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diff --git a/21781.txt b/21781.txt new file mode 100644 index 0000000..a6c30d4 --- /dev/null +++ b/21781.txt @@ -0,0 +1,7587 @@ +The Project Gutenberg eBook, Text Book of Biology, Part 1: Vertebrata, by +H. G. Wells + + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + + + + +Title: Text Book of Biology, Part 1: Vertebrata + + +Author: H. G. Wells + + + +Release Date: June 8, 2007 [eBook #21781] + +Language: English + +Character set encoding: ISO-646-US (US-ASCII) + + +***START OF THE PROJECT GUTENBERG EBOOK TEXT BOOK OF BIOLOGY, PART 1: +VERTEBRATA*** + + +E-text Prepared by "Teary Eyes" Anderson +and Dedicated To Destanie; +With Hopes Her Dream of Becoming A veterinarian Comes True + +Special Thanks to Deborah Furness of the University College London +for her help, and research, in learning about this book, and helping +me understand it better. Spellchecked with www.thesolutioncafe.com + + + +Note: Project Gutenberg also has an HTML version of this + file which includes the original illustrations. + See 21781-h.htm or 21781-h.zip: + (https://www.gutenberg.org/dirs/2/1/7/8/21781/21781-h/21781-h.htm) + or + (https://www.gutenberg.org/dirs/2/1/7/8/21781/21781-h.zip) + + +Transcriber's Note: + + I try to edit my e-texts so they can easily be used with voice + speech programs, I believe blind people and children should also + be able to enjoy the many books now available electronically. I + use the -- for an em-dash, with a space either before or after + it depending on its usage. This helps to keep certain programs + from squishing the words together, such as down-stairs. Also to + help voice speech programs I've enclosed upper case text + between - and _ (-UPPER CASE TEXT_), and used underscores to + show chapter and section headers. I also added a second + contents that shows the other sections of this e-text. This + e-text was made with a "Top can" text scanner, with a bit of + correcting here and there. + + This book is volume one of two. It was later reworked by A. M. + Davies in 1898 under the title "Text-book of Zoology", + then revised and rewritten by J. T. Cunningham about 1909 and + W. H. Leigh-Sharpe around 1932. Although these editions gave + Wells the main credit, most of Wells' writing and all his + drawings were removed; only his rough outline seems to have + been used. It was re-published by University Tutorial Press. + + The First Edition, as well as The Second and Revised Edition + (with dissections redrawn by Miss A. C. Robbins) are used in this + e-text. The First Edition had some small minor errors, as well as + dissection abbreviations that are shown on the Dissection Sheets, + but no mention of them was listed in the text. Certain figures on + the Dissections Sheets are missing (such as Figures 1, 2, 4, with + no mention to a 3, as if Mr. Wells drew a Figure 3 but found it + was not needed and removed it from the book). Rather then leaving + it as is, I put {} marks around my notes saying things like + {No Figure 3}. For the "Second and Revised Edition" Wells was + able to change some of these errors and missing parts, but many of + the same printing tablets were used and with almost each addition + other things were removed, (in one instance one entire section + from a chapter), and many of the helpful suggestions were + shortened or removed so other things could be explained more. In + an ideal version of the book both could have been used, but with + reprinting the entire book from the first to the second editions + almost as many things were lost as were gained, so I've tried to + indicate where both text go separate paths with the following; + [Second Edition only text] and -First Edition only text,- and also + {Lines from Second Edition only.} and {Lines from First Edition + only.} were more then just a sentence is added or removed. Other + things to notice is how some words are spelt or punctuated + differently throughout the book, such as; + Blood Vessels + Blood-Vessels + Bloodvessels + I've tried to keep these as close to the original book as possible. + + + + +University Correspondence College Tutorial Series. + +-Text-Book Of Biology._ + +by + +H. G. Wells, +Bachelor of Science, London., Fellow of the Zoological Society. +Lecturer in Biology at University Tutorial College. + +With An Introduction by G. B. Howes, +Fellow of the Linnean Society, Fellow of the Zoological Society. +Assistant Professor of Zoology, Royal College of Science, London. + + + + + + + +Part 1.-- Vertebrata. + + + +Contents + +Introduction + +Preface + +The Rabbit-- +1. External Form and General Considerations +2. The Alimentary Canal of the Rabbit +3. The Circulation +4. The Amoeba, Cells and Tissue +5. The Skeleton +6. Muscle and Nerve +7. The Nervous System +8. Renal and Reproductive Organs +9. Classificatory Points +10. Questions and Exercises + +The Frog-- +1. General Anatomy +2. The Skull of the Frog (and the Vertebrate Skull generally) +3. Questions on the Frog + +The Dog-Fish-- +1. General Anatomy +2. Questions on the Dog-Fish + +Amphioxus-- +1. Anatomy +2. The Development of Amphioxus +3. 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 2} + +Key for Dissection Sheets, and Abbreviations + + + + + + +-Introduction_ + +In the year 1884 I was invited to give tuition by correspondence, in +Biology. Although disposed at the time to ridicule the idea of +imparting instruction in natural science by letter, I gladly accepted +the opportunity thus afforded me of ascertaining for myself what +could and could not be accomplished in that direction. Anyone +familiar with the scope of biological enquiry, and the methods of +biological instruction, will not need to be reminded that it is only by +the most rigorous employment of precise directions for observation, +that any good results are to be looked for at the hand of the +elementary student. True to this principle, I determined to issue to +my correspondence pupils rigid instructions, and to demand in return +faithful annotated drawings of facts observed in their usage. In the +case of two among the few students who passed through my hands, +the result far exceeded my most sanguine anticipations. The notes +sent in by one of them-- a man working at a distance, alone and +unaided-- far excelled those wrung from many a student placed +under the most favourable surroundings; and their promise for the +future has been fulfilled to the utmost, the individual in question +being now a recognised investigator. It thus became clear that, +not-with-standing the complex conditions of work in the biological +field, tuition by correspondence would suffice to awaken the latent +abilities of a naturally qualified enquirer. The average members of a +University Correspondence Class will be found neither better nor +worse than those of any other, and they may therefore pass +unnoticed; if however, the correspondence system of tuition may +furnish the means of arousing a latent aptitude, when the +possibilities of other methods of approach are excluded-- and in +so doing, of elevating the individual to that position for which he was +by nature qualified, ensuring him the introduction to the one sphere +of labour for which he was born-- it will have created its own defence, +and have merited the confidence of all right-thinking people. The +plucking of one such brand from the burning is ample compensation +for the energy expended on any number of average dullards, who but +require to be left alone to find their natural level. + +Mr. Wells' little book is avowedly written for examination purposes, +and in conformity with the requirements of the now familiar "type +system" of teaching. Recent attempts have been made to depreciate +this. While affording a discipline in detailed observation and +manipulation second to that of no other branch of learning, it +provides for that "deduction" and "verification" by which all science +has been built up; and this appears to me ample justification for its +retention, as the most rational system which can be to-day adopted. +Evidence that its alleged shortcomings are due rather to defective +handling than to any inherent weakness of its own, would not be +difficult to produce. Although rigid in its discipline, it admits of +commentatorial treatment which, while heightening the interest of +the student, is calculated to stimulate alike his ambition and his +imagination. That the sister sciences of Botany and Zoology fall +under one discipline, is expressed in the English usage of the term +"Biology." 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 both; and, +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 domain, is to-day undeniable. These +and other allied considerations, 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 which, like this one, are specially written for the biological +neophyte. Although the student is the wiser for the actual +observation of the fact of nature, he becomes the better only when +able to apply them, as for example, by the judicious construction of +elementary generalizations, such as are introduced into the pages of +this work. So long as these generalizations, regarded as first +attempts to deduce "laws" in the form of "generalized statement of +facts based observation," are properly introduced into an elementary +text-book, intended for the isolated worker cut off from the lecture +room, their intercalation is both healthy and desirable. + +Mr. Wells has kept these precepts constantly in mind in the +preparation of his work, and in the formulation of his plans for its +future extension, thereby enhancing the value of the book itself, and +at the same time, discouraging the system of pure cram, which is +alien to the discipline of biological science. + +G. B. Howes +Royal College of Science, +South Kensington; +November 30, 1892. + + + +-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 student. In the +ideal world there is a plentiful supply of such teachers, and easy +access to their teaching, but in this real world only a favoured few +enjoy these advantages. Through causes that cannot be discussed +here, a vast number of solitary workers are scattered through the +country, to whom sustained help in this form is impossible, or +possible only in days stolen from a needed vacation; and to such +students especially does this book appeal, as well as to those more +fortunate learners who are within reach of orderly instruction, but +anxious to save their teachers' patience and their own time by some +preliminary work. + +One of the most manifest disadvantages of book-work, under the +conditions of the solitary worker, is the rigidity of its expressions; if +the exact meaning is doubtful, he can not ask a question. This has +been kept in view throughout; the writer has, above all, sought to be +explicit-- has, saving over-sights, used no uncommon or technical +term without a definition or a clear indication of its meaning. + +In this study of Biology, the perception and memory of form is a very +important factor indeed. Every student should draw sketches of his +dissections, and accustom himself to copying book diagrams, in +order to train his eye to perception of details he might otherwise +disregard. The drawing required is within the reach of all; but for +those who are very inexperienced, tracing figures is a useful +preliminary exercise. + +By the time the student has read the "Circulation of the Rabbit" +(Sections 34 to 49), he will be ready to begin dissection. It is +possible to hunt to death even such a sound educational maxim as +the "thing before the name," and we are persuaded, by a +considerable experience, that dissection before some such +preparatory reading is altogether a mistake. At the end of the book +is a syllabus (with suggestions) for practical work, 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 examination, and at an enormous +disadvantage, as regards the number of hours available, in +comparison with the leisurely students of a University laboratory. +This syllabus may, perhaps by itself, serve a useful purpose in some +cases, but in this essential part of the study the presence of some +experienced overlooker to advise, warn, and correct, is at first almost +indispensable. + +A few words may, perhaps be said with respect to the design of this +volume. It is manifestly modelled upon the syllabus of the +Intermediate Examination in Science of London University. That +syllabus, as at present constituted, appears to me to afford +considerable scope for fairly efficient biological study. The four types +dealt with in this book are extremely convenient for developing the +methods of comparative anatomy and morphological embryology. +Without any extensive reference to related organisms, these four +forms, and especially the three vertebrata, may be made to explain +and illustrate one another in a way that cannot fail to be educational +in the truest sense. After dealing with the rabbit, therefore, as an +organic mechanism, our sections upon the frog and dog-fish, and +upon development, are simply statements of differences, and a +commentary, as it were, upon the anatomy of the mammalian type. +In the concluding chapter, a few suggestions of the most elementary +ideas of it is hoped to make this first part of our biological course +complete in itself, and of some real and permanent value to the +student. And the writer is convinced that not only is a constant +insistence upon resemblances and differences, and their import, +intellectually the most valuable, but also the most interesting, and +therefore the easiest, way of studying animal anatomy. That chaotic +and breathless cramming of terms misunderstood, tabulated +statements, formulated "tips," and lists of names, in which so many +students, in spite of advice, waste their youth is, I sincerely hope, as +impossible with this book as it is useless for the purposes of a +London candidate. On the other hand, our chief endeavour has been +to render the matter of the book clear, connected, progressive, and +easily assimilable. In the second part Plants, Unicellular Organisms, +and Invertebrata will be dealt with, in a wider and less detailed view +of the entire biological province. + + {Lines from First Edition only.