 coupled creative ability with a plentiful lack of ideas1 and so perforce repeated itself with impotent variations4 ,on the other hand1 we have the supposition that these are family likenesses1 and the marks of a common ancestry4 ,this is the opinion now accepted by all zoologists of repute4 ,section #de4 ,it must not be for a moment imagined that it is implied that rabbits are descended from frogs1 or frogs from dog-fish1 but that these three forms are remote cousins1 derived from some ancient and far simpler progenitor4 ,but since both rabbit and frog pass through phases like the adult condition of the dog-fish1 it seems probable that the dog-fish has remained more like the primordial form than these two1 and similarly1 the frog than the rabbit4 ,section #df4 ,hence we may infer that the mammals were the last of the three groups1 of which we have taken types1 to appear upon the earth1 and that the fishes preceded1 the amphibia4 ,workers in an entirely independent province1 that of palaeontology1 completely endorse this supposition4 ,the first ,vertebrata to appear in the fossil history of the world are fishes2 fish spines and placoid scales 7compare dog-fish7 appear in the ,ordovician rocks4 ,in the coal measures come the amphibia2 and in the ,permo-triassic strata1 reptile-like mammals4 ,in the ,devonian rocks1 which come between the ,silurian and the coal measures1 we find very plentiful remains of certain fish called the dipnoi1 of which group three genera still survive2 they display1 in numberless features of their anatomy1 transitional characters between true fish and amphibia4 ,similarly1 in the ,permian come mammal-like reptiles1 that point also downward to the amphibia4 ,we find1 therefore1 the story told by the ovum written also in the rocks4 ,section #dg4 ,now1 when this fact of a common ancestry is considered1 it becomes necessary to explain how this gradual change of animal forms may have been brought about4 ,section #dh4 ,two subcontrary propositions hold of the young of any animal4 ,it resembles in many points its parent4 ,it differs in many points from its parent4 ,the general scheme of structure and the greater lines of feature are parental1 inherited2 there are also novel and unique details that mark the individual4 ,the first fact is the law of inheritance2 the second1 of variation4 ,section #di4 ,now the parent or parents1 since they live and breed1 must be more or less1 but sufficiently1 adapted to their conditions of living-- more or less fitted to the needs of life4 ,the variation in the young animal will be one of three kinds3 it will fit the animal still better to the conditions under which its kind live1 or it will be a change for the worse1 or it is possible to imagine that the variation-- as in the colour variations of domesticated cats-- will affect its prospects in life very little4 ,in the first case1 the probability is that the new animal will get on in life1 and breed1 and multiply above the average2 in the second1 it is probable that1 in the competition for food and other amenities of life1 the disadvantage1 whatever it is1 under which the animal suffers will shorten its career1 and abbreviate the tale of its offspring2 while1 in the third case1 an average career may be expected4 ,hence1 disregarding accidents1 which may be eliminated from the problem by taking many cases1 there is a continual tendency among the members of a species of animals in favour of the proportionate increase of the individuals most completely adapted to the conditions under which the species lives4 ,that is1 while the conditions remain unchanged1 the animals1 considered as one group1 are continually more highly perfected to live under those conditions4 ,and under changed conditions the specific form will also change4 ,section #ej4 ,the idea of this process of change may be perhaps rendered more vivid by giving an imaginary concrete instance of its working4 ,in the jungles of ,india1 which preserve a state of things which has existed for immemorial years1 we find the tiger1 his stripes simulating jungle reeds1 his noiseless approach learnt from nature in countless millions of lessons of success and failure1 his perfectly powerful claws and execution methods2 and1 living in the same jungle1 and with him as one of the conditions of life1 are small deer1 alert1 swift1 light of build1 inconspicuous of colour1 sharp of hearing1 keen-eyed1 keen-scented-- because any downward variation from these attributes means swift and certain death4 ,to capture the deer is a condition1 of the tiger's life1 to escape the tiger a condition of the deer's2 and they play a great contest under these conditions1 with life as the stake4 ,the most alert deer almost always escape2 the least so1 perish4 ,section #ea4 ,but conditions may alter4 ,for instance1 while most of these deer still live in the jungle with tigers1 over a considerable area of their habitat1 some change may be at work that thins the jungle1 destroys the tigers in it1 and brings in1 let us say1 wolves1 as an enemy to the deer1 instead of tigers4 ,now1 against the wolves1 which do not creep1 but hunt noisily1 and which do not spring suddenly upon prey1 but follow by scent1 and run it down in packs1 keen eyes1 sharp ears1 acute perceptions1 will be far less important than endurance in running4 ,the deer1 under the new conditions1 will need coarser and more powerful limbs1 and a larger chest2 it will be an advantage to be rough and big1 instead1 of frail and inconspicuous1 and the ears and eyes need not be so large4 ,the old refinements will mean weakness and death2 any variation along the line of size and coarseness will be advantageous4 ,slight and delicate deer will be continually being killed1 rougher and stronger deer continually escaping4 ,and so gradually1 under the new circumstances1 if they are not sufficient to exterminate the species1 the finer characteristics will be eliminated1 and a new variety of our old jungle deer will arise1 and1 if the separation and contrast of the conditions is sufficiently great and permanent1 we may1 at last1 in the course of ages1 get a new kind of deer specifically different in its limbs1 body1 sense organs1 colour1 and instincts1 from the deer that live in the jungle4 ,and these latter will1 on their side1 be still continually more perfected to the jungle life they are leading4 ,section #eb4 ,take a wider range of time and vaster changes of condition than this1 and it becomes possible to imagine how the social cattle-- with their united front against an enemy1 fierce onslaught1 and their general adaptation to prairie life-- have differentiated from the ancestors of the slight and timid deer2 how the patient camel1 with his storage hump1 water storage1 and feet padded against hot sand1 has been moulded by the necessity of desert life from the same ancestral form4 ,and so we may work back1 and link these forms1 and other purely vegetarian feeders1 with remoter cousins1 the ancestral hogs4 ,working in this way1 we presently get a glimpse of a possible yet remoter connection of all these hoofed and mainly vegetarian animals1 with certain central types that carry us across to the omnivorous1 and1 in some cases1 almost entirely vegetarian bears1 and to the great and prosperous family of clawed1 meat-eaters4 ,and thus we elucidate1 at last1 a thread of blood relationship between the1 at present1 strongly contrasted and antagonistic deer and tiger1 and passing thence into still wider generalizations1 it would be possible to connect the rabbit playing in the sunshine1 with the frog in the ditch1 the dog-fish in the sea-waters and the lancelet in the sand4 ,for the transition from dog-fish to rabbit differs from the transition from one species of deer to another only in magnitude3 it is an affair of vast epochs instead merely of thousands of years4 ,section #ec4 ,it would1 however1 be beyond the design of this book to carry our demonstration of the credibility of a common ancestry of animals still further back4 ,but we may point out here that it is not a theory1 based merely upon one set of facts1 but one singularly rich in confirmation4 ,we can construct1 on purely anatomical grounds1 a theoretical pedigree4 ,now the independent study of embryology suggests exactly the same pedigree1 and the entirely independent testimony of palaeontology is precisely in harmony with the already confirmed theory arrived at in this way4 ,section #ed4 ,it is in the demonstration of this wonderful unity in life1 only the more confirmed the more exhaustive our analysis becomes1 that the educational value and human interest of biology chiefly lies4 ,in the place of disconnected species of animals1 arbitrarily created1 and a belief in the settled inexplicable1 the student finds an enlightening realization of uniform and active causes beneath an apparent diversity4 ,and the world is not made and dead like a cardboard model or a child's toy1 but a living equilibrium2 and every day and every hour1 every living thing is being weighed in the balance and found sufficient or wanting4 ,our little book is the merest beginning in zoology2 we have stated one or two groups of facts and made one or two suggestions4 ,the great things of the science of ,darwin1 ,huxley1 ,wallace1 and ,balfour remain mainly untold4 ,in the book of nature there are written1 for instance1 the triumphs of survival1 the tragedy of death and extinction1 the tragi-comedy of degradation and inheritance1 the gruesome lesson of parasitism1 and the political satire of colonial organisms4 ,zoology is1 indeed1 a philosophy and a literature to those who can read its symbols4 ,in the contemplation of beauty of form and of mechanical beauty1 and in the intellectual delight of tracing and elucidating relationships and criticising