} + -In this volume, we study four organisms, and chiefly in their + relation to each other; in the next, we shall study a number of + organisms largely in relation to their environment. In this part + our key note is the evidence of inheritance; in our second part + it will be of adaptation to circumstances.- + +This book will speedily, under the scrutiny of the critical reader, +reveal abundant weakness. For these the author claims the full +credit. For whatever merit it may posses, he must however, +acknowledge his profound indebtedness to his former teacher, +Professor Howes. Not only has the writer enjoyed in the past the +privilege of Professor Howes' instruction and example, but he has, +during the preparation of this work, received the readiest help, +advise, and encouragement from him-- assistance as generous as it +was unmerited, and as unaffected as it was valuable. + + {Lines from Second Edition only.} + [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 reviewers, and the help they have him in improving + this maiden effort. To no one is there vouchsafed such a facility in + the discovery of errors in a book as to its author, so soon as it has + passed beyond his power of correction. Hence the general tone of + encouragement (and in some cases the decided approval) of the + members of this termination to a period of considerable remorse and + apprehension.] + +I have been able through their counsel, and the experience I have had +while using this book in teaching, to correct several printer's errors +and to alter various ambiguous or misleading expressions, as well as +to bring the book up to date again in one or two particulars. + +My thanks are particularly due to my friend Miss Robbins, who has +very kindly redrawn the occasionally rather blottesque figures of the +first edition. Not only have these plates gained immensely in grace +and accuracy, 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 appreciate. + +H. G. Wells +November, 1892. {First Edition.} +December, 1893. {Second Edition.} + + + +-The Rabbit._ + +1. _External Form and General Considerations._ + +Section 1. It is unnecessary to enter upon a description of the +appearance of this familiar type, but it is not perhaps superfluous, as +we proceed to consider its anatomy, to call attention to one or two +points in its external, or externally apparent structure. Most of our +readers know that it belongs to that one of two primary animal +divisions which is called the vertebrata, and that the distinctive +feature which place it in this division is the possession of a spinal +column or backbone, really a series of small ring-like bones, the +vertebrae (Figure 1 v.b.) strung together, as it were, on the main +nerve axis, the spinal cord (Figure 1 s.c.). This spinal column can be +felt along the neck and back to the tail. This tail is small, tilted up, +and conspicuously white beneath, and it serves as a "recognition +mark" to guide the young when, during feeding, an alarm is given +and a bolt is made for the burrows. In those more primitive (older and +simpler-fashioned) vertebrata, the fishes, the tail is much large and +far more important, as compared with the rest of the body, than it is +in most of the air-inhabiting vertebrates. In the former it is invariably a +great muscular mass to propel the body forward; in the latter it may +disappear, as in the frog, be simply a feather-bearing stump, as in +the pigeon, a fly flicker, as in the cow or horse, a fur cape in squirrel, +or be otherwise reduced and modified to meet special requirements. + + +Section 2. At the fore end, or as English zoologists prefer to say, +anterior end, of the vertebral column of the rabbit, is of course the +skull, containing the anterior portion of the nerve axis, the brain +(Figure 1 br.). Between the head and what is called "the body," in +the more restricted sense of the word, is the neck. The neck gives +freedom of movement to the head, enables the animal to look this +way and that, to turn its ears about to determine the direction of a +sound, and to perform endless motions in connexion with biting and +so forth easily. We may note that in types which swim through the +water, the neck dose not appear-- in the fish and frog, for instance-- +and the head simply widens out as one passes back to the body. +The high resistance offered by water necessitates this tendency to a +cigar or ship outline, just as it has determined the cigar shape of the +ordinary fish torpedo. + + +Section 3. In the body of the rabbit, as examined from the outside, +we can make out by feeling two distinct regions, just as we might in +the body of a man; anteriorily a bony cage, having the ribs at the +sides, a rod-like bone in the front, the sternum (Figure 1 -st.-, [stm.]), +and the backbone behind, and called the chest or thorax; and +posteriorily a part called the abdomen, which has no bony protection +over its belly, or ventral surface. These parts together with the neck +constitute the trunk. As a consequence of these things, in the +backbone of the rabbit there are four regions: the neck, or cervical +part, consisting of seven vertebrae, the thoracic part of twelve joined to +ribs, the abdominal (also called the lumbar) region of seven without +ribs, and the tail or caudal of about fifteen. Between the lumbar and +caudal come four vertebrae, the sacral, which tend to run together into +a bony mass as the animal grows old, and which form a firm +attachment for the base of the hind limb. + + +Section 4. The thorax and abdomen are separated by a partition, the +diaphragm (Figure 1 dia.). This structure is distinctive of that class of +the vertebrata called mammals, and which includes man, most of +the larger and commoner land animals, and whales and manatee. +We shall find later that it is essentially connected with the perfection +of the air breathing to which this group has attained. Another +characteristic shared by all mammals, and by no other creature, is +the presence of hair. In birds we have an equally characteristic cover +in the feathers, the frog is naked, and the fishes we find either naked +skins or scales. + + +Section 5. The short strong fore limbs are adapted to the burrowing +habit, and have five digits; the hind limbs are very much longer and +muscular, enable the animal to progress rapidly by short leaps, and +they have four toes. 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 54 (5 in front, 4 behind) with the rabbit, and +observe that with the frog the reverse is the case. + + +Section 6. We may note here the meaning of certain terms we shall +be constantly employing. The head end of the rabbit is anterior, the +tail end posterior, the backbone side of the body-- the upper side in +life-- is dorsal, the breast and belly side, the lower side of the +animal, is ventral. If we imagine the rabbit sawn asunder, as it were, +by a plane passing through the head and tail, that would be the +median plane, and parts on either side of it are lateral, and left or +right according as they lie to the animal's left or right. In a limb, or in +the internal organs, the part nearest the central organ, or axis, is +proximal, the more remote or terminal parts are distal. For instance, +the mouth is anteriorly placed, the tongue on its ventral wall; the +tongue is median, the eyes are lateral, and the fingers are distal to +the elbow. The student must accustom himself to these words, and +avoid, in his descriptions, the use of such terms as "above," +"below," "outside," which vary with the position in which we conceive +the animal placed. + + +Section 7. So much for the general form; we may note a few facts of +general knowledge, in connection with the rabbit's life-activity. In a +day of the rabbit's life a considerable amount of work is done-- the +animal runs hither and thither, for instance; in other words, a certain +mass of matter is moved through space, and for that we know force +must be exerted. Whence comes the force? + + +Section 8. We find the rabbit occupies a considerable amount of its +time in taking in vegetable matter, consisting chiefly of more or less +complex combustible and unstable organic compounds. It is a pure +vegetarian, and a remarkably moderate drinker. Some but only a +small proportion, of the vegetable matter it eats, leaves its body +comparatively unchanged, in little pellets, the faeces, in the process +of defaecation. For the rest we have to account. + + +Section 9. We find, also, that the rabbit breathes air into its lungs, +which is returned to the atmosphere with a lessened amount of +oxygen, and the addition of a perceptible amount of carbon dioxide. +The rabbit also throws off, or excretes, a fluid, the urine, which +consists of water with a certain partially oxydised substance +containing nitrogen, and called urea, and other less important salts. +The organs within the body, by which the urine is separated, are +called the kidneys. + + +Section 10. Repeating these facts in other words, the rabbit takes +into its body complex and unstable organic compounds containing +nitrogen, carbon, hydrogen, a certain amount of oxygen, a small +quantity of sulphur, and still smaller amounts of other elements. It +also breathes in oxygen. + + +Section 11. It returns a certain rejected part of its food comparatively +unchanged. Besides this, it returns carbon dioxide and water, which +are completely oxydised, and very simple and stable bodies, and +urea-- a less completely oxydised compound, but a very simple one +compared with the food constituents. + + +Section 12. Now the chemist tells us that when a stable body is +formed, or when an unstable compound decomposes into simpler +stable ones, force is evolved. The oxydation of carbon, for instance, +in the fireplace, is the formation of the stable compound called +carbon dioxide, and light and heat are evolved. The explosion of +dynamite, again is the decomposition of an unstable compound. +Hence, we begin to perceive that force-- the vital force-- which keeps +the rabbit moving, is supplied by the decomposition and partial +oxydation of compounds continued in its food, to carbon dioxide, +water, urea, and smaller quantities of other substances. + + +Section 13. This is the roughest statement of the case possible, but it +will give the general idea underlying our next chapters. We shall +consider how the food enters the body and is taken up into the +system, how it is conveyed to the muscles in the limbs, to the nerve +centres, and to wherever work is done, to be there decomposed and +partially oxydised, and finally how the products of its activity-- the +katastases, of which the three principal are carbon dioxide, water, +and urea-- are removed from the body. + + +Section 14. There are one or two comparatively modern terms that +we may note here. This decomposition of unstable chemical +compounds, releasing energy, is called kataboly. A reverse process, +which has a less conspicuous part in our first view of the animal's life +action, by which unstable compounds are built up and energy +stored, is called anaboly. The katastases are the products of +kataboly. + + +Section 15. In an ordinary animal, locomotion and other activity +predominate over nutritive processes, which fact we may express, in +the terms just given, by saying that kataboly prevails over anaboly. +An animal, as we have just explained, is an apparatus for the +decomposition and partial oxydation of certain compounds, and +these are obtained either directly or indirectly-- through other animals, +in the case of meat-eaters-- from the vegetable kingdom. +As the student will learn early in his botanical reading, the typical +plant has, in its green colouring matter, chlorophyll, a trap to catch +the radiating energy of the sun, and to accomplish, by the +absorption of that energy, the synthesis (building up) of those +organic compounds which the animal destroys. The typical plant is, +on whole, passive and synthetic, or anabolic; the typical animal, +active and katabolic; and the excess of kataboly over anaboly in the +animal is compensated for by the anabolic work stored up, as it +were, by the plant, which is, directly or indirectly, the animal's food. + + + +2. _The Alimentary Canal of the Rabbit_ + +Section 16. Figure 1 represents the general anatomy of the rabbit, +but is especially intended to show the alimentary (= food) canal, +shortened to a certain extent, and with the proportions altered, in +order to avoid any confusing complications. It is evidently simply a +coiled tube-- coiled for the sake of packing-- with occasional +dilatations, and with one side-shunt, the caecum (cae.), into which +the food enters, and is returned to the main line, after probably +absorbent action, imperfectly understood at present. A spiral fold in +this cul-de-sac {bottom-of-sack}, which is marked externally by +constrictions, has a directive influence on the circulation of its +contents. The student should sketch Figure 1 once or twice, and +make himself familiar with the order and names of the parts before +proceeding. We have, in succession, the mouth (M.), separated from +the nasal passage (Na.) above the palate; the pharynx (ph.), where +the right and left nasal passages open by the posterior nares into +the mouth; the oesophagus (oes.); the bag-like stomach, its left +(Section 6) end being called the cardiac (cd.st.), and its right the +pyloric end (py.); the U-shaped duodenum (ddnm.) and the very long +and greatly coiled ileum (il.). The duodenum and ileum together form +the small intestine; and the ileum is dilated at its distal end into a +thick-walled sacculus rotundus (s.r.), beyond which point comes the +large intestine. The colon (co.) and rectum (r.) continue the main line +of the alimentary canal; but, at the beginning of the large intestine, +there is also inserted a great side-shunt, the caecum (cae.), ending +blindly in a fleshy vermiform appendix (v.ap.). The figure will indicate +how the parts are related better than any verbal description can. +Between the