appearances1 there is also for many a great reward in zoological study4 ,with an increasing knowledge of the facts of the form of life1 there gradually appears to the student the realization of an entire unity shaped out by their countless1 and often beautiful1 diversity4 ,and at last1 in the place of the manifoldness of a fair or a marine store1 the student of science perceives the infinite variety of one consistent and comprehensive ,being-- a realization to which no other study leads him at present so surely4 ,to the student who feels inclined to amplify this brief outline of ,vertebrate ,anatomy1 we may mention the following books3 ,wiedersheim's and ,parker's ,vertebrates1 ,huxley's ,anatomy of the ,vertebrata1 ,flower's ,osteology of the ,mammalia1 ,wallace's ,distribution1 ,nicholson and ,lyddeker's ,palaeontology 7,volume #b71 the summaries in ,rolleston's ,forms of ,animal ,life 7where a bibliography will be found71 and ,balfour's ,embryology4 ,but reading without practical work is a dull and unprofitable method of study4 ,questions on ,embryology ,all these questions were actually set at ,london ,university ,examinations4 ,in ,both ,editions4 #a4 ,describe the changes in the egg-cell which precede fertilization2 describe the process of fertilization and the formation of the primary cell-layers1 as exhibited1 in three of the animal types known to you4 ,what is the notochord1 and how is it developed in the frog8 #b4 ,describe the early stages in the development of the egg of the fowl as far as the closure of the neural groove4 ,how do you account for the primitive streak8 #c4 ,describe the cleavage and the surface appearances of the egg of the frog and of the rabbit1 up to the time when the first gill-slits appear in the embryo4 ,give illustrative diagrams of what you describe4 #d4 ,describe the structure and cleavage of the ovum 7a7 of the frog1 7b7 of the fowl1 and 7c7 of the rabbit4 7d7 ,explain as far as possible the differences in the cleavage of these three eggs4 7e7 ,point out how the embryo is nourished in each case1 and 7f7 describe the constitution of the placenta in the rabbit4 #e4 7a7 ,what are the protovertebrae8 7b7 ,how does the notochord originate in the frog8 7c7 ,how are the vertebrae laid down in the tadpole8 7d7 ,describe the vertebral column of the adult frog4 7e7 ,in what important respects do the centra of the vertebrae of the frog1 the dog-fish1 and the rabbit differ from one another8 #f4 ,give an account of the more important features in the development of the frog4 #g4 ,what temporary organs are developed in the embryo frog which are absent from the embryo bird and mammal1 and what in the two latter which are absent from the former8 #h4 ,draw diagrams1 with the parts named1 of the heart and great arteries of the frog1 giving descriptions only in so far as is necessary to explain your diagrams2 trace the development of these structures in the tadpole2 point out particularly in which of the embryonic visceral 7branchial7 arches the great arteries of the adult run4 #i4 ,trace the history of the post-oral gill-slits and their accompanying cartilaginous bars and vascular arches in the frog1 fowl1 and rabbit4 #aj4 ,give a short account1 with illustrative figures1 of the mode of formation of the primary germinal layers in amphioxus and in the frog4 ,what explanation can you give of the differences between the two cases8 #aa4 ,give a short account1 with diagrammatic figures1 of the principal changes which occur in the circulatory and respiratory organs during the metamorphosis of the tadpole into the frog4 #ab4 ,how do protozoa differ from higher animals 7metazoa7 as regards 7a7 structure1 7b7 reproduction8 ,compare the process of fission in an amoeba with the segmentation of the ovum in amphioxus1 pointing out the resemblances and differences between the two cases4 -,miscellaneous ,questions ,most of these questions were actually set at the ,biological ,examinations of ,london ,university4 ,in ,both ,editions4 #a4 ,describe 7a7 the digestive1 7b7 the circulatory1 7c7 the excretory1 and 7d7 the reproductive organs of the amphioxus4 #b4 ,describe the stomach and intestines of the dog-fish and rabbit1 and point out in what way their differences are connected with diet4 #c4 ,describe the mechanism of respiration in the adult frog1 and contrast it with that of the tadpole4 #d4 ,give an account of the structure of the epidermis and its outgrowths in the frog and the rabbit4 #e4 ,describe the organs of circulation 7heart and main arteries and veins7 and respiration in the frog in its mature and immature states4 #f4 ,give a brief account of the physiology of respiration4 ,describe fully the means by which respiration is effected in the following animals3-- frog1 amphioxus1 rabbit1 and dog-fish4 #g4 ,describe the minute structure of the blood of the rabbit1 frog1 and amphioxus4 #h4 ,describe and illustrate by means of sketches the chief points of difference between the skeleton of the rabbit as a typical mammal1 and that of the common frog as a typical amphibian4 #i4 7a7 ,explain what is meant by the term central nervous system4 7b7 ,describe the tissue elements which enter into its composition4 7c7 ,explain1 as far as you can1 the function of each structure described4 7d7 ,how is the central nervous system developed in the frog1 and 7e7 in the rabbit8 7f7 ,what conclusions may be drawn from the facts stated as to the origin of the central nervous system in evolution8 #aj4 ,give an account of the structure 7including histology7 and of the functions of the spinal cord and spinal nerves of the frog4 #aa4 ,give a description of the minute structure and chemical characters of the following tissues as seen in the frog3-- cartilage1 bone1 muscle4 ,from which of the primary cell-layers of the embryo are they respectively developed8 #ab4 ,what substance is excreted by the renal organ of a frog1 and what relation does this substance bear to the general life of the organism8 ,describe the parts by which similar excretion is believed to be effected in amoeba1 hydra1 earthworm1 mussel1 and lobster4 #ac4 ,describe1 with illustrative sketches1 the structure of the connective tissue1 cartilage1 and muscular tissue of a frog4 ,also describe the structure of the muscular tissue of the lobster and snail4 #ad4 ,give in account of the more important features in the development of the frog4 #ae4 ,describe and compare the structure of the renal organs in a frog and a rabbit4 #af4 ,give an account of the structure of the genito-urinary organs of the frog4 ,compare these organs of the frog with those of the dog-fish and of the rabbit4 ,distinguish in each case the conditions of the two sexes1 and describe briefly the microscopic structure and development of the ova and of the spermatozoa4 #ag4 ,describe1 with diagrams1 the arrangement of the urinary and generative organs in the male of 7a7 the rabbit1 7b7 the dog-fish1 and 7c7 the frog2 7d7 point out the most important differences between them4 #ah4 7a7 ,describe the structure of the ovarian egg of the rabbit1 7b7 and of the pigeon1 7c7 and of the frog2 7d7 from what part of the embryo do they originate8 7e7 ,what is the structure and origin of the ovarian follicle in the rabbit1 and 7f7 of the ovarian stroma8 7g7 ,what is the granulosa and what the zona pellucida8 #ai4 ,describe the pre-segmentation changes1 mode of impregnation1 and early stages of development in the ovum of the frog1 as far as the closure of the neural canal4 #bj4 ,illustrate1 with diagrams1 from the structure of typical organisms1 the principle of repetition of similar parts4 -,note on ,making ,comparisons ,students preparing for examinations are frequently troubled by comparison questions4 ,tabulation is often recommended1 but we are inclined to favour a rather more flexible plan of marking off differences and resemblances4 ,in tabulation a considerable loss of time is occasioned by writing down the features of both the things compared1 and this is a serious consideration for the examinee4 ,we advise him therefore1 first1 if he possibly can1 to draw side by side and in corresponding positions the two things under consideration1 and then1 going over them in a methodical way1 to state simply the difference between each homologous part4 ,we append as examples three test answers actually submitted 7with figures7 in ,correspondence work3-- #a4 ,compare the brain of the frog with that of the rabbit4 ,in the frog's fore-brain-- ,the olfactory lobes are fused in the middle line4 ,there is no corpus callosum1 nor is there a middle commissure to the third ventricle4 ,the cerebral hemispheres are not convoluted1 and1 looked at from the dorsal aspect1 do not hide the thalamencephalon and mid-brain4 ,the pineal gland lies in the cranial wall and not deeply between the hemispheres1 and its stalk is longer and tilts forward4 ,in the mid-brain-- ,the optic lobes are two1 instead of being corpora quadrigemina1 and hollow4 ,in the hind-brain-- ,the cerebellum is a very small transverse band1 and has no lateral parts4 ,the medulla is relatively larger4 ,there are no spinal accessory nor hypoglossal nerves to the brain4 #b4 ,compare the vertebrae of dog-fish1 rabbit1 and frog4 ,the centra of the dog-fish are -opistho- amphi-coelous 7i4e41 hollow at either end74 ,the centra of the rabbit are flat-faced4 ,the centra of the frog are procoelous 7hollow in front74 ,the notochord persists between the centra in the dog-fish and rabbit1 within the centra in frog4 ,the centra of the rabbit have epiphyses1 absent in the dogfish and frog4 ,the transverse processes of the rabbit typically bear ribs4 ,short ribs occur in the dog-fish1 but their homology with those of the rabbit is doubtful4 ,the frog has no ribs4 ,the interneural plates are peculiar to