coiling alimentary tube and the body walls is a space, +into which the student cuts when he begins dissecting; this is the +peritoneal cavity (pt.). A thin, transparent membrane, the mesentery, +holds the intestines in place, and binds them to the dorsal wall of +this peritoneal space. + + +Section 17. The food stuffs of an animal, the unstable compounds +destined ultimately to be worked into its life, and to leave it again in +the form of katastases (Section 13), fall into two main divisions. The +first of these includes the non-nitrogenous food stuffs, containing +either carbon together with hydrogen and oxygen in the proportion of +H2O (the carbo-hydrates), or carbon and hydrogen without oxygen +(the hydrocarbons). The second division consists of the nitrogenous +materials, containing also carbon, hydrogen, a certain amount +of oxygen, sulphur, and possibly other elements. Among the +carbohydrates, the commonest are starch and cellulose, which are +insoluble bodies, and sugar, which is soluble. The hydrocarbons, +fats, oils, and so on, form a comparatively small proportion of the +rabbit's diet; the proverb of "oil and water" will remind the student +that these are insoluble. The nitrogenous bodies have their type in +the albumen of an egg; and muscle substance and the less modified +living "protoplasm" of plants, a considerable proportion of the +substance of seeds, bulbs, and so on, are albuminous bodies, or +proteids. These also are insoluble bodies, or when soluble, will not +diffuse easily through animal membranes. + + +Section 18. Now the essential problem which the digestive canal of +the rabbit solves is to get these insoluble, or quasi-insoluble, bodies +into its blood and system. They have to pass somehow into the +circulation through the walls of the alimentary canal. In order that a +compound should diffuse through a membrane, it must be both +soluble and diffusible, and therefore an essential preliminary to the +absorption of nutritive matter is its conversion into a diffusible soluble +form. This is effected by certain fluids, formed either by the walls of +the alimentary canal or by certain organs called glands, which open +by ducts into it; all these fluids contain small quantities of organic +compounds of the class called ferments, and these are the active +agents in the change. The soluble form of the carbohydrates is +sugar; proteids can be changed into the, of course, chemically +equivalent but soluble and diffusible the peptones; and fats and oils +undergo a more complicated, but finally similar change. + + +Section 19. We shall discuss the structure and action of -a gland- +[glands] a little more fully in a subsequent chapter. Here we will +simply say that they are organs forming each its characteristic fluid +or secretion, and sending it by a conduit, the duct, to the point +where its presence is required. The saliva in our mouths, tears, and +perspiration, are examples of the secretions of glands. + + +Section 20. In the month of the rabbit the food is acted upon by the +teeth and saliva. The saliva contains ptyalin, a ferment converting +starch into sugar, and it also serves to moisten the food as it is +ground up by the cheek teeth. It does not act on fat to any +appreciable extent. The teeth of the rabbit are shown in Figure XVIII., +Sheet 4. The incisor teeth in front, two pairs above and one pair below +(i.), are simply employed in grasping the food; the cheek teeth-- the +premolars (pm.) and molars (m.) behind-- triturate the food by a +complicated motion over each. Their crowns are flat for this purpose, +with harder ridges running across them. + + +Section 21. This grinding up of the food in the mouth invariably +occurs in herbivorous animals, where there is a considerable amount +of starch and comparatively little hydrocarbon in the food. By finely +dividing the food, it ensures its intimate contact with the digestive +ferment, ptyalin. In such meat-eaters as the cat and dog, where little +starchy matter and much fat is taken, the saliva is, of course, of less +importance, and this mastication does not occur. The cheek teeth of +a dog ({Section 91}), and more so of a cat, are sharp, and used for +gnawing off fragments of food, which are swallowed at once. +Between the incisors and premolars of a dog come the +characteristic biting teeth, or canines, absent in the rabbit. + + +Section 22. The student will probably ask why the cheek teeth, +which are all similar in appearance, are divided into premolars and +molars. The rabbit has a set of milk molars-- a milk dentition-- which +are followed by the permanent teeth, just as in man. Those cheek +teeth of the second set, which have predecessors in the first series, +are called premolars; the ones posterior to these are the molars. + + +Section 23. After mastication, the food is worked by the tongue and +cheeks into a saliva-soaked "bolus" and swallowed. The passage +down the oesophagus is called deglutition. In the stomach it comes +under the influence of the gastric juice, formed in little glandular pits +in the stomach wall-- the gastric (Figure VIII. Sheet 3) and pyloric +glands. This fluid is distinctly acid, its acidity being due to about +one-tenth per cent {of a hundred} of hydrochloric acid, and it +therefore stops any further action of the ptyalin, which can act only +on neutral or slightly alkaline fluids. The gastric juice does not act on +carbo-hydrates or hydrocarbons to any very noticeable degree. Its +essential property is the conversion of proteids into peptones, and the +ferment by which this is effected is called pepsin. Milk contains +a peculiar soluble proteid, called casein, which is precipitated by a +special ferment, the rennet-ferment, and the insoluble proteid, the +curd, thus obtained is then acted on by the pepsin. In the +manufacture of cheese, the rennetferment obtained, from the +stomach of a calf is used to curdle the milk. + + +Section 24. After the food has undergone digestion in the stomach it +passes into the duodenum, the U-shaped loop of intestine +immediately succeeding the stomach. The duodenum is separated +from the stomach by a ring-like muscular valve, the pylorus; this +valve belongs to the class of muscles called sphincters, which, under +ordinary circumstances, are closed, but which relax to open the +circular central aperture. The valve at the anus, which retains the +faeces, is another instance of a sphincter. + + +Section 25. The food at this stage is called chyme; it is an acid and +soup-like fluid-- acid through the influence of the gastric juice. The +temperature of the animal's body is sufficiently high to keep most of +the fat in the food melted and floating in oily drops; much of the +starch, has been changed to sugar, and the solid proteids to soluble +peptones, but many fragments of material still float unchanged. + + +Section 26. It meets now with the bile, a greenish fluid secreted by +that large and conspicuous gland the liver. The bile is not simply a +digestive secretion, like the saliva or the gastric juice; it contains +matters destined to mix in, and after a certain amount of change to +be passed out of the body with, the faeces; among these +substances, of which some portion is doubtless excretory, are +compounds containing sulphur-- the bile salts. There is also a +colouring matter, bili verdin, which may possibly also be excretory. If +the student will compare Sections 10 and 11, he will notice that in +those paragraphs no account is taken of the sulphur among the +katastases, the account does not balance, and he will at once see +that here probably is the missing item on the outgoing side. The bile, +through the presence of these salts, is strongly alkaline, and so +stops the action of the gastric juice, and prepares for that of the +pancreas, which can act only in an alkaline medium. The fermentive +action of the bile is trifling; it dissolves fats, to a certain extent, and +is antiseptic, that is, it prevents putrefaction to which the chyme +might be liable; it also seems to act as a natural purgative. + + +Section 27. The bile, as we shall see later, is by no means the sole +product of the liver. + + +Section 28. The pancreatic juice, the secretion of the pancreas is +remarkable as acting on all the food stuffs that have not already +become soluble. It emulsifies fats, that is, it breaks, the drops up +into extremely small globules, forming a milky fluid, and it +furthermore has a fermentive action upon them; it splits them up into +fatty acids, and the soluble body glycerine. The fatty acids combine +with alkaline substances (Section 26) to form bodies which belong to +the chemical group of Soaps, and which are soluble also. The +pancreatic juice also attacks any proteids that have escaped the +gastric juice, and converts them into peptones, and any residual +starch into sugar. Hence by this stage, in the duodenum, all the food +constituents noticed in Section 17 are changed into soluble forms. +There are probably, three distinct ferments in the pancreatic juice +acting respectively on starch, fat, and proteid, but they have not +been isolated, and the term pancreatin is sometimes used to +suggest the three together. + + +Section 29. A succus entericus, a saliva-like fluid secreted by +numerous small glands in the intestine wall (Brunner's glands, +Lieberkuhnian follicles), probably aids, to an unknown but +comparatively small extent, in the digestive processes. + + +Section 30. The walls of the whole of the small intestine are engaged +in the absorption of the soluble results of digestion. In the +duodenum, especially, small processes, the villi project into the +cavity, and being, like the small hairs of velvet pile, and as thickly +set, give its inner coat a velvety appearance. In a villus we find +(Figure IX., Sheet 3) a series of small blood-vessels and with it +another vessel called a lacteal. The lacteals run together into larger +and larger branches until they form a main trunk, the thoracic duct, +which opens into the blood circulation at a point near the heart; but +of this we shall speak further later. They contain, after a meal, a fluid +called chyle. + + +Section 31. Emulsified fats pass into the chyle. Water and diffusible +salts certainly pass into the vein. The course taken by the peptones +is uncertain, but Professor Foster favours the chyle in the case of +the rabbit-- the student should read his Text-book of Physiology, +Part 2, Chapter 1, Section 11, if interested in the further discussion +of this question. + + +Section 32. The processes that occur in the remaining portions of +the alimentary canal are imperfectly understood. The caecum is so +large in the rabbit that it must almost certainly be of considerable +importance. In carnivorous animals it may be so much reduced as to +be practically absent. An important factor in the diet of the +herbivorous animals, and one absent from the food of the carnivora, +is that carbohydrate, the building material of all green-meat- [food], +cellulose, and there is some ground for thinking that the caecum is +probably a region of special fermentive action upon it. The pancreatic +juice, it may be noted, exercises a slight digestive activity upon this +substance. + + +Section 33. Water is most largely absorbed in the large intestine, +and in it the rejected (mainly insoluble) portion of the food gradually +acquires its dark colour and other faecal characteristics. + + + +3. _The Circulation_ + +Section 34. 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 body. This is one of the functions of the +series of structures-- heart and blood-vessels, called the circulation, +circulatory system, or vascular system. It is not the only function. +The blood also carries the oxygen from the lungs to the various parts +where work is done and kataboly occurs, and it carries away the +katastases to the points where they are excreted-- the carbon +dioxide and some water to the lungs, water and urea to the kidneys, +sulphur compounds of some kind to the liver. + + +Section 35. The blood (Figure 4, Sheet 2) is not homogeneous; +under the low power of the microscope it may be seen to consist of-- + + (1.) a clear fluid, the plasma, in which float-- + + (2.) a few transparent colourless bodies of indefinite and changing + shape, and having a central brighter portion, the nucleus with a still + brighter dot therein the nucleolus-- the white corpuscles (w.c.), and + + (3.) flat round discs, without a nucleus, the red corpuscles (r.c.), + greatly more numerous than the white. + + +Section 36. The chyle of the lacteals passes, as we have said, by +the thoracic duct directly into the circulation. It enters the left vena +cava superior (l.v.c.s.) near where this joins the jugular vein (ex.j.) +(see Figure 1, Sheet 2, th.d.) and goes on at once with the rest of +the blood to the heart. The small veins of the villi, however, which also +help suck up the soluble nutritive material, are not directly +continuous with the other body veins, the systemic veins; they +belong to a special system, and, running together into larger and +larger branches, form the lieno gastric (l.g.v.) and mesenteric (m.v.) +veins, which unite to form the portal vein (p.v.) which enters the liver +(l.v.) and there breaks up again into smaller and smaller branches. +The very finest ramifications of this spreading network are called the +(liver) capillaries, and these again unite to form at last the hepatic +vein (h.v.) which enters the vena cava inferior (v.c.i.), a median +vessel, running directly to the heart. This capillary network in the +liver is probably connected with changes requisite before the +recently absorbed materials can enter the general blood current. + + +Section 37. The student has probably already heard the terms vein +and artery employed. In the rabbit a vein is a vessel bringing blood +towards the heart, while an artery is a vessel conducting it away. +Veins are thin-walled, and therefore flabby, a conspicuous purple +when full of blood, and when empty through bleeding and collapsed +sometimes difficult to make out in dissection. They are formed by +the union of lesser factors. The