the dog-fish in this comparison4 #c4 ,compare the skull of the dog with that of the frog4 ,the ,brain ,case-- ,of the frog is a cylindrical box1 from which the otic capsules project conspicuously on either side4 ,it contains only two ossifications in its cartilaginous substance 7the sphen-ethmoid and the ex-occipital71 being protected by the membrane bones1 the parieto-frontals above and the parasphenoid below4 ,in the mammal it is enormously inflated1 and the otic capsules are imbedded in its wall4 ,there are supra- and basi- as well as ex-occipital bones2 the para-sphenoid is 78 entirely7 gone1 and its place is taken by the basi- and pre-sphenoids1 and the lateral walls contain fresh paired ossifications1 the ali- and orbito-sphenoids-- all cartilage bones4 ,the sphenethmoid is perhaps represented in part by the ethmoid4 ,as a result of the inflation of the brain-case1 the squamosal1 which slopes downward and outward in the frog1 and overlies the cartilaginous suspensorium 7quadrate cartilage71 has become a constituent of the brain-case wall1 and slopes downwardly and in4 ,jaw ,suspension-- ,the point of attachment of the jaw has shifted outward1 and the original suspensorial cartilage 7the quadrate7 has taken on a new and minor function as the incus of the middle ear-- the squamosal superseding it as the suspensory part4 ,lower ,jaw-- ,distinct bones in the frog2 one mass in the dog4 ,otic ,capsule-- ,position as specified4 ,one centre of ossification in the frog forming pro-otic2 several fuse together and form periotic of the dog4 ,there is no bulla and no external ear in the frog4 ,palate-- ,in the frog the posterior nares open into the front of the mouth4 ,in the dog the maxillae and palatines send plates down and in 7the palatine plates7 to cut off a nasal passage from the rest of the buccal chamber1 and carry the posterior nares back to the pharynx1 thus cutting the vomers off from the mouth roof4 ,the pterygoids in the dog are much reduced1 and do not reach back to the suspensorium4 ,the frog has no lachrymal bone4 -,syllabus ,of ,practical ,work ,we would impress upon the student at the outset the importance of some preliminary reading before dissection is undertaken4 ,no one would dream of attempting to explore a deserted city without some previous study of maps and guide-books1 but we find again and again students undertaking to explore the complicated anatomy of a vertebrated animal without the slightest1 or only the slightest1 preparatory reading4 ,this is entirely a mistake4 ,a student should be familiar with the nomenclature of the structures he contemplates examining1 he should have some idea of their mutual relations and functions1 or his attention will inevitably be diverted by the difficulty of new names and physiological questionings to the neglect of his dissection1 and that careful observation of form and mutual position which is the essential object of dissection4 ,on the other hand1 it is equally necessary-- perhaps more so-- to warn students against the bookish fallacy1 and to assure them of the absolute impossibility of realizing biological facts from reading alone4 ,practical work can alone confirm and complete the knowledge to which the text-book is the guide4 ,in scientific teaching it may sometimes be convenient for the thought to precede the thing1 but until the thing has been dealt with the knowledge gained is an unsatisfactory and unstable possession4 ,for such dissection as the subject-matter of this book requires1 the following appliances will be needed3-- 7a7 ,two or three scalpels of various sizes4 7b7 ,scissors1 which must taper gradually1 have straight blades1 and be pointed at the ends1 and which must bite right up to the tips 7or they are useless74 ,two pairs1 small and large1 are advisable4 7c7 ,forceps1 which must hold firmly1 and meet truly at the points4 7d7 ,two needles set in wooden handles4 7e7 ,an ordinary watchmaker's eye-glass is very helpful1 but not indispensable4 7f7 ,a dissecting dish-- an ordinary pie dish will do-- into which melted paraffin wax has been poured1 to the depth of1 say1 three-quarters of an inch1 and allowed to solidify4 7,this wax may be blackened by mixture with lampblack4 ,if the wax floats up at any time1 it can1 of course1 be remelted4 ,or it may be loaded with lead47 7g7 ,a rough table or board 7for the rabbit and dog-fish74 7h7 ,blanket pins1 and ordinary pins4 7i7 ,a pickle or other wide-mouthed jar1 and some common1 methylated spirit4 7j7 ,a microscope1 with low power of #a inch or #a_/b inch1 and high power #a_/f inch or #a_/d inch4 ,glass slips and cover glasses1 and a bottle of very weak 7#a per cent47 solution of salt4 ,animals for dissection may be obtained from the recognised dealers1 who usually advertise in such scientific periodicals as ,nature1 ,natural ,science1 and ,knowledge4 ,sinel 7naturalist1 ,jersey7 is the most satisfactory dealer in dog-fish in our experience2 ,bolton 7,malvern7 will supply ,amphioxus through the post2 frogs and rabbits may be obtained anywhere4 ,the tame variety of rabbit is quite satisfactory for the purpose of dissection4 ,the following notes may possibly be of some use to the student2 they follow the lines of work arranged by the author for the evening classes of the ,university ,tutorial ,college1 classes considerably restricted as regards time1 when compared with ordinary laboratory workers4 ,most of the sections below occupied about three hours1 but for a student working alone they are more likely to take four or five1 and even then it is not probable that they will be so satisfactory as if performed under skilled supervision4 ,there are many points extremely difficult to convey verbally which are elucidated at once by actual demonstration upon a specimen4 ,each of these dissections should be repeated1 and it is well if a different condition of the type is selected for the repetition-- an old one if the first specimen was immature1 a female if the first was a male4 -,the ,rabbit ,may be killed by chloroform1 or potassium cyanide1 or drowned4 ,it may also be readily suffocated with house-hold gas4 ,it should be killed immediately before use1 as otherwise the gastric juice attacks the wall of the stomach1 and the dissection is1 in consequence1 rendered extremely disagreeable4 ,a very young rabbit is unsatisfactory as regards the genitalia1 but otherwise there is no objection to a little one1 and it has this advantage-- that it may be immersed more conveniently under water1 in a large pie dish1 for purposes of fine dissection4 ,the external features of the animal should be examined3 eyelids1 whiskers and teeth1 toes1 anus1 perineal space on either side of the same1 urogenital opening1 and position of the ribs1 vertebral column1 and limb girdles beneath the skin should be made out4 ,then the animal should be pinned out through the legs1 the ventral surface uppermost1 the skin opened up along the middle line from pelvic girdle to symphyses of jaw2 separated from the body wall below by means of the handle of a scalpel1 and turned back2 and then the abdominal wall should be cut into and two flaps pinned back to expose its contents4 ,note the xiphisternum4 ,the caecum and colon will be recognised 7,section #af72 the stomach1 the right and left central1 and left lateral lobes of the liver will probably be apparent2 and the urinary bladder 7especially if distended7 in the middle line behind4 ,without any further dissection1 but simply by turning the parts over1 all the structures of the abdomen in ,figure #a1 ,sheet #a1 will be identified4 ,seek especially for and note particularly1 the gall bladder1 bile duct1 and portal vein1 pancreatic duct1 sacculus rotundus1 vermiform appendix1 ureters 7by pulling urinary bladder forward71 genital ducts 7looping over ureters71 spleen1 kidneys1 and adrenals4 ,the vena cava inferior is seen dorsally4 ,the genital duct guides the student to the genital gland2 if the subject is a male1 the testes may be exposed by dissection1 or by pulling the vas deferens gently the scrotal sac will be turned inside out1 and the testes brought into view4 ,the ovary lies exposed without dissection posterior to the kidney4 ,examine all this carefully1 and make small sketches of points of interest-- the duodenal loop and the pyloric end of the stomach1 for instance2 the meeting of colon1 caecum1 and sacculus rotundus again2 or the urinary bladder and adjacent parts4 ,note the dorsal aorta and vena cava and their connexions behind4 7,compare figure of circulation47 ,cut through pelvic girdle1 and remove one hind leg1 to see bladder and genital ducts better 7compare ,sheet #aj74 ,wash away any blood that may flow4 ,turn all the intestines over to the animal's right1 and see the dorsal aorta and vena cava inferior of the abdomen1 the inferior mesenteric artery1 and the spermatic 7or ovarian7 artery 7compare1 of course1 with figure in book74 ,in front1 immediately dorsal to the spleen1 is a variable quantity of lymphoidal tissue1 which must be very carefully cleared to see the superior mesenteric and coeliac arteries4 ,separate ,spigelian lobe from stomach1 and look for vagus nerve descending by oesophagus1 solar plexus around the superior mesenteric artery1 and thrown up very distinctly by the purple vena cava inferior beneath1 and the splanchnic nerve4 ,to see the abdominal sympathetic behind1 gently remove the peritoneum that lies on either side of the aorta2 blood-vessels will be seen running in between the vertebral bodies1 and the sympathetic chain1 with its ganglia1 made out very distinctly1 as it runs across them longitudinally4 ,now cut oesophagus just in front of stomach1 and cut the rectum1 cut through the mesentery supporting the intestine1 and remove and unravel alimentary canal2 cut open1 wash out1 