portal breaks up into lesser branches +within the liver. Arteries have thick muscular and elastic walls, thick +enough to prevent the blood showing through, and are therefore pale +pink or white and keep their round shape. + + +Section 38. The heart of the rabbit is divided by partitions into four +chambers: two upper thin-walled ones, the auricles (au.), and two +lower ones, both, and especially the left, with very muscular walls, +the ventricles (vn.). The right ventricle (r.vn.) and auricle (r.au.) +communicate, and the left ventricle (l.vn.) and auricle (l.au.). + + +Section 39. The blood coming from all parts of the body, partly +robbed of its oxygen and containing much carbon dioxide and other +katastases, enters the right auricle of the heart through three great +veins, the median vena cava inferior from the posterior parts of the +body, and the paired venae cavae superiores from the anterior. With +the beating of the heart, described below, it is forced into the right +ventricle and from there through the pulmonary artery (p.a.) seen in +the figure passing under the loop of the aorta (ao.) to the lungs. + + +Section 40. The lungs (lg. Figure 1, Sheet 1) are moulded to the +shape of the thoracic cavity and heart; they communicate with the +pharynx by the trachea (tr. in Figure 1, Sheet 1) or windpipe, and +are made up of a tissue of continually branching and diminishing +air-tubes, which end at last in small air-sacs, the alveoli. The +final branches of the pulmonary arteries, the lung capillaries, lie +in the walls of these air-sacs, and are separated from the air by an +extremely thin membrane through which the oxygen diffuses into, +and the carbon dioxide escapes from, the blood. + + +Section 41. The mechanism of respiration will be understood by +reference to Figure 3, Sheet 2. It will be noted, in dissecting that the +lungs have shrunk away from the walls of the thorax; this +collapse occurs directly an aperture is made in the thorax wall, and +is in part due to their extreme elasticity. In life the cavity of the +thorax forms an air-tight box, between which and the lungs is a +slight space, the pleural cavity (pl.c.) lined by a moist membrane, +which is also reflected, over the lungs. The thorax wall is muscular +and bony, and resists the atmospheric pressure on its outer side, 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 them. In +inspiration (or breathing-in) the ribs are raised by the external +intercostal (Anglice, between-ribs, e.i.c.m.) and other allied +muscles, and the diaphragm (dia.) contracts and becomes flatter; +the air is consequently sucked, in as the lungs follow the movement +of the thorax wall. In expiration the intercostals and diaphragm relax +and allow the elastic recoil of the lungs to come into play. The +thoracic wall is simultaneously depressed by the muscles of the +abdominal area, the diaphragm thrust forwards, as the result of the +displacement and compression of the alimentary viscera thus +brought about. (r.r.r. in the Figure mark ribs.) + + +Section 42. The oxygen and carbon dioxide are not carried in +exactly the same way by the blood. The student will know from his +chemical reading that neither of these gases is very soluble, but +carbon dioxide is sufficiently so in an alkaline fluid to be conveyed +by the liquid plasma. The oxygen however, needs a special portative +mechanism in the colouring matter of the red corpuscles, the +haemoglobin, with which it combines weakly to form +oxy-haemoglobin of a bright red colour, and decomposing easily in +the capillaries (the finest vessels between the arteries and veins), to +release the oxygen again. The same compound occurs in all true +vertebrata, and in the blood-fluid of the worm; in the crayfish a similar +substance, haemocyanin, which when oxygenated is blue, and when +deoxydized colourless, discharges the same function. + + +Section 43. The blood returns from the lungs to the left auricle (l.au.) +by the pulmonary veins, hidden in the Figure by the heart, passes +thence to the thick-walled left ventricle (l.vn.), and on into the aorta +(ao.). + + +Section 44. The beating of the heart is, of course, a succession of +contractions and expansions of its muscular wall. The contraction, +or systole, commences at the base of the venae cavae and passes +to the auricles, driving the blood before it into the ventricles, which +then contract sharply and drive it on into the aorta or pulmonary +artery; a pause and then a dilatation, the diastole follows. The flow of +the blood is determined in one direction by the various valves of the +heart. No valves occur in the opening of the superior cavae but an +imperfect one, the Eustachian valve, protects the inferior cava; the +direction of the heart's contraction prevents any excessive back-flow +into the veins, and the onward, tendency is encouraged by the suck +of the diastole of the ventricles. Between the left ventricle and auricle +is a valve made up of two flaps of skin, the mitral valve, the edges of +the flaps being connected with the walls of the ventricle through the +intermediation of small muscular threads, the chordae tendinae, +which stretch across its cavity to little muscular pillars, the papillary +muscles; these attachments prevent the mitral valve from flapping +back into the auricle, and as the blood flows into and accumulates in +the ventricle it gets behind the flaps of the valve and presses its +edges together. When the systole of the ventricle occurs, the +increased, tension of the blood only closes the aperture the tighter, +and the current passes on into the aorta, where we find three +watch-pocket valves, with the pocket turned away from the heart, +which are also closed and tightened by any attempt at regurgitation +(back-flow). A similar process occurs on the right side of the heart, +but here, instead of a mitral valve of two flaps between auricle and +ventricle, we have a tricuspid valve with three. The thickness of the +muscular walls, in view of the lesser distance through which it has to +force the blood, -are- [is] less for the right ventricle than the left. + + +Section 45. The following are the chief branches of the aorta. The +student should be able to follow them with certainty in dissection; +they are all displayed in the Figure; but it must not be imagined for a +moment that familiarity with this diagram will obviate the necessity +for the practical work; (in.) is the innominate artery; it forks into +(s.cl.a.) the right subclavian, and (r.c.c.) the right common carotid. +Each carotid splits at the angle of the jaw into an internal and an +external branch. The left common carotid, (l.c.c.) arises from the +base of the innominate,* (l.s.cl.a.) the left subclavian, directly from +the aorta. The aorta now curves round to the dorsal middle line, and +runs down as seen in Figure 1, Sheet 1 (d.ao.) and Figure 1, Sheet +2 (d.ao.). Small branches are given off to the ribs, and then comes +the median coeliac (coe.a.) to the stomach and spleen, the median +superior mesenteric (s.mes.a.) to the main portion of the intestine, +and the inferior mesenteric (p.m.a.) to the rectum. Note that no veins +to the inferior vena cava correspond to these arteries-- the blood they +supply going back by the portal vein (p.v.). The paired renal arteries +(r.a.) supply the kidneys, and the common iliacs (c.il.a.) the hind +legs, splitting into the internal iliacs (i.il.a.) and the femoral (f.). + + {Lines from Second Edition only.} + [The student should note that the only arteries in the middle line + are those supplying the alimentary canal.] + + {Lines from First Edition only.} + * -The figure is inaccurate, and represents the left common carotid + as arising from the aortic arch.- + + +Section 46. The distribution of the veins of the rabbit has only a +superficial parallelism with arteries. The chief factors of vena cava +inferior are the hepatic vein (h.v.), which receives the liver blood, +the renal veins (r.v.), from the kidneys, the ilaeo-lumbar, from the +abdominal wall, and the external (e.il.v.) and internal ilias (i.il.v.); +with the exception of the renal veins none of these run side by side +with arteries. The superior cavae (r. and l.v.c.s.) are formed by the +union of internal (i.j.) and external jugular (e.j.) veins with a +subclavian (s.cl.v.) from the fore limb. The term pre-caval vein is +sometimes used for superior cava. The attention, of the student is called +to the small azygos vein (az.) running into the right vena cava superior, +and forming the only asymmetrical (not-balancing) feature of the veins +in front of the heart; it brings blood back from the ribs of the thorax +wall, and is of interest mainly because it answers to an enormous +main vessel, the right post-cardinal sinus, in fishes. There are +spermatic arteries and veins (s.v. and a.) to the genital organs. All +these vessels should be patiently dissected out by the student, and +drawn. + + +Section 47. Between the final branches of the arteries and the first +fine factors of the veins, and joining them, come the systemic +capillaries. These smallest and ultimate ramifications of the +circulation penetrate every living part of the animal, so that if we +could isolate the vascular system we should have the complete form +of the rabbit in a closely-meshed network. It is in the capillaries that +the exchange of gases occurs and that nutritive material passes out +to the tissues and katastases in from them; they are the essential +factor in the circulatory system of the mammal-- veins, arteries, and +heart simply exist to remove and replace their contents. The details of +the branching of the pulmonary artery and the pulmonary veins need +not detain us now. + + +Section 48. Summarising the course of the circulation, starting from +the right ventricle, we have-- pulmonary artery, pulmonary capillaries, +pulmonary vein, left auricle, left ventricle, aorta, arteries, and systemic +capillaries. After this, from all parts except the spleen and alimentary +canal, the blood returns to systemic veins, superior or inferior cavae, +right auricle, and right ventricle. The blood from the stomach spleen, +and intestines however, passes via {through} the portal vein to the liver +capillaries and then through the hepatic vein to inferior cava, and so +on. Material leaves the blood to be excreted in lungs, kidneys, by the +skin (as perspiration), and elsewhere. New material enters most +conspicuously; + + (a) by the portal veins portal veins and + + (b) by the thoracic duct and left superior cava. + + +Section 49. The following table summarises what we have learnt up to +the present of the physiology of the Rabbit, considered as a +mechanism using up food and oxygen and disengaging energy:-- + + -Air_ {Nitrogen... returned unchanged.} + {Oxygen... through Pulmonary Vein to--} {see 3.} + + -Food_ {Carbo-Hydrates (Starch, Sugar, Cellulose.)} Sugar. + {Protein.} {Peptones.} + {Fat (little in Rabbit.)} {Glycerine, and fatty acids in soups.} + {Rejected matter got rid of in Defaecation.} + + 1a. {Chyle in Lacteals going via {through} Thoracic Duct and Left + Superior Cava to--} {see 2.} + + 1b. {Veins of Villi--} + {Portal Vein--} + {Liver--} + {Hepatic Vein and Inferior Cava to--} {see 2.} + + 2. {Right side of heart; then to lungs, and then to--} {see 3.} + + 3. {Left side of heart; whence to Systemic Arteries and Capillaries.} + + 4. {The tissues and -Kataboly_.} + + 5. {Urea (?Liver) Kidney and Sweat Glands} + {CO2} {Lungs} + {H2O} {Lungs, Kidney, Sweat Glands} + {Other Substances} {Mainly by [Kidney,] Liver and Alimentary Canal} + + + +4. _The Amoeba. Cells, and Tissue_ + +Section 50. We have thus seen how the nutritive material is taken into +the animal's system and distributed over its body, and incidentally, we +have noted how the resultant products of the creature's activity are +removed. The essence of the whole process, as we have already +stated, is the decomposition and partial oxydation of certain complex +chemical compounds to water, carbon dioxide, a low nitrogenous +body, which finally takes the form of urea, and other substances. We +may now go on to a more detailed study, the microscopic study, or +histology, of the tissues in which metaboly and kataboly occur, but +before we do this it will be convenient to glance for a moment at +another of our animal types-- the Amoeba, the lowest as the rabbit is +the highest, in our series. + + +Section 51. This is shown in Figure III., Sheet 3, as it would appear +under the low power of the microscope. We have a mass of a clear, +transparent, greyish substance called protoplasm, granular in +places, and with a clearer border; within this is a denser portion called +the nucleus, or endoplast (n.), which, under the microscope, by +transmitted light, appear brighter, and within that a still denser spot, +the nucleolus (ns.) or endoplastule. The protoplasm is more or less +extensively excavated by fluid spaces, vacuoles; one clearer circular +space or vacuole, which is invariably present, appears at intervals, +enlarges gradually, and then vanishes abruptly, to reappear after a +brief interval; this is called the contractile vacuole (c.v.). The +amoeba is constantly changing its shape, whence its older name of +the Proteus animalcule, thrusting out masses of its substance in +one direction, and withdrawing from another, and hence slowly +creeping about. These thrust-out parts, in its outline, are called +pseudopodia (ps.). By means of them it gradually creeps round and +encloses its food. Little particles of nutritive matter are usually to be +detected in the homogeneous protoplasm of its body; commonly +these are surrounded by a drop of water taken in with them, and the +drop of water is then called a food vacuole. The process of taking in +food is called ingestion. The amoeba, in all probability, performs +essentially the same chemical process as we have summarised in +Sections 10, 11, 12; it ingests food, digests it in the food vacuoles +and builds it up into its body protoplasm, to undergo kataboly and +furnish the force of its motion-- the contractile vacuole, is probably +respiratory and