and examine caecum and stomach4 ,bleeding to a considerable extent is inevitable1 chiefly from the portal vein4 ,the liver had better remain if the same rabbit is to serve for the second dissection4 ,second ,dissection4-- ,skin front of thorax and neck4 ,note subclavian veins running out to fore limbs-- avoid cutting these4 ,cut through ribs and remove front of thorax1 to expose its contents2 cut up middle line of neck1 and clear off small muscle bands1 to expose bloodvessels2 pick away carefully whatever is left of thymus gland2 make out structure of heart and blood-vessels1 as described1 in ,chapter #c2 note larynx and trachea4 ,now proceed to the examination of the nerves of this region4 ,see phrenic nerve1 by vena cava inferior1 and between heart and lungs1 and sympathetic1 running over the heads of the ribs4 ,by the common carotids will be found the large white vagus nerve1 the greyish sympathetic1 and a small branch of ,x41 the depressor4 ,make out branches of ,x4 named in text4 ,the big white cervical spinal nerves will be evident dorsally4 ,clear forward into the angle between the jaw and the bulla tympani1 to see ,,xii4 and ,,xi42 ,,ix4 will be found1 lying deeper1 dorsal to the carotid artery and body of the hyoid4 ,compare with figure given of this4 ,skin the cheek1 and see ,,vii4 running over it4 ,cut through malar and remove it2 cut through lower jaw-bone and turn it back1 to see the third branch of the fifth nerve on its inner side2 examine the muscles of eyeball1 and remove it1 to expose the first and second branches of ,v4-- the latter is especially deep within orbit4 ,remove1 open1 wash out1 and examine the heart4 ,shave off the dorsal wall of cranium1 to expose hemispheres of brain1 and then put the head in strong spirit for a week or so4 ,with a second rabbit1 this dissection may advantageously be varied by removing the lower jaw1 cutting -up- through soft palate1 and observing openings of the ,eustachian tubes4 ,the tonsils 7on the ventral side of the soft palate7 must not confused with these4 ,the heart should also be cut out1 washed out and examined 7,compare ,sections #ch1 #dd47 ,third ,dissection4-- 7,before this is performed the mammalian skull should have been studied and examined47 ,take the head of a rabbit1 the brain of which has been hardened by spirit1 and carefully remove cranium2 be particularly careful in picking away the periotic bone1 on account of the flocculi of cerebellum4 ,it is difficult to avoid injury to the pituitary body embedded in the basisphenoid bone4 ,examine with the help of ,sheet #h4 ,make the sections there indicated4 -,the ,frog ,may be killed by drowning in dilute methylated spirit1 or by chloroform4 ,take a recently-killed frog1 and examine a drop of its blood1 spread out on a glass slip1 under the microscope2 compare it with your own4 ,before using the high power1 put a cover glass over the object1 of course4 ,scrape the roof of the mouth of the frog gently1 to obtain ciliated epithelium2 and mount in very weak salt solution-- the cilia will still be active4 ,squamous epithelium may be seen by the student similarly scraping the interior of his own cheek4 ,take a piece of muscle from one of the frog's limbs1 tease out with needles upon a glass slip1 and examine4 ,to see the striations clearly1 the high power will be needed4 ,compare a piece of muscle from the wall of the alimentary canal4 ,similarly examine nerve and connective tissue4 ,first ,dissection4-- ,pin out the frog in a dissecting dish1 ventral surface uppermost1 and cover with water4 ,open up the skin along the mid-ventral line4 ,note the large sub-cutaneous lymph spaces1 the pelvic and pectoral girdles1 and the anterior abdominal vein4 ,cut into the body cavity on one side of this latter1 cut across in front of where the vein dips down to liver1 and peel the body wall away from it4 ,the xiphisternum will probably be cut in this operation4 ,in early spring the females are greatly distended with ova1 and the greater portion of the ovary may1 with advantage1 be removed4 ,the oviduct is dead white then1 and larger and much more in evidence than the 7pinkish7 intestine even4 ,turn over the viscera1 and compare with ,sheet #aa2 one lung is often found greatly inflated1 and then projects back into the body cavity2 the stomach is1 in some cases1 pushed forward and hidden behind the shoulder girdle4 ,observe the allantoic bladder1 the spleen1 gall bladder1 portal vein1 and pancreas4 ,by squeezing the gall bladder gently1 the bile duct will be injected with bile1 and will be apparent if the stomach is turned over4 ,the oesophagus1 just in front of the stomach1 should be cut through1 and the rectum1 and the mesentery and alimentary canal supported by it1 removed4 ,this will expose the urogenital organs4 7,vide ,figures given47 ,these vary greatly1 especially in the females1 at different seasons4 ,the condition figured would be seen in late autumn1 or winter4 ,in spring females are often found copulating with males1 and then the ovary itself is inconspicuous1 while the lower part of the oviduct is enormously distended with ova1 so as to be mistaken sometimes for the ovary by those who fail to note that the ova are enclosed by a thin semi-transparent skin 7wall of oviduct74 ,the vena cava inferior is seen between the kidneys and the renal portal vein beside the ureter4 ,cutting through the mesentery supporting the kidney laterally1 the dorsal aorta is exposed1 and on either side of it the sympathetic chain and rami communicantes1 often tinged with black pigment4 ,this black pigment is a frequent but variable feature of the frog's anatomy1 and usually dapples or blackens the testes1 and also sometimes darkens the otherwise pale pink arteries4 ,behind the kidneys the sciatic plexus also becomes visible4 ,careful drawings should be made4 ,cut off the head of the frog1 shave off top of brain case1 and put the head in strong spirit4 ,second ,dissection4-- ,a fresh frog is required4 ,pin out under water as before1 and open up body cavity4 ,now carefully remove the muscle from the ventral portion of the shoulder girdle1 to expose the clavicles and coracoids4 ,cut away xiphisternum1 and then cut through clavicles and coracoids on either side1 and remove ventral part of shoulder girdle1 to expose the heart4 ,open out the cut portions of body wall and pin4 ,the veins going towards the heart should now1 with a little examination1 be evident4 ,make out the external jugular1 the innominate1 and its two branches1 and the pulmo-cutaneous and vena cava superior4 ,clear by carefully picking away any shreds of semi-transparent tissue4 ,make out1 by feeling1 the position of the hyoid body1 and of its anterior cornua4 ,note the hypoglossal nerve 7first spinal7 running ventral to this1 and the ninth cranial nerve1 running parallel to it but dorsal to the hyoid-- hidden therefore by the hyoid1 and reappearing in front4 ,the vagus may also be made out less distinctly1 running postero-ventrally towards the heart4 ,by clearing the muscle by the rumus of the jaw1 ,,vii4 may be seen1 and the third branch of ,v41 running across the jaw at about the middle of its length4 ,pick off the thin transparent pericardium from the heart very carefully1 and proceed to cut away all the veins made out4 ,the truncus arteriosus may then be followed up as it branches4 ,note all the branches shown in the figures in this book4 ,the precise position of the vessels will vary to a certain extent with the attitude in which the frog is pinned4 ,the cutaneous artery will prevent the student following up the aortic arch until it is cut2 then the arch may be followed round until it meets its fellow to form the dorsal aorta4 ,note the sympathetic again4 ,make careful drawings of all this4 ,cut off lower jaw1 and note posterior nares and ,eustachian openings4 ,if time allows1 remove the heart1 and examine by cutting open and washing4 7,compare1 ,section #dd7 ,remove eyeball1 to see the first and second branches of the fifth nerve1 and the ,vidian 7i4e4 palatial7 branch of the seventh4 ,third ,dissection4-- ,read the account of the frog's skull carefully4 ,take the head of a recently killed frog and drop into boiling water for a minute4 ,then pick off1 very carefully1 muscle1 connective tissue1 nerves1 and etc41 to clear the cranio-facial apparatus2 examine the bones1 compare with figures given in this book1 and draw4 ,take the head1 which has been in spirit a fortnight or so1 pick away cranium1 and compare brain with figures given4 ,examine ventricles1 by taking sections1 after drawings have been made4 -,the ,dog-,fish- ,first ,dissection4-- ,examine external characters1 nasal grooves-- no internal nares-- fins1 spiracle1 scales passing over lips1 and cloaca4 ,cut off tail below the cloacal opening4 ,the males are distinguished by the large claspers along the inner edge of the pelvic fin4 ,open up body cavity4 ,usually this is in a terrible mess in the fish supplied by dealers1 through the post-mortem digestion of the stomach4 ,wash out all this under a stream of water from a tap or water-bottle4 ,frequently the testes are washed out of the male in this operation and ova from the loose ovaries in the female4 ,now compare with figure given in this book1 allowing for the collapse of the stomach1 if it has occurred4 ,cut through the oesophagus and rectum1 and remove alimentary canal from body2 cut open and wash out the intestine1 and examine spiral valve4 ,now make a careful examination of the cloaca and its apertures1 and dissect away the peritoneum hiding the kidney4 ,in the female find the opening of the oviducts in front of the liver4 ,remove liver1 and cut off body now behind pectoral fin4 ,before throwing tail and hinder part of body away1 note the myotomes of body wall1 the notochord