perhaps excretory, accumulating and then, by its +"systole" (compare Section 44), forcing out of its body, the water, +carbon dioxide, urea, and other katastases, which are formed +concomitantly with its activity. The amoeba reproduces itself in the +simplest way; the nucleus occasionally divides into two portions and +a widening fissure in the protoplasm of the animal's body separates +one from the other. It is impossible to say that one is the parent cell, +and the other the offspring; the amoeba we merely perceive, was one +and is now two. It is curious to note, therefore, that the amoeba is, +in a sense, immortal-- that the living nucleus of one of these minute +creatures that we examine to-day under a microscope may have +conceivably drawn, out an unbroken thread of life since the remotest +epochs of the world's history. Although no sexual intercourse can be +observed, there is reason to believe that a process of supposed +"cannabalism," in which a larger amoeba may occasionally engulph +a smaller one, is really a conjugative reproductive process, and +followed by increased vitality and division. + + +Section 52. Now if the student will compare Section 35, he will see +that in the white blood corpuscles we have a very remarkable +resemblance to the amoeba; the contractile vacuole is absent, but we +have the protoplasmic body, the nucleus and nucleolus, and those +creeping fluctuations of shape through the thrusting out and +withdrawal of pseudopodia, which constitute "amoeboid" motion. They +also multiply, in the same way, by division. + + +Section 53. It is not only in the white corpuscle of the blood that we +find this resemblance; in all the firmer parts of the body we find, on +microscopic examination, similar little blebs of protoplasm, and at an +early stage of development the young rabbit is simply one mass of +these protoplasmic bodies. Their division and multiplication is an +essential condition, of growth. Through an unfortunate accident, these +protoplasmic blebs, which constitute the living basis of the animal +body, have come to be styled "cells," though the term "corpuscles" is +far more appropriate. + + +Section 54. The word is "cell" suggests something enclosed by firm +and definite walls, and it was first employed in vegetable histology. +Unlike the typical cells of animals, the cells of most plants are not +naked protoplasm, but protoplasm enclosed in a wall of substance +(cell wall) called cellulose. The presence of this cellulose cell wall, +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 indeed, the primary distinction between the +vegetable and the animal kingdoms, as ordinarily considered. + + +Section 55. Throughout life, millions of these cells retain their +primary characters, and constitute the white corpuscles of blood, +"phagocytes," and connective tissue corpuscles; others again, +engage in the formation of material round themselves, and lie, in +such cases, as gristle and bone, embedded in the substance they +have formed; others again, undergo great changes in form and internal +structure, and become permanently modified into, for instance, nerve +fibres and muscle substance. The various substances arising in this +way through the activity of cells are called tissues, the building +materials of that complex thing, the animal body. Since such a +creature as the rabbit is formed through the co-operation of a vast +multitude of cells, it is called multicellular; the amoeba, on the other +hand, is unicellular. The rabbit may be thus regarded as a vast +community of amoeboid creatures and their products. + + +Section 56. Figure IV., Sheet 3 represents, diagrammatically, +embryonic tissue, of which, to begin with, the whole animal +consists. The cells are all living, capable of dividing and similar, but as +development proceeds, they differentiate, some take on one kind of +duty (function), and some another, like boys taking to different +trades on leaving school, and wide differences in structure and +interdependence become apparent. + + +Section 57. It is convenient to divide tissues into three classes, +though the divisions are by no means clearly marked, nor have they +any scientific value. The first of these comprises tissues composed +wholly, or with the exception of an almost imperceptible cementing +substance, of cells; the second division includes the skeletal tissues, +the tissue of mesentery, and the connective and basement tissue of +most of the organs, tissues which, generally speaking, consist of a +matrix or embedding substance, formed by the cells and outside of +them, as well as the cells themselves; and, thirdly, muscular and +nervous tissue. We shall study the former two in this chapter, and +defer the third division until later. + + +Section 58. The outer layer of the skin (the epidermis), the inmost +lining of the alimentary canal, the lining of the body cavity, and the +inner linings of blood-vessels, glands, and various ducts constitute our +first division. The general name for such tissues is epithelium. When +the cells are more or less flattened, they form squamous epithelium +(Figure VI.) such as we find lining the inside of a man's cheek (from +which the cells sq.ep. were taken) or covering the mesentery of +various types-- sq.end. are from the mesentery (Section 16) of a frog. +A short cylindroidal form of cell makes up columnar epithelium, seen +typically in the cells covering the villi of the duodenum (Figure V.). +This epithelium of the villi has the outer border curiously striated, and +this is usually spoken of as leading towards "ciliated" epithelium, to +be described immediately. The epithelium of the epididermis is +stratified-- that is to say, has many thicknesses of cells; the deeper +layers are alive and dividing (stratum mucosum), while the more +superficial are increasingly flattened and drier as the surface is +approached (stratum corneum) and are continually being rubbed off +and replaced from below. + + +Section 59. In the branching air-tubes of the lung, the central canal of +the spinal cord, and in the ureters of the rabbit, and in most other +types, in various organs, we find ciliated epithelium (Figure VII.). +This is columnar or cubical in form, and with the free edge curiously +modified and beset with a number of hair-like processes, the cilia, by +which, during the life of the cell, a waving motion is sustained in one +direction. This motion assists in maintaining a current in the contents +of ducts which are lined with this tissue. The motion is independent of +the general life of the animal, so long as the constituent cell still +lives, and so it is easy for the student to witness it himself with a +microscope having a 1/4-inch or 1/6-inch objective. Very fine cilia may +be seen by gently scraping the roof of a frog's mouth (the cells figured +are from this source), or the gill of a recently killed mussel, and +mounting at once in water, or, better, in a very weak solution of +common salt. + + +Section 60. The lining of glands is secretory epithelium; the cells +are usually cubical or polygonal (8, g.ep.), and they display in the +most characteristic form what is called metabolism. Anaboly (see +Section 14) we have defined, 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 protoplasm; +kataboly is a chemical running down; metaboly is a more general +term, covering all vital chemical changes. The products of the action +of a glandular epithelium are metabolic products, material derived from +the blood is worked, up within the cell, not necessarily with +conspicuous gain or loss of energy, and discharged into the gland +space. The most striking case of this action is in the "goblet cells" +that are found among the villi; these are simply glands of one cell, +unicellular glands, and in Figure V. we see three stages in their +action: at g.c.1 material (secretion) is seen forming in the cell, at +g.c.2 it approaches the outer border, and at g.c.3 it has been +discharged, leaving a hollowed cell. Usually however, the escape of +secreted matter is not so conspicuous, and the gland-cells are +collected as the lining of pits, simple, as in the gastric, pyloric, and +Lieberkuhnian glands (Figure VIII., Sections 23, 29), or branching +like a tree or a bunch of grapes (Figure r.g.), as in Brunner's glands +(Section 29) the pancreas, and the salivary glands. The salivary +glands, we may mention, are a pair internal to the posterior ventral +angle of the jaw, the sub-maxillary; a pair anterior to these, the +sub-lingual; a pair posterior to the jaw beneath the ear, the parotid, +and a pair beneath the eye, the infra orbital. + + +Section 61. The liver is the most complicated gland in the body +(Figure X.). The bile duct (b.d.) branches again and again, and ends at +last in the final pits, the lobuli (lb.), which are lined with secretory +epithelium, and tightly packed, and squeeze each other into polygonal +forms. The blood supply from which the bile would appear to be +mainly extracted, is brought by the portal vein, but this blood is +altogether unfit for the nutrition of the liver tissue; for this latter +purpose a branch of the coeliac artery, the hepatic serves. Hence in +the tissue of the liver we have, branching and interweaving among +the lobuli, the small branches of the bile duct (b.d.), which carries +away the bile formed, the portal vein (p.v.), the hepatic artery (h.a.), +and the hepatic vein (h.v.). (Compare Section 45.) Figure X.b shows +a lobule; the portal vein and the artery ramify round the lobules-- are +inter-lobular, that is (inter, between); the hepatic vein begins in +the middle of the lobules (intra-lobular), and receives their blood. +(Compare X.a.) Besides its function in the manufacture of the +excretory, digestive, and auxiliary bile, the liver performs other +duties. It appears to act as an inspector of the assimilation material +brought in by the portal vein. The villi, for instance, will absorb +arsenic, but this is arrested and thrown down in the liver. A third +function is the formation of what would seem to be a store of +carbo-hydrate, glycogen, mainly it would appear, from the sugar in the +portal vein, though also, very probably, from nitrogenous material, though +this may occur only under exceptional conditions. Finally, the nitrogenous +katastates, formed in the working of muscle and nerve, and returned +by them to the blood for excretion, are not at that stage in the form of +urea. Whatever form they assume, they undergo a further metabolism +into urea before leaving the body, 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 urea. + + +Section 62. Similar from a physiological point of view, to the secretory +glands which form the digestive fluids are those which furnish +lubricating fluids, the lachrymal gland, and Harderian glands +in the orbit internally to the eye, and posterior and anterior to it +respectively, the sebaceous glands (oil glands) connected with the +hair, and the anal and perineal glands. The secretions of excretory +glands are removed from the body; chief among them are the sweat +glands and kidneys. The sweat glands are microscopic tubular +glands, terminating internally in a small coil (Figure VIII. s.g.) and +are scattered thickly over the body, the water of their secretion being +constantly removed by evaporation, and the small percentage of salt +and urea remaining to accumulate as dirt, and the chief reasonable +excuse for washing. The kidney structure is shown diagrammatically +in Figure 5, Sheet 7. A great number of branching and straight looped, +tubuli (little tubes) converge on an open space, the pelvis. Towards +the outer layers (cortex) of the kidney, these tubuli terminate in little +dilatations into which tangled knots of blood-vessels project: the +dilatations are called Bowman's capsules (B.c.), and each coil of +bloodvessel a glomerulus (gl.). In the capsules, water is drained from +the blood; in the tubuli, urea and other salts in the urine are secreted +from a branching network of vessels. + + +Section 63. In all the epithelial tissues that we have considered we +have one feature in common: they are cells, each equivalent to the +amoeba, that have taken on special duties-- in a word, they are +specialists. The amoeba is Jack of all trades and a free lance; the +protective epidermal cell, the current-making ciliated cell, the bile or +urea-making secretory cell, is master of one trade, and a soldier in a +vast and wonderfully organized host. We will now consider our second +kind of cell in this organization, the cell of which the especial aim is +the building round it of a tissue. + + +Section 64. The simplest variety in this group is hyaline (i.e. glassy) +cartilage (gristle). In this the formative cells (the cartilage +corpuscles) are enjellied in a clear structureless matrix (Figure XII.), +consisting entirely of organic compounds accumulated by their +activity. Immediately round the cell lies a capsule of newer material. +Some of the cells have recently divided (1); others have done so less +recently, and there has been time for the interpolation of matrix, as +at 2. In this way the tissue grows and is repaired. A thin layer of +connective tissue (see below), the perichondrium, clothes the +cartilaginous structure. + + +Section 65. Connective tissue (Figure XIII) is a general name for a +group of tissues of very variable character. It is usually described as +consisting typically in the mammals of three chief elements felted +together; of comparatively unmodified corpuscles (c.c.), more or less +amoeboid, and of fibres which are elongated, altered, and distorted +cells. The fibres are of two kinds: yellow, branching, and highly elastic +(y.e.f.), in consequence of which they fall into sinuous lines +in a preparation, and white and inelastic ones (w.i.f.), lying in parallel +bundles. Where the latter element is entirely dominant, the connective +tissue is tendon, found especially at the point of attachment of +muscles to the parts they work. Some elastic ligaments are almost +purely yellow fibrous tissue. A loose interweaving of the three +elements is areolar tissue, the chief fabric of mesentery, membrane, +and the dermis (beneath the epidermis). With muscle it is the material +of the walls of the alimentary canal and bloodvessels, and generally it +enters into, binds together, and holds in place other tissue. The +connective tissue of fishes displays the differentiation of fibres in a far +less distinct manner. + + +Section 66. Through connective tissues wander the phagocytes, +cells that are difficult to distinguish, if really distinct, from the white +blood corpuscles. These cells possess a remarkable freedom; they +show an initiative of their own, and seem endowed with a +subordinate individuality. They occur in great numbers in a tissue +called, botryoidal tissue (Figure XIV.), which occurs especially in +masses and patches along the course of the alimentary canal, in its +walls. The tonsils, swellings on either side of the throat, are such +masses, and aggregates occur as visible patches, the Peyer's +patches, on the ileum. It also constitutes the mass of the vermiform +appendix and the wall of the sacculus rotundus; and in the young +animal the "thymus gland," ventral to the heart, and less entirely, the +"thyroid gland," ventral to the larynx, are similar structures, which +are reduced or disappear as development proceeds. It is evident that +in these two latter cases the term "gland" is somewhat of a +misnomer. The matrix of botryoidal tissue is a network of stretched +and hollowed connective tissue cells-- it is not a secretion, as +cartilage matrix appears to be. During digestion, the phagocytes prowl +into the intestine, and ingest and devour bacteria, that might +otherwise give rise to disease. In inflammation, we may note here, +they converge from all directions upon the point wounded or irritated. +They appear to be the active agents in all processes of absorption +(see osteoclasts under bone), and for instance, migrate into and +devour the tissue of the tadpole's tail, during its metamorphosis to the +adult frog. + + +Section 67. Within the connective tissue cells fat drops may be +formed, as in Figure XV. Adipose tissue is simply connective tissue +loaded with fat-distended cells. The tissue is, of course, a store form +of hydro-carbon (Section 17) provided against the possible +misadventure of starvation. With the exception of some hybernating +animals, such store forms would seem to be of accidental importance +only among animals, whereas among plants they are of invariable and +necessary occurrence. + + +Section 68. We now come to Bone, a tissue confined to the +vertebrata, and typically shown only in the higher types. As we +descend in the scale from birds and mammals to lizards, amphibia +(frogs and toads) and fish, we find cartilage continually more +important, and the bony constituent of the skeleton correspondingly +less so. In such a type as the dog-fish, the skeleton is entirely +cartilaginous, bone only occurs in connection with the animal's +scales; it must have been in connection with scales that bone first +appeared in the vertebrate sub-kingdom. In the frog we have a +cartilaginous skeleton overlaid by numerous bony scutes (shield-like +plates) which, when the student comes to study that type, he will +perceive are equivalent to the bony parts of such scales as occur in +the dog-fish, sunk inward, and plating over the cartilage; and in the +frog the cartilage also is itself, in a few places, replaced by bony +tissue. In the adult rabbit these two kinds of bone, the bone overlying +what was originally cartilage (membrane bone), and the bone +replacing the cartilage (cartilage bone) have, between them, +practically superseded the cartilage altogether. The structure of the +most characteristic kind of bone will be understood by reference to +Figure XVI. It is a simplified diagram of the transverse section of +such a bone as the thigh bone. M.C. is the central marrow cavity, +H.v., H.v. are cross sections of small bloodvessels, the Haversian +vessels running more or less longitudinally through, the bone in +canals, the Haversian canals. Arranged round these vessels are +circles of the formative elements, the bone corpuscles or +osteoblasts (b.c.) each embedded in bony matrix in a little bed, the +lacuna, and communicating one with another by fine processes +through canaliculi in the matrix, which processes are only to be +seen clearly in decalcified bone (See Section 70). The osteoblasts are +arranged in concentric series, and the matrix is therefore in concentric +layers, or lamellae (c.l.). Without and within the zone of Haversian +systems are (o.l. and i.l.), the outer and inner lamellae. The bone is +surrounded by connective tissue, the periosteum. In addition to this +compact bone, there is a lighter and looser variety in which spicules +and bars of bony tissue are loosely interwoven. Many flat bones, the +bones of the skull, for instance, consist of this spongy bone, plated +(as an electro spoon is plated) with compact bone. + + +Section 69. Among the bony bars and spicules of spongy bone +occurs the red marrow-- which must not be confused with the yellow +marrow, the fatty substance in the central cavity of long bones. In this +red marrow are numerous large colourless cells, which appear +to form within their substance and then liberate red blood corpuscles. +This occurs especially in the spongy bone within the ribs. + + +Section 70. The matrix of bone differs from that of cartilage or of +most other tissues in consisting chiefly of inorganic salts. The chief of +these is calcium phosphate, with which much smaller quantities +of calcium carbonate, and magnesium phosphate and carbonate +occur. These inorganic salts can be removed by immersion of the +bone in weak hydrochloric acid, and a flexible network of connecting +tissue, Haversian vessels, bone corpuscles, and their processes +remains. This is decalcified bone alluded to above. + + +Section 71. In the very young rabbit, the limb bones, vertebral column, +and many of the skull bones are simply plates and bars of cartilage; +the future membrane bones, however are planned out in connective +tissue. The development of the latter is simple, the connective +tissue corpuscles functioning by a simple change of product as +osteoblast. The development of the cartilage bones, however, is more +complicated. Figure XVII., represents, in a diagrammatic way, the +stages in the conversion of a cartilaginous bar to bone. To begin with, +the previously sporadically-arranged (scattered anyhow) corpuscles +(u.c.c.) are gathered into groups in single file, or in other words, into +"columnar" groups (as at c.c.). The matrix becomes clouded with +inorganic salts of lime, and it is then said to be calcified. This +calcified cartilage then undergoes absorption-- it must not be +imagined for a moment that bone is calcified cartilage. Simultaneous +with the formation of the cavities (s.) due to this absorption, +connective tissue (p.c.i.) from the surrounding perichondrium (p.c.) +grows into the ossifying* bar. It is from this connective tissue that the +osteoblasts (o.b.) arise, and bone is built up. Throughout life a bone is +continually being absorbed and reformed by the activity of the +osteoblasts. An osteoblast engaged in the absorption instead of the +formation of bone is called an osteoclast. + +* The formation of bone is called ossification. To ossify is to become +bony. + + +Section 72. The great thing to notice about this is that cartilage does +not become bone, but is eaten into and ousted by it; the osteoblasts +and osteoclasts replace entirely the cartilage corpuscles, and are +not derived from them. + + +Section 73. We may mention here the structure of the spleen +(Figure 1, Sheet 1). It consists of a connective tissue and muscular +coating, with an internal soft matrix much resembling botryoidal +tissue, traversed by fibrous trabeculne (= beams, planks) containing +blood-vessels, and the whole organ is gorged with blood, particularly +after meals. The consideration of its function the student may +conveniently defer for the present. + + +Section 74. Here also, we may notice the lymphatics, a series of +small vessels which return the overflow of the blood serum from the +capillaries, in the nutrition of the tissues in all parts of the body, +to the thoracic duct (see Section 36), and the general circulation. At +intervals their course is interrupted by gland-like dilatations, the +lymphatic glands, in which masses of rapidly dividing and growing +(proliferating) cells occur, of which, doubtless, many are detached and +become, first "lymph corpuscles," and, when they reach the veins, +white blood corpuscles. + + + +5. _The Skeleton_ + +Section 75. We are now in a position to study the rabbit's skeleton. +We strongly recommend the student to do this with the actual bones +at hand-- they may be cleared very easily in a well-boiled rabbit. This +recommendation may appear superfluous to some readers, but, as a +matter of fact, the marked proclivity of the average schoolmaster for +mere book-work has put such a stamp on study, that, in nine cases +out of ten, a student, unless he is expressly instructed to the +contrary, will go to the tortuous, and possibly inexact, descriptions +of a book for a knowledge of things that lie at his very finger-tips. We +have not written, this chapter to give a complete knowledge of the +skeleton, but simply as an aid in the actual examination of the bones. + + +Section 76. We may take the skeleton under five headings. There is +the central axis, the chain of little bones, the vertebrae, threaded +on the spinal cord (see Figure 1 and Section 1); the thorax, the box +enclosed by ribs and sternum; the fore-limb and bones connected +with it (pectoral girdle and limb), and the hind-limb and bones +connected with it (pelvic girdle). Finally there is the skull, but +following the London University syllabus, we shall substitute the +skull of the dog for of that of the rabbit, as more typically mammalian +(Section 4). + + +Section 77. In Section 3 (which the student should refer to) we have +a division of the vertebrae into four varieties. Of these most +representative is the thoracic. A thoracic vertebra (Figure 4, Sheet +5, T.V.) consists of a central bony mass, the body or centrum (b.), +from which there arises dorsally an arch, the neural arch (n.a.), +completed by a keystone, the neural spine (n.s.); and coming off +laterally from the arch is the transverse process (tr.p.). Looking at +the vertebra sideways, we see that the arch is notched, for the exit +of nerves. Jointed to the thoracic vertebrae on either side are the ribs +(r.). Each rib has a process, the tuberculum, going up to articulate +with the transverse process, and one, the capitulum articulating +between the bodies of two contiguous vertebrae. The facets for the +articulation of the capitulum are indicated in the side view by +shading. At either end of the body of a vertebra of a young rabbit are +bony caps, the epiphyses (ep.), separated from the body by a plane +of unossified cartilage (indicated, by the dots). These epiphyses to +the vertebral bodies occur only among mammals, and are even absent +in some cases within the class. In the adult rabbit they have ossified +continuously with the rest of the body. + + +Section 78. A cervical vertebra (C.V.) seems, upon cursory +inspection, to have no rib. The transverse processes differ from those +of thoracic series in having a perforation, the vertebrarterial canal, +through which the vertebral artery runs up the neck. A study of the +development of these bones shows that the part marked f.r. ossifies +separately from the rest of the transverse process; and the form of the +equivalent structures in certain peculiar lower mammals and in +reptiles leaves no doubt that f.r. is really an abbreviated rib; fused up +with the transverse process and body. The two anterior cervical +vertebrae are peculiar. The first (at.) is called the Atlas-- the figure +shows the anterior view-- and has great articular faces for the +condyles (Section 86) of the skull, and a deficient centrum. The next +is the axis, and it is distinguished by an odontoid peg (od.p.), which +fits into the space where the body of the atlas is deficient. In +development the centrum of the axis ossifies from one centre, and the +odontoid, peg from another, which at that time occupies the position +of centrum of the atlas. So that it would seem that the atlas is a +vertebra minus a centrum, and the axis is a vertebra plus a centrum, +added at the expense of the atlas. + + +Section 79. The lumbar vertebrae (l.v.) are larger, and have cleft +transverse processes, each giving rise to an ascending limb, the +metapophyses, and a descending one. The latter (generally spoken of +as the transverse processes) point steeply downward, and are +considerably longer than those of thoracic series. The sacral +vertebrae (s.v.) have great flattened transverse propcesses for +articulation with the ilia. The caudal vertebrae (c.v.) are gradually +reduced to the mere elongated centra, as we proceed, towards the tip +of the tail. + + +Section 80. All the vertebrae join with their adjacent fellows through +the intermediation of certain intervertebral pads, and also articulate by +small processes at either end at the upper side of the arch, +the zygapophyses. The normals to the polished facets of these point, +in the case of the anterior zygapophyses, up and in (mnemonic: +ant-up-in), and in the case of the posterior, down and out. The student +should make this, and the other features of vertebrae, out upon actual +specimens. + + +Section 81. The thorax is bounded dorsally by the vertebral column, +and ventrally by the sternum. The sternum consists of segments, +the sternebrae (st.); anteriorly there is a bony manubrium (mb.), +posteriorly a thin cartilaginous plate, the xiphisternum (xi.). Seven +pairs of ribs articulate by cartilaginous ends (sternal ribs) with the +sternum directly, as indicated in the figure; five (false) ribs are joined, +to each other and to the seventh, and not to the sternum directly. +The last four ribs have no tuberculum (Section 77). + + +Section 82. The fore-limb (pectoral