and vertebral body1 neural canal1 and1 in the tail1 the haemal canal4 7,see ,section #i the ,dog-fish7 ,lines from ,first ,edition only4 -,the relation of the vertebral bodies to the notochord may be very well seen by taking successive slices1 about one-tenth of an inch thick1 through the vertebral body4 ,the cartilage is hard and semi-transparent1 the notochord jelly-like1 least at the centres of the centra1 and at a maximum intervertebrally4- ,the notochord is a soft jelly4 ,cut away the ventral part of the pectoral girdle1 to open pericardium4 ,with a seeker1 make out the pericardio peritoneal opening4 ,cut into the sinus venous1 and run seekers into the ,cuvierian and hepatic sinuses4 ,cut open the ,cuvierian and posterior cardinal sinuses1 and run seekers into their affluents4 ,dissect along the truncus arteriosus to afferent branchials4 ,cut away the heart and oesophagus2 run a seeker up the dorsal aorta and cut along it from the ventral side to subclavian and efferent branchial arteries4 ,skin the top of the head4 ,note1 while doing this1 the yellow1 jelly-like sense-tubuli beneath the skin4 ,shave off top of brain-case1 and leave the head in spirit for a week or so4 ,second ,dissection4-- ,place the head with the ventral side downward1 skin all the dorsal surface as yet unskinned4 ,refer to book for precise position of the anterior cardinal sinus1 and then cut down through body wall into this just over gill slits4 ,the tenth nerve will become visible1 with its slit branches athwart the floor of the sinus4 ,clear to make this more evident1 and make out its lateral line and visceral branches1 and the ninth nerve4 ,lines from ,second ,edition only4 ,the pharyngo-branchials may be felt beneath the sinus4 ,run a seeker from the dorsal aorta to the efferent branchials4 ,proceed now to orbit1 and1 without any dissection beyond the removal of skin1 make out recti and oblique muscles of eyeball1 and the optic1 third and fourth nerves4 ,cut through these structures carefully and remove1 exposing nerves seven1 and five1 as described and figured in the text4 ,examine the otic capsule by taking successive slices through it to show the labyrinth of the ear4 -,remove the dorsal wall of the skull to obtain a dorsal view of brain4 ,if this is sufficiently hard1 examine it2 if not1 return it to spirit for a more convenient occasion4- ,examine brain4 -,amphioxus ,two specimens of this type should be obtained4 ,it should be examined entire by the naked eye and with the low power of the microscope4 ,immersion1 in glycerine will render it more transparent2 or it may be cleared with oil of cloves1 put up temporarily in that1 or permanently in ,canada balsam4 ,one specimen should then be pinned out in the dissecting dish1 ventral side uppermost1 and the atrium opened to expose liver and pharynx4 ,a part of the pharynx may be examined with the low power to see the form of the gill slits4 ,the second specimen should be soaked in turpentine for some time1 and then dropped into melted paraffin wax4 ,transverse sections may then be cut with a razor1 the paraffin wax removed from these by solution in turpentine1 the turpentine in its turn dissolved out by alcohol1 and the sections1 after immersion in oil of cloves1 may be transferred to ,canada balsam for examination and preservation4 ,this work should not be attempted until some practical histological work has been done in botany1 and it may be altogether avoided by the purchase of stained and mounted sections4 -,development ,laboratory work in this portion of the science is not usually undertaken by elementary students of biology1 but the reader will probably find it helpful1 in the realization of the facts given in this book1 to look out for frog spawn1 in ,february and ,march1 and to catch and examine tadpoles of various sizes4 ,a small dissecting dish may be made by pouring melted paraffin wax into one of those shallow china pots chemists use for cold-cream1 and tadpoles may be pinned out with entemologists' pins and dissected with needles4 ,but this is a work of supererogation4 ,partially incubated hen's eggs may be obtained at a small cost almost anywhere1 and the later stages profitably examined and dissected under warm water4 ,for a clear understanding of the allantois and amnion1 this last is almost indispensable4 ,a few microscopic slides of sections of embryonic chicks should also be compared with our rough diagrams4 -,key for ,dissection ,sheets1 and ,abbreviations4 ,sheet #a ,figure #a4 ,main facts of the ,rabbit's ,anatomy 7diagrammatic74 an41 anus4 a4ao41 arch of the aorta4 au41 auricle4 a4r41 ad-renal body4 br41 brain4 b4d41 bile duct4 brch41 bronchus4 cd4st41 cardiac end of stomach4 co41 colon4 cae41 caecum4 ddnm41 duodenum4 d4ao41 dorsal aorta4 dia41 diaphragm4 ep41 epiglottis4 g4d41 genital duct 7either sex74 il41 ileum4 in4art41 innominate artery4 k41 kidney4 lg41 lung4 lv41 liver4 l41 larynx4 l4s4c41 l4c4c4 left common carotid artery4 m41 mouth4 na41 nasal passage4 oes41 oesophagus4 p4v41 pyloric valve4 p4d41 pancreatic duct4 pt41 peritoneal cavity4 r41 rectum4 st41 stomach4 stm41 sternum4 s4r41 sacculus rotundus4 s4c41 spinal cord4 tr41 trachea4 ur41 ureter4 ur4b41 urinary bladder4 v4b41 a vertebral body4 v4ap41 vermiform appendix4 v4v41 v4p4 velum palatium4 v41 ventricle of heart4 v4c4i41 vena cava inferior4 ,figure #b4 ,the ,liver 7diagrammatic74 g4b41 the gall bladder4 r4l41 r4c41 l4l441 l4c41 right lateral and central1 and left lateral and central1 lobes respectively4 sp41 the ,spigelian lobe 7fits into angle of stomach and oesophagus74 ,illustration3 ,diagram ,sheet #a4 ,sheet #b ,figure #a4 ,the ,rabbit's ,circulation 7see footnote to ,section #de74 7,throughout l4 indicates left1 r4 right4 ,vessels without r4 or l4 prefixed are median47 -99 ,the figure is inaccurate at one point2 l4c4c4 should spring from the base of inn4 ,see ,sheet #i4- ,first ,edition only text ao4a41 aortic arch4 au41 auricle4 az4v41 7p4c4 in ,figure #b71 azygos vein4 c4c41 common carotid4 c4il4a41 common iliac artery4 coe4a41 coeliac artery4 d4ao41 dorsal aorta4 e4il4v41 external iliac vein4 e4ju41 external jugular vein4 f41 femoral artery4 h4v41 hepatic vein4 inn41 innominate artery4 in4j41 internal jugular vein4 i4il4a41 internal iliac artery4 i4il4v41 internal iliac vein4 k41 kidney4 lv41 liver4 l4g4v41 lienogastric vein 7portal74 m4v41 mesenteric 7portal system74 p4m4a41 posterior mesenteric artery4 p4v41 main portal vein4 p4a4 pulmonary artery4 r41 rectum4 r4a41 renal artery4 r4v41 renal vein4 s4v41 and a41 spermatic 7or ovarian7 vein and artery 7to genital organ74 s4mes4a41 superior mesenteric artery4 s4-cl4a41 subclavian artery4 s4-cl4v41 subclavian vein4 v4c4s41 vena cava superior4 v4c4i41 vena cava inferior4 v4 or vn41 ventricle4 ,figure #b4 ,figure of ,circulation 7simplified7 illustrating certain points in development to be referred to later4 ,figure #c4 ,respiration4 ,see text1 ,section #da4 ,figure #d4 ,blood4 ,see text1 ,section #ce4 ,illustration3 ,diagram ,sheet #b4 ,sheet #c ,histological ,figures1 #a4 ,no numbers ,i41 or ,,ii4 ,figure ,,iii4 ,an amoeba4-- n41 nucleus4 ns41 necleolus4 c4v41 contractile vacuole4 ,figure ,,iv4 ,embryonic tissue from the blastoderm of a chick4 ,figure ,v4 ,columnar epithelium4-- g4c4#a1 g4c4#b1 g4c4#c1 successive phases in the development of a goblet cell4 ,figure ,,vi4 g4end41 is geminating endothelium2 the cells divide and apparently drop off to become white corpuscles in the lymph current4 sq4end41 squamous endothelium from the mesentery4 sq4ep41 squamous epithelium 7from the mucous membrane within the cheek74 st41 are opening 7stomata7 communicating between the lymphatics in the mesentery and the peritoneal 7coelomic7 space4 ,figure ,,vii4 ,ciliated epithelium from the roof of the frog's mouth4 ,figure ,,viii4 ,forms of glands4-- g4ep41 is a gastric gland from the stomach2 trs41 below1 is cross section4 ,this is one of the simplest types of gland4 s4g41 a sweat gland1 is also a simple tube1 but convoluted below4 r4g41 is a racemose gland1 such as the pancreas1 ,brunner's or the salivary glands4 ,the kidney1 we shall see later1 is simply an aggregate of branching tubuli 7,sheet #g74 ,figure ,,ix4 ,a duodenal villus4-- lac41 the lacteal4 v41 the vein4 ,figure ,x4a4 ,diagram of liver structure4-- b4d41 the inter-lobular bile duct4 h4a41 the hepatic artery1 bringing blood to oxygenate and nourish the liver tissue1 and similarly distributed4 h4v41 the hepatic vein taking blood from the liver to the heart1 its twigs commencing in the lobuli 7intra-lobular74 lb4 lb41 lobuli4 p4v41 the portal vein bringing blood1 from which substances are to be elaborated1 into the liver1 and breaking up between the lobuli 7inter-lobular74 ,figure ,x4b4 ,a diagram of the appearance of an injected liver lobule as seen in section under the microscope4 ,illustration3 ,diagram ,sheet #c4 ,sheet #d ,histological ,diagrams1 #b4 ,figure ,,xi4 ,a blood capillary4 ,white corpuscles are migrating through the walls into the tissues 7compare ,section #ff74 ,figure ,,xii4 ,hyaline cartilage 7,section #fd74 ,figure ,,xiii4 c4c41 connective tissue corpuscle4 w4i4f41 white inelastic fibres4 y4e4f41 yellow elastic fibres4 ,figure ,,xiv4 ,botryoidal tissue 7,section #ff74 ,figure ,,xv4 ,development of a fat drop4-- f4d41 fat drop1 in a connective tissue corpuscle2 c4c41 in the formation of adipose tissue 7,section #fg74 ,figure ,,xvi4 ,diagrammic cross section of a long bone4-- b4c41 bone corpuscle in a lacuna4 ,h4v41 ,haversian vessel 7in the ,haversian canal7 surrounded by concentric lamellae of bone1 c4l41 and together with these and zones of bone corpuscles1 called a ,haversian system4 i4l41 inner lamellae4 m4c41 medullary canal full of yellow marrow4 o4l41 outer lamellae4 p4o41 periosteum4 ,figure ,,xvii4 ,to illustrate bone development 7,section #ga74 ,figure ,,xviii4 ,dentition of rabbit1 incisors #b_/a1 canine #j_/j1 premolar #c_/b1 molar #c_/c4 ,illustration3 ,diagram ,sheet #d4 ,sheet #e4 ,diagram of the ,rabbit's ,bones4 ,to be compared with the real things4 ,d and ,d' show the fore and hind limbs1 to illustrate their homology4 ,d is in the embryonic position4 ,the radius and tibia are1 at an early stage in development1 on the anterior edge of their respective limbs2 the ulna and fibula1 posterior2 the former are spoken of as preaxial in position1 the latter as postaxial4 ,but in the adult the humerus is twisted so that the proximal end of the radius lies at the outer side of the elbow1 whence it crosses the ulna1 so that its distal end is inside1 while the femur is also twisted round1 so that the entire tibia is internal4 ,figures #a and #b4 -,limbs4-- a4c41 acetabulum4 acr41 acromion4 as41 astragulus4 c41 carpus4 ca41 calcaneum4 co41 coracoid4 coty41 cotyloid bone4 fb41 fibula4 fe41 femur4 g41 glenoid cavity 7for head of humerus74 hd41 head of femur4 hum41 humerus4 i41 ilium4 is41 ischium4 m4c41 meta-carpals4 na41 navicular4 o41 olecranon process of ulna4 o4f41 olfactory fossa4 pb41 pubis4 r41 radius4 u41 ulna4 ,figure #c4 -,sternum4-- ,mb41 manubrium4 r#a41 r#b41 and etc41 sternal ribs4 st41 sternebrae4 xi41 ,xiphisternum4 ,figure #d4 ,vertebrae4-- ,at41 ,atlas4 ,ax41 axis4 c41 b4 centrum4 ,c4,v41 caudal vertebra4 c4v41 ,cer4,v4 cervical vertebra4 ep41 epiphysis4 f4r41 fused rib 7in cervical vertebrae74 ,l4,v41 ,lumbar vertebra4 m41 metapophysis 7of lumbar vertebra74 n4a41 neural arch4 n4s41 neural spine4 r41 rib4 ,s4,v41 sacral vertebra4 ,t4,v41 ,thoracic4 tr4p41 transverse process4 v4a4c41 vertebrarterial canal4 z41 zygapophysis4 ,illustration3 ,diagram ,sheet #e4 ,sheet #f4 ,the ,skull of ,canis499-- #a4 ,dorsal4 #b4 ,ventral4 #c4 ,right ,lateral ,aspect4 #d4 ,section a little to the left of the nasal septum4 #e4 ,lower jaw 7smaller7 #f4 ,hyoid apparatus4 ,lines from ,first ,edition only4 -99,a ,fox in this case4 ,the skull is quite like that of a ,dog1 but it has the advantage of more distinct sutures between the bones4- a4n41 anterior nares4 a4s41 ali-sphenoid4 b4h41 body of the hyoid4 b4o41 basi-occipital4 b4sp41 basi-sphenoid4 c41 condyle of the skull4 c4#a1 c4#d1 canines4 c4f41 condylar foramen 7for ,,xii474 c4h41 cerato-hyal4 ,e4f41 ,eustachian foramen4 e4h41 epihal4 -e4n41 or a4n41 the anterior nares4- e4o41 exoccipital4 eth41 ethmoid4 e4t41 ethmo-turbinal4 f41 frontal4 f4l4a41 foramen lacerum anterius4 f4l4m41 foramen lacerum medium4 f4l4p41 foramen lacerum posterius 7for ,,ix41 ,x41 ,,xi474 ,f4,m41 or f4m41 foramen magnum4 f4o41 foramen ovale4 f4r41 foramen rotundum4 i41 incisors4 ju41 jugal4 m41 molars4 m4t41 maxillo-turbinal4 mx41 maxilla4 na41 nasal4 n4t41 nasal turbinal4 o4f41 optic foramen4 o4s41 orbito-sphenoid4 p41 or pal41 palatine4 pa41 parietal4 p4m41 pre-maxilla4 p4m4#a1 p4m4#d1 premolars4 p4n41 posterior nares4 p4sp41 pre-sphenoid4 pt41 pterygoid4 s4h41 stylo-hyal4 s4m4f41 stylo-mastoid foramen 7for ,,vii474 s4o41 supra-occipital4 sq41 squamosal4 s4t41 sectorial tooth4 t4h4 thyro-hyal4 vo41 -black line indicating position of- vomer4 z4p41 zygomatic process of squamosal4 ,illustration3 ,diagram ,sheet #f4 ,sheet #g ,figure #a4 ,striated muscle fibre 7of the ,rabbit71 ruptured to show sarcolemma4 e4p41 its end plate4 ,k4m41 membrane of ,krause4 n41 nucleus4 nv41 nerve4 sc41 sarcolemma4 s4e41 sarcous elements4 ,figure #b4 ,cardiac muscle4 ,figure #c4 ,unstriated muscle fibres4 ,figure #d4 ,diagram of the ,skin4 b4v41 blood vessel4 d41 areolar tissue of the dermis 7mesoblastic74 s4c41 stratum corneum1 and s4m41 stratum mucosum of the epidermis4 s4g41 sweat gland4 t4c41 tactile corpuscle4 ,figure #e4 ,to illustrate ,kidney structure4-- a4b4v41 and e4b4v41 afferent and efferent blood-vessels1 of which the latter go to break up upon the tubli4 ,b4c41 one of ,bowman's capsules of the cortex2 ur4t41 the uriniferous tubule running from it into the medulla1 where it loops and branches2 around it branches a blood-vessel1 of which the latter go to break up upon the tubuli4 c41 cortex4 g41 glomerulus1 a knot of blood-vessels in the capsule4 m41 medulla4 p41 pelvis4 ur41 ureter4 ,the water of the urine is probably filtered off in the capsule1 the urea and other salts secreted by the tubuli4 ,no ,figure #f4 ,figure #g4 ,the ,auditory structures of the ,rabbit 7diagram74 ,see text1 ,section #aae4 ,figure #h4 ,the ,eye 7diagram74 ,see text1 ,section #aaa4 ,figure #i4 ,the ,retina 7diagram74 ,see text1 ,section #aab4 ,illustration3 ,diagram ,sheet #g4 ,sheet #h ,the ,brain of the ,rabbit4-- #a4 ,in median section4 #b4 ,from above1 with the top of the right hemisphere sliced off horizontally at the level of the corpus callosum4 #c4 ,a deeper section through the thalamencephalon1 corresponding to ,b in 7#a74 #d4 ,under-view of the brain4 #e4 ,diagram referred to in the text and for comparison with ,sheet #g1 #c;b41 and ,sheet #ah1 #b4 ,figures #a-e4 ar41 arrow in the iter4 a4c41 the anterior commissure1 a thickening of the anterior wall of the third ventricle4 c4c41 corpus callosum4 c4 cb41 crura cerebri4 c4h41 cerebral hemispheres4 c4q41 corpora quadrigemina4 f4cbm4 7right71 flocculus of the cerebellum4 l4h41 left cerebral hemisphere 7ch474 l4l41 lateral lobe of cerebellum4 m4c41 middle commissure4 m4o41 medulla oblongata4 op41 optic nerve4 o4l41 olfactory lobe4 o4th41 7right71 optic thalamus4 p4c41 posterior commissure 7thickening of postero-dorsal wall of the third ventricle74 p4g41 pineal gland4 pt41 pituitary body4 p4,v41 pons ,varolii4 s4c41 thin roof of the fourth ventricle4 v4cbm41 vermis of cerebrum4 v4l41 lateral ventricle4 ,figure #d4 ,nerves4-- ,i41 ,olfactory4 ,,ii41 ,optic4 ,,iii41 ,oculo-motor4 ,,iv41 ,patheticus4 ,v41 ,trigeminal4 ,,vi41 ,abducens4 ,,vii41 ,facial 7portio dura74 ,,viii41 ,auditory 7portio mollis74 ,,ix41 ,gustatory 7glossopharyngeal4 ,x41 ,pneumogastric or vagus4 ,,xi41 ,spinal ,accessory4 ,,xii41 ,hygoglossal4 ,figure #f4 ,the ,spinal ,cord in section4-- c4c41 the central canal4 d4f41 the dorsal fissure4 d4n41 the dorsal nerve root2 g41 its ganglion4 v4f41 the ventral fissure4 v4n41 the ventral nerve root4 ,note that in ,figure #a the central canal is continuous with the fourth ventricle4 ,figure #g4 ,histological elements4-- g4c41 multipolar ganglion cell4 n41 nucleus of a medullated nerve4 a4c41 its axis fibre4 s4,s41 7sheath of ,schwann71 medullary sheath interrupted at intervals by n4,r41 the nodes of ,ranvier4 n4m4f41 a non-medullated fibre4 ,illustration3 ,diagram ,sheet #h4 ,sheet #i4 -,the ,nerves of the ,rabbit4 ,figure ,i4 ,rough sketch of dissection of the neck from the left ventral aspect4-- ,the bands of muscle between hyoid1 mandible1 and sternum1 and the thymus gland carefully cleared4 lr41 is the larynx1 and b41 the balla4 s4m4g41 the right sub-maxillary gland 7the left has been removed74 ,the nerves are numbered4 l4r4l4n41 r4r4l4n4 is the left recurrent laryngeal looping under that solid connection between the pulmonary artery 7p4a47 and ao41 the aortic arch1 which was an open tube in the embryo1 the ductus arteriosus4 hy41 is the hyoid with its posterior cornua4 ph4n41 is the phrenic nerve4 r4r4l4n41 l4r4l4n4 is the right recurrent looping under the sub-clavian4 s4c4g41 is the super or cervical ganglion of the sympathetic 7sym472 s4l4n41 is the left superior laryngeal1 and g4 the left depressor branch of x4 z41 is the ramus descendens noni of the twelfth nerve4 ,in early development the heart lay just beneath the pharynx in the position of the larynx 7compare ,dog-fish and ,frog72 as the neck elongated1 the heart shifted back with its vessels1 and so the long loop of the recurrent laryngeal comes to be drawn out in this singular way4 ,figure ,,ii4 ,diagram of orbit to show ,v4#a orbit-nasal1 ,v4#b the maxillary1 and ,v4#c the mandibular branch of ,v4 ,in order to show these in dissection1 the malar must be cut away1 and the eye and glands of the orbit removed4 s4r41 e4r4 p4r41 i4r41 and a4r41 cut ends of the superior1 external 7or posterior71 inferior1 and anterior 7or internal7 recti muscles4 s4o41 and i4o41 the superior and inferior obliques4 ,figure ,,iii4 ,general diagram of the ,rabbit's cranial nerves4 ,figure ,,iv4 ,rough sketch of dissection of the nerves and blood-vessels dorsal to stomach4-- ,the stomach turned over to the animal's right1 the ,spigelian liver lobe cleared from the oesophagus1 the mesentery supporting spleen and hiding solar plexus picked off1 and the mesentery hiding sympathetic cleared4 coe4art41 coeliac artery1 and s4m4a41 superior mesenteric artery4 coe4g4 coeliac1 and s4m4g41 superior mesenteric ganglion4 ,the two together form the solar plexus4 l4abd4sym41 left abdominal sympathetic 7in the actual dissection1 the right would also be visible74 l4a4r41 left adrenal4 l4sp4n41 left splanchnic nerve4 r4art41 renal artery4 r4v41 renal vein4 st41 the stomach1 and sp41 the spleen4 x41 the vagus on oes41 the oesophagus4 ,illustration3 ,diagram ,sheet #i4 ,sheet #aj4 -,reproductive ,organs of the ,rabbit4 ,figure #a4 ,the ,male4 ,figure #b4 ,the ,female ,organs4 7,the symbols below the figures indicate the sex47 pb41 is the pubic symphysis which has been cut through4 ,r41 the rectum1 with r4g41 the rectal gland1 and a41 the anus4 t41 the tail4 r4ur41 the right ureter4 l4ur41 the left ureter4 ur4b41 the urinary bladder4 ,in the ,male ep41 the epididymis4 ,p41 the penis4 pp41 the prepuce4 scr41 the scrotal sac1 containing these2 r4v4d41 the right vas deferens4 ,t41 is the testis4 u4m41 the uterus masculinus4 ,in the ,female c4ut1 the left cornu uteri4 ,f4t41 the left ,fallopian tube4 ov41 is the ovary1 with a ,graafian follicle1 ,g4,f4 ,v41 the vagina4 v4b41 the vestibule4 ,figure #c4 ,diagram of ovary with stages in the development of a ,graafian follicle #a1 #b1 #c1 #d1 #e1 see text1 ,section #acg4 ,the arrow indicates the changes in position of the developing follicles4 ,illustration3 ,diagram ,sheet #aj4 ,sheet #aa4 ,figure #a4 ,general dissection of ,frog 7male74 ,figure #b4 ,the heart and great vessels laid open4 ,figure #c4 ,the circulatory system from the side4 ,figure #d4 ,blood4 n41 nucleus4 r4c41 red corpuscle 7oval and nucleated74 w4c41 white corpuscle ,small figure of ,frog in left-hand corner is to show position of heel1 h4 ,reference ,letters4 all4b41 allantoic bladder 7 urinary bladder74 c4ad41 corpus adiposum4 cl4c41 cut end of the right clavicle4 d41 duodenum4 g4b41 gall bladder4 il41 ileum4 k41 kidney4 l4au41 left auricle4 l4g41 lung4 l4int41 large intestine4 l4s4v41 longitudino-spiral valve4 ,l4v41 ,liv41 liver4 pan41 pancreas4 r4au41 right auricle4 sp41 spleen4 st41 stomach4 ,t41 testis4 t4a41 truncus arteriosus4 ur41 urogenital duct4 v41 ventricle of heart4 ,arteries 7white74 ao41 aorta4 c4a41 carotid arch4 c4g4 c4gl41 carotid gland4 coe41 coeliac4 cu41 -and pa41- cutaneous4 d4ao41 dorsal aorta4 e4c41 lingual artery4 i4c41 internal carotid4 l4a4a41 left aortic arch4 pa41 and p41 pulmonary4 p4c4 p4cu41 pulmo-cutaneous4 r4a4a41 right aortic arch4 s4cl41 sub-clavian4 t4a41 truncus arteriosus4 ,veins of the ,caval ,system -7black7-4 b4v41 brachial 7from fore limb74 e4j41 external jugular4 h4v41 hepatic vein4 i4j41 internal jugular4 in4v41 innominate vein4 l4v4c4s41 left vena cava superior4 p4v41 cutaneous vein4 s4cl4v41 sub-clavian vein s4s4r41 sub-scapular vein4 v4c4i41 vena cava inferior4 ,veins of the ,portal and ,renal ,portal ,systems -7shaded7-4 a4ad41 and a4ab4v41 anterior abdominal vein4 b4v41 and p4v41 united are called the sub-clavian vein4 l4fm41 left femoral4 l4p41 left pelvic4 l4r4p41 7and r4p47 left renal portal4 l4sc41 left sciatic4 p4v41 portal vein4 -7,the anterior abdominal is coloured black in ,figure #a47- ,the cutaneous artery in the above figures is turned back4 ,in dissection it will be found to lie over and hide the dorsal-ward sweep of the aortic arch4 ,illustration3 ,diagram ,sheet #aa4 ,sheet #ab4 ,figure #a4 ,upper view of the ,frog's brain4 ,figure #b4 ,under view of the same4 ,figure #c4 ,the same-- median section4 ,figure #d4 ,the distribution of the ,frog's nerves4 ,compare ,sheet #i1 ,figure ,,iii4 ,the shaded part in #d is the -otic capsule- tympanum4 ,the hyoid apparatus is roughly represented in black to show its relation to ,,ix4 7dorsal to it7 and sp4 #a 7ventral74 ,compare nerves ,,ix and ,,xii in ,sheet #i4 ,the nerves are numbered4 cb41 the cerebellum4 c4h41 cerebral hemispheres4 f4t41 filum terminale4 g4tr41 ganglion on the fifth nerve4 l4t41 lamina terminalis4 mb41 mid-brain4 md41 medulla oblongata4 o4l41 optic lobes4 pin41 pineal gland4 pit41 pituitary body4 r4h41 olfactory lobes 7rhinencephalon74 th4c41 thalamencephalon4 sp4#a1 first spinal nerve4 sp4#b1 #c1 brachial plexus to fore limb4 ,figure #e4 ,the spinal column 7and pelvic girdle7 of the ,frog4 ,figure #e;b4 ,vertebrae4 ,figure #f4 ,the pectoral girdle and limb1 dorsal view4 ,figure #g4 ,the pelvic girdle and right limb from the side4 7l4h4 shows the position of the right lymph hearts-- they are paired47 as41 astragalus4 b41 body4 c41 calcar 78 a sixth digit74 cal41 calcaneum4 cl41 clavicle overlying a procoracoid cartilage4 co41 coracoid4 f41 fibula4 ,,fe41 femur4 h41 humerus4 il41 ilium4 is41 ischium4 o4st41 omosternum4 pu41 pubis4 r41 radius4 sc41 scapula4 s4sc41 supra-scapula4 s4v41 sacral vertebra4 t41 tibia4 t4p41 transverse process4 ul41 ulna4 ur41 urostyle4 x41 xiphisternum4 z41 zygapophysis4 #a1 #b1 and etc41 first1 second1 and etc41 digits4 ,d4 and ,d'4 are simplified diagrams of the limbs for comparison with the similar ones of the ,rabbit4 ,in each girdle we have a dorsal ossification 7scapula1 ilium7 and two ventral parts 7pubis and procoracoid cartilage1 ischium and coracoid71 and at the meeting-place of the three in each case the proximal bone of the limb 7humerus1 femur7 articulates4 ,illustration3 ,diagram ,sheet #ab4 ,sheet #ac4 -,urogenital ,organs of the ,frog4 ,figure #a4 ,the ,male4 ,figure #b4 ,the ,female4 ,the oviduct removed on the animal's left1 and the ovary on its right4 ,organs common to both sexes4-- al4b41 allantoic bladder4 c4ad41 corpus adiposum4 cl41 cloaca4 int41 intestine4 ,k41 kidney4 lg41 7dotted outline of7 lung4 oes41 oesophagus4 r4p4v41 renal portal vein4 st41 stomach4 ,in the ,male4-- ,t41 testis4 v4e41 vasa efferentia4 u4g4d41 urogenital duct4 p41 prostate gland4 ,in the ,female4-- adr41 adrenal4 f4t41 fallopian tube 7anterior part of oviduct74 99 its opening4 o4d41 oviduct 7letters on the opening -uterine portion-74 ov41 ovary4 ur41 ureter4 7,this would be the condition about midwinter47 ,in ,march o4d4 will be either enormously distended with eggs1 or large1 flabby1 and empty1 and ov4 will be small and brownish1 without any large eggs2 the ovary gradually recovers its size through the summer4 ,figure #c4 ,spermatozoa attached to the parent cell 7g4e47 from the lining epithelium of the testis1 and one free4 fl41 the flagellum4 ,illustration3 ,diagram ,sheet #ac4 ,sheet #ad -,skull ,structure and ,development of the ,frog4 ,figure #a4 ,i41 ,,ii41 early and late stages of the ,tadpole's chindrocranium4 ,diagrammatic4 ,figure #b4 ,dorsal view of a young ,frog's cranium-- the membrane bones removed4 ,diagrammatic4 ,figures #c and #d4 ,dorsal and ventral views1 respectively1 of the ,frog's skull-- the lower jaw removed4 ,figure #e4 ,side view of the ,frog's skull4 ,figure #f4 ,median section of the brain case4 ,figure #g4 ,the hyoid apparatus4 ,figure #h4 ,i41 ,,ii41 ,,iii41 progressive stages of the ,tadpole's skull from the side4 ,after ,w4 ,k4 ,parker4 ,figure #i4 ,f41 side and hind views of the ,frog's skull4 ,d41 the same of the ,dog4 ,roughly diagrammatic4 ,n4,b4-- ,in all cartilage is dotted1 cartilage bone cross-barred1 and membrane bone1 white4 ,in ,figure #d1 pt41 should be cross-barred2 and in #e1 th4h4 plain4 a4c41 anterior cornu of hyoid 7 ,,ch47 -not lettered1 in ,figure #e-4 a4o41 antorbital cartilage4 ar41 angulo-splenial -7,on ,frog ,section #cd1 for ,articulare read -,angulo-,splenial7-4 -b41 parachordal part of brain box-4 b4c41 brain case4 b4h41 body of hyoid4 b4r41 branchial arches4 ,,ch a4c4 c4t41 cornua trabeculi4 d41 dentary4 e41 eye4 ,e4,n41 external nares4 e4o41 exoccipital bone4 f41 fenestra 7membranous part of cranial wall74 -f4p41 fronto-parietal4- h4m41 hyomandibular cleft ,eustachian tube and ear drum4 mb41 mandible4 ,m4,c41 ,meckel's ,cartilage4 m4mk41 mento-,meckelian bone4 m4p41 mouth passage4 mx41 maxilla4 n4c41 notochord4 n4o41 nasal organ4 n4p41 nasal passage4 ot41 or o4c41 otic 7auditory7 capsule4 pal41 palatine bone4 ,,pal41 hard palate of ,mammal4 p4c41 parachordal4 p4f41 parieto-frontal -see f4p4- p4m41 premaxilla4 ,p4,n41 internal nares4 p4o41 prootic bone4 p4p41 palato-pterygoid cartilage4 psph41 parasphenoid bone4 pt41 pterygoid bone4 q41 quadrate cartilage4 q4j41 quadrato-jugal4 s4e41 sphenethmoid bone4 sq41 squamosal4 t41 trabecular part of brain box4 t4c41 trabecula4 th4h41 thyrohyal4 ,illustration3 ,diagram ,sheet #ad4 ,sheet #ae ,figure #a4 ,dissection of -,male- ,female ,dog-,fish to show alimentary canal1 the pericardium also being opened and the cloaca slit up4 ,above is also seen the dorsal view of the head4 ,figure #b4 ,the pelvic girdle and fin skeleton of a male4 ,no ,figure #c1 in ,first ,edition4 ,figure #d4 ,the spiral valve in the colon4 ,figure #c1 in ,second ,edition4 a4p41 abdominal pore4 aur41 -auricle- atrium of heart4 b4d41 bile duct4 b4pt41 basi-pterygium4 -cl41 clasper4- cl4c41 -its- the supporting cartilage of the clasper4 co41 colon4 d'dnm41 duodenum4 e41 the eye4 g4bl41 gall bladder4 g4s41 gill slits4 ,l4,lv41 left lobe of liver4 ,m4,lv41 middle lobe of liver4 olf41 olfactory opening4 pan41 pancreas4 pcd41 pericardial wall4 pel4g41 the pelvic girdle4 p4p41 arrow through pericardio-peritoneal canal4 r4g41 rectal gland4 ,r4,liv41 right lobe4 sp41 spiracle4 spl41 spleen4 st41 the stomach4 s4v41 sinus venosus4 u4g4p41 uro-genital pore4 v41 ventricle4 ,illustration3 ,diagram ,sheet #ae4 ,sheet #af4 ,figure #a4 ,circulation of the ,dog-,fish4 ,figure #b4 ,simplified and more typical fish circulation1 in which the posterior cardinals have not coalesced in the median line4 ,the ,cuvierian veins the vena cava superior of the higher type2 the posterior cardinal is represented by the azygos vein in the ,rabbit4 ,compare ,sheet #bd1 ,figure #g1 and ,sheet #b1 ,figure #b4 ,figure #c4 ,side view of the pericardium4 a4br41 afferent branchial artery4 a4c4s41 anterior cardinal sinus 7 internal jugular vein74 au41 atrium 7auricle7 7 the two auricles of higher forms74 b4a41 bulbus arteriosus4 c4a41 conus arterious4 cd4 a41 caudal artery4 cd4v41 caudal vein4 c4s41 ,cuvierian sinus4 d4a41 dorsal aorta4 ,e41 eye4 e4br41 efferent branchial arteries4 g4s41 in position of gill slits4 h4br4a41 hypobranchial artery4 ,h4,s41 hepatic sinus4 i4j4s41 inferior jugular sinus 7 external jugular vein74 ,k41 kidney4 ,l4,v41 lateral vein4 oe4s41 ventral wall of oesophagus4 ,p4,c4,c41 pericardial cavity4 ,p4,c4,s41 posterior cardinal sinus4 p4p4c41 pericardio-peritoneal canal4 ,p4,v41 portal vein4 r4p4v41 reno-portal vein4 s4c4v41 subclavian vein4 ,vn41 ventricle4 -v4s4v41 inferior 7 external7 jugular vein-4 ,figure #d4 ,skeleton of pectoral limb1 and girdle4-- g41 the girdle 7also in ,figure #c74 m4p41 meso-pterygium4 mt4p41 meta-pterygium4 p4p41 pro-pterygium4 sc41 its dorsal portion4 ,illustration3 ,diagram ,sheet #af4 ,sheet #ag -,the ,uro-genital ,organs of the ,dog-,fish4 ,figure #a4 ,the ,female1 the oviduct of the left side cut away1 -and an egg case in the oviduct4- ,figure #b4 ,the ,male4 ,the rectum is removed in both cases1 and the silvery peritoneum dissected off from the kidneys4 ,figure #c4 ,a generalized diagram of the uro-genital organs4-- ,all references in text4 ,ms41 the mesonephros1 is the epididymis in the male1 and is reduced in the female2 ,ms4d41 its duct1 is the vas deferens in the male1 and persists only as the urinary receptacle in the female4 ,mt4 and ,mt4d41 the metanephros and metanephric duct1 become the functional kidney and ureter in both sexes4 ,g4 is the gonad 7reproductive gland71 and ,m4,l4 the animal's middle line 7median plane74 -,ps4-1 ,pr41 the pronephros1 is never developed in the ,dog-fish2 ,p4d41 its supposed duct1 is the oviduct of the female1 and is suppressed in the male4 ,illustration3 ,diagram ,sheet #ag4 ,sheet #ah4 ,figure #a4 ,the ,dog-,fish ,brain1 dorsal view4 ,figure #b4 ,median section of the same4 ,to the right a more diagrammatic figure4 ,the nerves are numbered3-- ,,br#a1 ,,br#b1 ,,br#c1 ,,br#d branches of ,x forking over the second to the fifth gillslit4 cb41 cerebellum4 h4s4c41 horizontal semi-circular canal of ear1 exposed by the slicing down of the otic mass4 ,,lat41 lateral-line branch of ,x4 m4o41 medulla oblongata4 oph41 ophthalmic nerve 7,v4#a,,vii4#a74 op4l41 optic lobe4 pit41 pituitary body4 pr4c41 prosencephalon 7cerebral hemisphere74 rh41 olfactory lobe 7rhinencephalon74 r4t41 -its- restiform tracts of medulla4 -st-4 ,s4,p4,g41 stalk of the pineal gland4 th41 thalamencephalon4 th4c41 thalamencephalon4 -ut41 the utriculus1 seen through the semi-transparent cartilage-4 ,vid41 the ,vidian branch of ,,vii4 ,visc41 visceral branch of ,x4 ,figure #c4 ,diagram of the ear of a fish4 ,the structure of this is easily made out by clearing otic capsule and cutting slices of the cartilage in the ,dog-,fish 7e4g41 ,figure #a1 h4s4c474 amp41 their ampullae4 a4v4c41 p4v4c41 h4c41 anterior1 posterior1 horizontal canal respectively4 amp41 the ampullae4 d4e41 the ductus endo-lymphaticus4 -sac41 the sacculus2 c41 a small outgrowth of the latter1 corresponding to the rabbit's cochlea-4 -ut41 the utriculus-4 ,figure #d4 ,the cranium and branchial bars of a ,dog-,fish4 ,the groove in the otic capsule connects the orbital and anterior cardinal sinuses4 ,a4,c4,s41 position of the anterior cardinal sinus 7dotted outline74 c41 the vertebral centra4 c4b41 the cerato-branchial4 c4h41 the cerato-hyal4 e4b41 epi-branchial4 ex4b41 extra-branchial4 h4,m41 the hyo-mandibular4 i4n4p41 inter-neural plate4 ,m4,c41 ,meckel's 7lower jaw7 bar4 ,na4,c41 the nasal capsule4 n4p41 neural plate4 n4s41 neural spine4 ,ot4,c41 the otic capsule4 ph4b41 the pharyngo-branchial4 ,p4pt41 the palato-pterygoid bar 7upper jaw bar74 p4s41 pre-spiracular ligament1 containing a cartilaginous nodule4 r41 rib4 sp41 the position of the spiracle4 ,figure #e4 ,diagrams of a vertebral centrum4-- ,for reference letters1 see text 7,section #i74 ,no ,figure #f1 in ,first ,edition4 ,figure #f4 ,diagram for comparison with ,figure ,,iii41 ,sheet #i4 ,illustration3 ,diagram ,sheet #ah4 ,sheet #ai4 ,figure #a4 ,amphioxus1 seen from the right side4 a----b shows the natural size4 ,the animal is supposed to be clarified1 and mounted in some highly refracting medium1 so that it is practically transparent2 ,i41 ,,ii41 ,,iii41 and etc41 refer to the section figured on ,sheet #bj4 ,figure #b4 ,amphioxus1 ,general ,dissection4 7,slightly altered from a figure by ,professor ,e4 ,r4 ,lankester47 ,the ventral atrial wall is removed4 ,the pharynx cut away from the dorsal body-wall1 and with the true ventral body-wall turned over to the 7animal's7 right4 ,the arrow a41 a41 passes through anus to intestine2 b41 b41 is thrust through the atrial pore to the atrial cavity4 ,note coe41 the body cavity4 ,references to the two figures4 an41 anus4 at41 atrial cavity4 at4w41 atrial wall4 at4p41 atrial pore4 a4d41 anterior dilatata of nervous system4 b4w41 body-wall4 b4t4,l41 brown tubes of ,lankester4 c4f41 ciliated funnel4 coe41 coelome4 c4ao41 cardiac aorta4 d4ao41 dorsal aorta 7paired74 d4ao'41 dorsal aorta median4 g41 gonads 7male or female genital gland74 hep41 hepatic vein4 in41 intestine4 i4w41 intestine wall4 lv41 liver4 m4f41 median fin4 n4c41 notochord4 p4v41 portal vein4 ph41 pharynx4 -p4s4-1 e4s4 pigment spot 7eye spot74 s4c41 spinal cord4 ,illustration3 ,diagram ,sheet #ai4 ,sheet #bj -,sections of ,amphioxus4 ,the ,roman numerals indicate the corresponding region in ,figure #a1 ,sheet #ai4 ,the lettering is identical2 but note1 in addition2 br4c41 branchial canal4 c4f41 ciliated funnel4 d4c4c41 dorsal coelomic canal4 end41 endostyle4 ep41 epipleur4 e4s41 eye spot4 h4p41 hypopharyngeal grove4 h4vn41 for hepatic vein4 o4c41 oral cavity 7or hood74 ,illustration3 ,diagram ,sheet #bj4 ,sheet #ba4 -,phases in the ,development of ,amphioxus4 ,figures #a1 #b1 #c1 #d4 ,phases in segmentation4 ,figure #e4 ,the blastosphere4 ,figure #f4 ,the gastrula in section1 anterior end to the right4 ,figure #g4 i4 ,dorsal view post gastrula stage4 ,figure #g4 ii4 ,diagrammatic section of the same in the position indicated by the transverse line in #g1 i4 ,figure #h4 ,diagrammatic section of a later stage4 coe4p41 the coelomic pouches4 n4c41 the notochord4 n4p41 the neural plate4 ,figure #i4;i4 ,still later section4 ,figure #i4 ii4 ,diagrammatic view of late embryo4 ,figures #aj1 #aa1 #ab illustrate the formation of the atrium as a median ventral invagination1 at4 ,illustration3 ,diagram ,sheet #ba4 ,sheet #bb4 -,the ,development of the ,frog4 ,these diagrams must be studied with the text4 ,they should be compared with the corresponding ones of ,amphioxus as indicated below4 ,figures #a1 #b1 #c4 ,stages in segmentation 7compare #a1 #b 1#c of ,sheet #ba ,amphioxus74 ,figure #d4 ,blastosphere stage 7compare #e1 ,amphioxus74 ,this1 on a smaller scale4 ,the cells on the ventral side are so much larger because distended with yolk4 ,figure #e4 ,gastrula stage in section 7compare #f1 ,amphioxus74 ,the ,frog on a smaller scale than ,amphioxus4 ,figure #f4 ,dorsal view of gastrula 7compare #g1 ,amphioxus74 ,figure #g4 ,part of a transverse section of developing tadpole1 corresponding to ,figure #h of ,amphioxus4 ,figures #h and #i4 ,diagrammatic longitudinal sections of tadpoles 7compare #i4 ii4 of ,amphioxus74 ,y4 represents a mass of yolk cells4 ,figure #aj4 ,side view of young tadpole1 showing external gills 7e4g47 and suckers 7s474 ,note the ventral bulging due to the yolk4 ,figure #aa4 ,ventral view of a later tadpole4 op41 the operculum4 int41 coiling intestine4 ,figure #ab4 ,head of still later tadpole in horizontal section to show atrial chamber formed by operculum4 int4g41 internal gills4 ,l41 developing lungs4 ,figure #ac4 ,diagrammatic cross-section of the mid-dorsal part of an embryonic vertebrate4 ao41 aorta4 ,b4,c41 ,bowman's capsule4 coe41 coelom4 d4g41 ganglion on dorsal root of spinal nerve4 gl41 -its branch- arteriole to form glomerulus4 g4r41 genital ridge4 ,i41 intestine4 ,m4,d41 ,mullerian duct4 ns4 nst41 nephrostome4 n4c41 notochord2 -n4s4-1 n4sh4 its sheath4 s4c41 neural canal4 ,w4,d41 ,wolffian duct4 ,illustration3 ,diagram ,sheet #bb4 ,sheet #bc4 -,the ,development of the ,fowl4 ,figure #a4 ,diagram of the early ovum4 ,the section below is a small portion of the blastodermic area4 b4d41 blastoderm4 y41 the undivided yolk4 s4c41 the segmentation between the blastoderm and yolk4 ,compare s4c4 in ,sheet #bb1 ,figure #d4 ,figure #b4 ,area pellucida about the sixteenth hour4 ,the figure below is the central part of the section indicated by the transverse line1 and showing the primitive streak 7p4s474 ,figure #c4 ,area pellucida about the twenty-first hour4 ,two sections through a and b below4 ,figure #d4 ,about the twenty-fifth hour2 surface view2 longitudinal section to right and transverse above4 ,figure #d;b4 ,diagrammatic rendering of same stage 7compare ,figure #i of ,frog and #i4;ii4 ,amphioxus74 ,this will be most clearly understood if the reader look at ,sheet #bb1 ,figure #i1 and imagine ,y4 enormously increased1 and the embryo sinking into it4 ,epiblast1 ep41 -line of dashes- black line4 ,mesoblast1 dotted4 ,hypoblast1 -black- line of dashes4 pp41 the pleuro-peritoneal cavity4 ,figure #e and #f illustrate formation of amnion 7a47 and allantois 7all474 #f is about the fourth day4 ,illustration3 ,diagram ,sheet #bc4 ,sheet #bd4 -,the ,development of the ,fowl4 ,figure #a4 ,chick about the -fifth- third day4 ,at this stage the chick lies on its left side in the yolk4 ,for lettering of blood vessels1 see 7#g7 below4 i41 the intestine4 u4v41 the yolk sac4 v4v41 the vitelline veins4 al41 the allantois4 ,figure #b4 ,chick about sixth day4 ,figure #c4 ,development of heart4 ,figure #d4 ,development of the eye4 ,figure #e4 ,chick about the sixteenth day4 ,a4,m4 is the amnion surrounding the embryo4 ,note particularly how the allantois 7al47 has spread over surface of shell and how the yolk sac is shrivelled4 ,figure #f4 ,figures to illustrate the relative function and importance of allantois and yolk sac in bird and mammal4 ,in the fowl1 however1 the blood-vessels of the allantois also probably absorb the albumen of the egg1 and may excrete urea into the egg-space4 ,figure #g4 ,simplified figure of the embryonic circulation1 for comparison with the similar figures annexed to ,dog-,fish and ,rabbit4 ,lines from ,second ,edition only4 ,a4,c41 anterior cardinal4 ,ao41 ,aorta4 ,br#d1 sixth aortis arch 7fourth branchial74 ,c4,s4 ,cuvierian sinus4 ,h41 the heart4 ,i4,c41 inferior cava4 ,p4,c41 posterior cardinal vein4 ,tr4,a41 truncus arteriosus4 v4v41 vitelline vein4 ,figure #h4 ,chick on the nineteenth day4 ,illustration3 ,diagram ,sheet #bd4