limb) consists of an upper arm +bone, the humerus (hum.) the distal end of which is deeply +excavated by the olecranon fossa (o.f.) as indicated by the dotted +lines; of two bones, the ulna (u.) and radius (r.) which are firmly bound +by ligament in the position of the figure (i.e., with the palm of the hand +downward, "prone"); of a number of small bones (carpalia), the carpus +(c.); of a series of metacarpals (mc.); and of three digits (= fingers) +each, except the first, or pollex, of three small bones-- the phalanges, +only the proximal of which appear in the figure. The ulna has a +hook-like head, the olecranon (o.) which distinguishes it easily from +the distally thickened radius. The limb is attached to the body through +the intermediation of the shoulder-blade (scapula, sc.) a flattened +bone with a median external ridge with a hook-like termination, the +acromion (acr.). There is also a process overhanging the glenoid +cavity (g.) wherein the humerus articulates, which process is called +coracoid (co.); it is ossified from two separate centres, and represents +a very considerable bone in the bird, reptile, and frog. Along the dorsal +edge of the scapula of the rabbit is unossified cartilage, which is +called the supra-scapula (s.sc.). In man there runs from the acromion +to the manubrium of the sternum a bone, the collar-bone or clavicle. +This is represented by a very imperfectly ossified rudiment in the +rabbit. The scapula and clavicle, the bones of the body connected +with the fore-limb, are frequently styled the pectoral girdle, or +shoulder-girdle; this name of girdle will appear less of a misnomer +when lower vertebrate types are studied. + + +Section 83. The hind limb and its body bones-- pelvic limb and +girdle-- are shown in Figure 2. The limb skeleton corresponds +closely with that of the fore-limb. The femur (fe.) answers to the +humerus, and is to be distinguished from it by the greater +distinctness of its proximal head (hd.) and by the absence of an +olecranon fossa from its distal end. The tibia (ti = the radius) is fused +for the distal half of its length with the fibula (fb. = ulna). A tarsus +(tarsalia) equals the carpus.* Two of the proximal tarsalia may be +noted: one working like a pulley under the tibia, is the astragalus +(as.); one forming the bony support of the heel, is the calcaneum +(ca.). There is a series of metatarsals, and then come four digits of +three phalanges each. + +* Such a resemblance as exists between one vertebra and another in +the rabbit, or between the humerus and the femur, is called serial +homology; the two things correspond with each other to the extent of +imperfect reduplication. "Homology" simply is commonly used to +indicate the resemblance between any two structures in different +animals, in origin and position as regards other parts. Thus the heart +of the rabbit and of the frog are homologous structures, corresponding +in position, and resembling each other much as two memory +sketches of one picture might do. + + +Section 84. The pelvic girdle differs from the pectoral in most land +vertebrata in being articulated with the vertebral column. This +difference does not exist in fishes. It consist in the rabbit of four +bones; the ilium (i.), the ischium (is.), the pubis (pb.), and the small +cotyloid bone-- the first two and the latter one meeting in the +acetabular fossa (ac.) in which the head of the femur works. The +pubes and ischia are fused along the mid-ventral line. Many +morphologists regard, the ilium as equivalent to, that is, strictly +corresponding in its relation, to the scapula, the pubis to the +cartilaginous substratum of the clavicle, and the ischium to the +coracoid. + + +Section 85. These bones will be studied at the greatest advantage +when dissected out from a boiled rabbit. Prepared and wired +skeletons, disarticulated skeletons, plates of figures, and written +descriptions are in succession more tedious and less satisfactory +ways to a real comprehension, of this matter. This chapter directs the +student's attention to the most important points in the study of the +skeleton, but it is in no way intended to mitigate the necessity of +practical work. It is a guide simply. + + +Section 86. The mammalian skull will be better understood after the +study of that of some lower vertebrate. We shall describe its main +features now, but their meaning will be much clearer after the lower +type is read. Our figures are of Canis. In section (Figure VI., Sheet 6), +we perceive a brain case (cranium) opening behind by a large +aperture, the foramen magnum (F.M.). In front of this is an extensive +passage, the nasal passage (E.N. to P.N.) which is divided from the +mouth by a bony floor, the palate, and which opens into the pharynx +behind at the posterior nares (P.N.) and to the exterior by the anterior +or external nares (E.N.). It is divided into right and left passages by a +middle partition, the nasal septum. Outside the skull, on its wings, is +a flask-like bone, the bulla tympani (b. in Figures 2 and 3), protecting +the middle ear, and from above this there passes an arch, the cheek +bone (ju. in Figures 1, 2, and 3), to the upper jaw, forming in front the +bony lower protection of the cavity containing the eye, the orbit. The +cheek arch, nasal passage, and jaws, form collectively the "facial +apparatus," as distinguished from the cranium, and the whole skull is +sometimes referred to as, the "cranio-facial apparatus." Two +eminences for articulation with the atlas vertebra, the condyles (c.), lie +one on each side of the lower boundary of the foramen magnum. + + +Section 87. The floor of the cranium consists of a series of cartilage +bones, the basi-occipital (b.o.), basi-sphenoid (b.sp.), pre-sphenoid +(p.sp.), and in front, the ethmoid (eth.), which sends down a median +plate, not shown, in the figure, to form the nasal septum between right +and left nasal passages. Like extended wings on either side of the +basi-occipital are the ex-occipital (e.o.) (the bone is marked +in Figure 4, but the letters are a little obscured by shading). +Similarly the ali-sphenoids (a.s.), are wings to the basi-, and the +orbito-sphenoids (o.s.), to the pre-sphenoid bone (p.sp.). Between the +ex-occipital and ali-sphenoid there is wedged in a bone, the periotic +(p.o.) containing the internal ear (Section 115). Above the foramen +magnum the median supra-occipital bone completes what is called +the occipital arch. A pair of parietals (pa.) come above the +ali-sphenoids, and a pair of frontals (f.) above the orbito-sphenoids. At +the side the brain case is still incomplete, and here the aquamosal +(sq.) enters into its wall. In the external view (Figure 3) the bulla hides +the periotic bone from without. The student should examine all four +figures for these bones before proceeding. + + +Section 88. The outer edge of the upper jaw and the cheek arch are +made up of three paired bones. First comes the premaxilla (p.m.) +(not p.m.1 or p.m.4), containing in the dog, the three incisors of either +side. Then comes the maxilla, bearing the rest of the teeth.* The jugal +or malar (ju.) reaches over from the maxilla to meet a zygomatic +process (= connecting outgrowth) (z.p.) of the squamosal bone. + +* In the dog a sabre-like canine (c.), four premolars (p.m.1 and +p.m.4) and two molars (m.1 and m.2). + + +Section 89. In the under view of the skull (Figure 2) it will be seen that +the maxilla sends in a plate to form the front part of the hard palate. +Behind, the hard palate is completed by the pair of palatine bones +(pal.), which conceal much of the pre- and orbito-sphenoid in the +ventral view, and which run back as ridges to terminate in two small +angular bones, the pterygoids (pt.) which we shall find represent much +more important structures in the lower vertebrata. + + +Section 90. The pre-maxillae and maxillae bound the sides of the +nasal passage, and it is completed above by a pair of splints, the +nasals. Along the floor of the nasal passage, on the middle line, lies a +splint of bone formed by the coalescence of two halves. It embraces +in a V-like groove the mesethmoid (nasal septum) above, and lies on +the palate. + + {Lines from First Edition only.} + -Its position is indicated by a heavy black line in 4, and it is + called, the vomer bone (vo.).- + + {Lines from Second Edition only.} + [In the frog it is represented by two laterally situated bones. + This is the vomer bone (vo.).] + +The nasal passages are partially blocked by foliated bony outgrowths, +from the inner aspect of their walls, which in life are covered with +mucous membrane, and increase the surface sensitive to smell. The +ethmoid ends in the ethmo-turbinal (e.t.); the nasal, the naso-turbinal +(n.t.); and the maxilla, the maxillo-turbinal (m.t.). In the anterior +corner of the orbit there is a bone, the lachrymal (lc. Figure 1), which +is hidden by the maxilla in the side view of the skull. + + +Section 91. The lower jaw (mandible) is one continuous bone in the +mammal. Three incisors bite against the three of the upper jaw. Then +comes a canine, four premolars, and three molars, the first of which is +blade-like (sectorial tooth), and bites against the similar sectorial +tooth (last premolar) of the upper jaw. The third molar is small. The +arrangement of tooth is indicated in the following dental formula:-- +I. 3.3/3.3, C. 1.1/1.1, P.M. 4.4/4.4, M. 2.2/3.3 + + +Section 92. Attached just behind the bulla above, and passing round +on either side of the throat to meet at the base of the tongue, is the +hyoid apparatus (Figure 6). The stylohyal (s.h.), epihyal (e.h.), and +ceratohyal (c.h.) form the anterior cornu of the hyoid. The body of the +hyoid (b.h.) forms a basis for the tongue. The posterior coruna (t.h.) of +the hyoid are also called the thyrohyals. + + +Section 93. The following table presents these bones in something +like their relative positions. 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 presphenoid, the +squamosal similarly overlying the periotic bone, and the jugal reaching +between them. Membrane bones are distinguished by capital letters. + + -Cranium_ + + -Nasal_ (paired), Ethmoid Bone (median), -Vomer_ + -Frontal_ (paired), -Lachrymal_ (paired), Orbito-sphenoid (paired), + Pre-sphenoid (median), Eye + -Parietal_ (Paired), Ali-sphenoid (paired), Basi-sphenoid (median)*, + Periotic Bone (paired) + -Bulla_ (paired) + Supra-occipital (median), Ex-occipital (paired), Basi-occipital + (median) + + -Upper Jaw_ + + -Pre-Maxilla_ (paired) + Palatine (paired) + Pterygoid (paired) + + -Lower Jaw_ + + -Maxilla_ (paired) + -Jugal_ (paired) + -Squamosal_ (paired) + + *In this table the small bones of the ear are simply indicated by an + asterisk. + + +Section 94. Hidden by the bulla, and just external to the periotic bone, +are the auditory ossicles, the incus, malleus, os orbiculare, +and stapes. These will be more explicitly treated when we discuss the +ear. + + +Section 95. When we come to the study of the nerves, we shall +revert to the skull, and treat of its perforations. The student should not +fail, before proceeding, to copy and recopy our figures, and to make +himself quite familiar with them, and he should also obtain and handle +an actual skull. For all practical purposes the skull of a +sheep or cat will be almost as useful as that of the dog. + + + +6. _Muscle and Nerve_ + +Section 96. We have, in the skeleton, a complicated apparatus of +parts hinged and movable upon one another; the agent moving these +parts is the same agent that we find in the heart walls propelling the +blood through the circulation, in the alimentary canal squeezing the +food along its course, and universally in the body where motion +occurs, except in the case of the creeping phagocytes, and the ciliary +waving of ciliated epithelium. This agent is muscle. We have, in +muscular tissue, a very wide departure from the structure of the +primordial cell; to use a common biological expression, a very great +amount of modification (= differentiation). Sheet 7 represents the +simpler kind of muscular tissue, unstriated muscle, in which the +cell character is still fairly obvious. The cells are fusiform +(spindle-shaped), have a distinct nucleus and faint longitudinal +striations (striations along their length), but no transverse +striations. + + +Section 97. In striated muscle extensive modifications mask the cell +character. Under a 1/4 inch objective, transverse striations of the +fibres are also distinctly visible, and under a much higher power we +discern in a fibre (Sheet 7) transverse columns of rod-like sarcous +elements (s.e.), the columns separated by lines of dots, the +membranes of Krause (k.m.), and nuclei (n.), flattened and +separated into portions, and lying, in some cases, close to the +sarcolemma (sc.) the connective tissue enclosing the fibre, in others +scattered throughout the substance of the fibre. The figure shows the +fibre ruptured, in order to display the sarcolemma; e.p. is the end +plate of a nerve (n.v.), and fb. are the fibrillae into which a fibre +may be teased. + + +Section 98. In the heart we have an intermediate kind of muscle +cardiac muscle (Figure 2), in which the muscle fibres branch; there +is apparently no sarcolemma, and the undivided nuclei lie in the +centre of the cell. + + +Section 99. Unstriated muscle is sometimes called involuntary, and +striated, voluntary muscle; but there is really not the connexion with +the will that these terms suggest. We have just mentioned that the +heart-muscle is striated, but who can alter the beating of the heart +by force of will? And the striated muscles of the limbs perform, +endless involuntary acts. It would seem that unstriated muscle +contracts slowly, and we find it especially among the viscera; in the +intestine for instance, where it controls that "peristaltic" movement +which pushes the food forward. Voluntary muscle, on the other hand, +has a sharp contraction. The muscle of the slow-moving snails, slugs, +and mussels is unstriated; all the muscle of the active insects and +crustacea (crabs, lobsters, and crayfish) is striated. Still if the student +bears the exception of the heart in mind, and considers muscles as +"voluntary" that his will can reach, the terms voluntary and involuntary +will serve to give him an idea of the distribution of these two types of +muscle in his own body, and in that of the rabbit. + + +Section 100. Muscular contraction, and generally all activity in the +body is accompanied by kataboly. The medium by which these +katabolic changes are set going and controlled is the nervous +system. The nervous system holds the whole body together in one +harmonious whole; it is the governing organization of the multicellular +community (Section 55), and the supreme head of the government +resides in the brain, and is called the mind. But just as in a political +state only the most important and most exceptional duties are +performed by the imperial body, and minor matters and questions of +routine are referred to boards and local authorities, so the mind +takes cognisance only of a few of the higher concerns of the animal, +and a large amount of the work of the nervous system goes on +insensibly, in a perfectly automatic way-- even much that occurs in +the brain. + + +Section 101. The primary elements in the tissue of the nervous +system are three; nerve fibres, which are simply conducting threads, +telegraph wires; ganglion cells, which are the officials of the system; +and neuroglia, a fine variety of connective tissue which holds these +other elements together, and may also possibly exercise a function in +affecting impressions. A message along a nerve to a ganglion cell is +an afferent impression, from a cell to a muscle or other external end +is an efferent impression. The passage of an impression may be +defined as a flash of kataboly along the nerve, and so every feeling, +thought, and determination involves the formation of a certain +quantity of katastases, and the necessity for air and nutrition. + + +Section 102. Unlike telegraph wires, to which they are often +compared, nervous fibres usually convey impressions only in one +direction, either centrally (afferent or sensory nerve fibres), or +outwardly (efferent or motor nerve fibres). But the so-called motor +nerve fibres include not only those that set muscles in motion, but +those that excite secretion, check impulsive movements, and govern +nutrition. + + +Section 103. Figure 7, Sheet 8, shows the typical structure of nervous +tissues. The nerve fibres there figured are bound together by +endoneurium into small ropes, the nerves, encased in perineurium. +There is always a grey axis cylinder (a.c.), which may (in medullated +nerves), or may not (in non-medullated or grey nerves) have a +medullary sheath (s.S.) interrupted at intervals by the nodes of +Ranvier (n.R.). Nuclei (n.) at intervals under the sheath indicate the +cells from which nerve fibres are derived by a process of elongation. +The nerves of invertebrata, where they possess nerves, are mostly +grey, and so are those of the sympathetic system of vertebrata, to +be presently described, g.c., g.c. are ganglion cells; they may have +many hair-like processes, usually running into continuity with +the axis cylinders of nerve fibres, in which case they are called +multi-polar cells, or they may be uni- or bi-polar. + + +Section 104. The simplest example of the action of the nervous +system is reflex action. For instance, when the foot of a frog, or the +hand of a soundly sleeping person, is tickled very gently, the limb is +moved away from the irritation, without any mental action, and entirely +without will being exercised. And when we go from light into darkness, +the pupil of the eye enlarges, without any direct consciousness of the +change of its shape on our part. Similarly, the presence or food in the +pharynx initiates a series of movements-- swallowing, the digestive +movements, and so on-- which in health are entirely beyond our +mental scope. + + +Section 105. A vast amount of our activities are reflex, and in such +action an efferent stimulus follows an afferent promptly and quite +mechanically. It is only where efferent stimuli do not immediately +become entirely transmuted into outwardly moving impulses that +mental action comes in and an animal feels. There appears to be a +direct relation between sensation and motion. For instance, the +shrieks and other instinctive violent motions produced by pain, +"shunt off" a certain amount of nervous impression that would +otherwise register itself as additional painful sensation. Similarly most +women and children understand the comfort of a "good cry," and its +benefit in shifting off a disagreeable mental state. + + +Section 106. The mind receives and stores impressions, and these +accumulated experiences are the basis of memory, comparison, +imagination, thought, and apparently spontaneous will. Voluntary +actions differ from reflex by the interposition of this previously stored +factor. For instance, when a frog sees a small object in front of him, +that may or may not be an edible insect, the direct visual impression +does not directly determine his subsequent action. It revives a number +of previous experiences, an image already stored of similar insects +and associated with painful or pleasurable gustatory experiences. +With these arise an emotional effect of desire or repulsion which, +passes into action of capture or the reverse. + + +Section 107. Voluntary actions may, by constant repetition, become +quasi-reflex in character. The intellectual phase is abbreviated away. +Habits are once voluntary and deliberated actions becoming +mechanical in this way, and slipping out of the sphere of mind. For +instance, many of the detailed movements of writing and walking +are performed without any attention to the details. An excessive +concentration of the attention upon one thing leads to +absent-mindedness, and to its consequent absurdities of +inappropriate, because imperfectly acquired, reflexes. + + +Section 108. This fluctuating scope of mind should be remembered, +more especially when we are considering the probable mental states +of the lower animals. An habitual or reflex action may have all the +outward appearance of deliberate adjustment. We cannot tell in any +particular case how far the mental comes in, or whether it comes in at +all. Seeing that in our own case consciousness does not enter +into our commonest and most necessary actions, into breathing and +digestion, for instance, and scarcely at all in the details of such acts +as walking and talking we might infer that nature was economical in +its use, and that in the case of such an animal as the Rabbit, which +follows a very limited routine, and in which scarcely any versatility in +emergencies is evident, it must be relatively inconsiderable. Perhaps +after all, pain is not scattered so needlessly and lavishly throughout +the world as the enemies of the vivisectionist would have us believe. + + + +7. _The Nervous System_ + +Section 109. A little more attention must now be given to the +detailed anatomy of the peripheral and central nerve ends. A nerve, as +we have pointed out, terminates centrally in some ganglion cell, either +in a ganglion or in the spinal cord or brain; peripherally there is a +much greater variety of ending. We may have tactile (touch) ends +of various kinds, and the similar olfactory and gustatory end organs; +or the nerve may conduct efferent impressions, and terminate in a +gland which it excites to secretion, in a muscle end-plate, or in fact, +anywhere, where kataboly can be set going and energy disengaged. +We may now briefly advert to the receptive nerve ends. + + +Section 110. Many sensory nerves, doubtless, terminate in fine ends +among the tissues. There are also special touch corpuscles, ovoid +bodies, around which a nerve twines, or within which it terminates. + + +Section 111. The eye (Figure 8) has a tough, dense, outer coat, the +sclerotic (sc.), within which is a highly vascular and internally +pigmented layer, the choroid, upon which the percipient nervous layer, +the retina (r.) rests. The chief chamber of the eye is filled with +a transparent jelly, the vitreous humour (v.h.). In front of the eye, the +white sclerotic passes into the transparent cornea (c.). The +epidermis is continued over the outer face of this as a thin, +transparent epithelium. The choroid coat is continued in front by a +ring-shaped muscle, the iris (ir.) the coloured portion of the eyes. This +iris enlarges or contracts its central aperture (the black pupil) +by reflex action, as the amount of light diminishes or increases. +Immediately behind this curtain is the crystalline lens (l.), the +curvature of the anterior face or which is controlled by the ciliary +muscle (c.m.). In front of the lens is the aqueous humour (a.h.). The +description of the action of this apparatus involves the explanation of +several of the elementary principles of optics, and will be found by +the student in any text-book of that subject. Here it would have no +very instructive bearing, either on general physiological considerations +or upon anatomical fact. + + +Section 112. The structure of the retina demands fuller notice. Figure +9 shows an enlarged, diagram of a small portion of this, the +percipient part of the eye. The optic nerve (o.n. in Figure 8) enters +the eye at a spot called the blind spot (B.S.), and the nerve fibres +spread thence over the inner retinal surface. From this layer of nerve +fibres (o.n. in Figure 9) threads run outward, through certain clear +and granular layers, to an outermost stratum of little rods (r.) and +fusiform bodies called cones (c.), lying side by side. The whole of +the retina consists of quite transparent matter, and it is this outermost +layer of rods and cones (r. and c.) that receives and records the visual +impression. 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 vertebrata, and it is a striking point +of comparison with the ordinary invertebrate eye. + + +Section 113. We may pause to call the student's attention to a little +point in the physiology of nerves, very happily illustrated here. The +function of a nerve fibre is the conduction of impressions pure and +simple; the light radiates through the fibrous layer of the retina without +producing the slightest impression, and at the blind spot, where the +rods and cones are absent, and the nerve fibres are gathered +together, no visual impressions are recorded. If there is any doubt as +to the existence of a blind spot in the retinal picture, the proof is easy. +Let the reader shut his left eye, and regard these two asterisks, fixing +his gaze intently upon the left-hand one of them. + + * * + +At a distance of three or four inches from the paper, both spots will be +focussed on his retina, the left one in the centre of vision, and the +right one at some spot internal to this, and he will see them both +distinctly. Now, if he withdraws his head slowly, the right spot will of +course appear to approach the left, and at a distance of ten or twelve +inches it will, in its approach, pass over the blind spot and vanish, to +reappear as he continues to move his head away from the paper. +The function of nerve fibres is simply conduction, and the nature of the +impressions they convey is entirely determined by the nature of their +distal and proximal terminations. + + +Section 114. Certain small muscles in the orbit (eye-socket) move the +eye, and by their action contribute to our perception of the +relative position of objects. There is a leash of four muscles rising +from a spot behind the exit of the optic nerve from the cranium to the +upper, under, anterior, and posterior sides of the eyeball. These are +the superior, inferior, anterior, and posterior recti. Running from the +front of the orbit obliquely to the underside of the eyeball is the +inferior oblique muscle. Corresponding to it above is a superior oblique. +A lachrymal gland lies in the postero-inferior angle of the orbit, and a +Handerian gland in the corresponding position in front. In addition to +the upper and lower eyelids of the human subject, the rabbit has a +third, the nictitating lid, in the anterior corner of the eye. + + +Section 115. The ear (Sheet VII.) consists of an essential organ of +hearing, and of certain superadded parts. The essential part is called +the internal ear, and is represented in all the true vertebrata (i.e., +excluding the lancelet and its allies). In the lower forms it is a hollow +membranous structure, embedded in a mass of cartilage, the otic +capsule; in the mammal the latter is entirely ossified, to form the +periotic bone. The internal ear consists of a central sac, from which +three semicircular canals spring. The planes of the three canals are +mutually at right angles; two are vertical, the anterior and posterior +(p.v.c.) vertical canals, and one is horizontal, the horizontal canal +(h.c.). There are dilatations, called ampullae, at the anterior base of +the anterior, and at the posterior base of the posterior and horizontal +canals. Indirectly connected with the main sac is a spirally-twisted +portion, resembling a snail shell in form, the cochlea. This last part +is distinctive of the mammalia, but the rest of the internal ear is +represented in all vertebrata, with one or two exceptions. The whole of +the labyrinth is membranous, and contains a fluid, the endolymph; +between the membranous wall of the labyrinth and the enclosing bone +is a space containing the perilymph. Strange as it may appear at first, +the entire lining of the internal ear is, at an early stage, continuous +with the general epidermis of the animal. It grows in just as a gland +might grow in, and is finally cut off from the exterior; but a +considerable relic of this former communication remains as a thin, +vertical blind tube (not shown in the figure), the ductus +endolymphaticus. + + +Section 116. The eighth nerve runs from the brain case (Cr.), into the +periotic bone, and is distributed to the several portions of this +labyrinth. In an ordinary fish this internal ear is the sole auditory organ +we should find; the sound-waves would travel through the water to the +elastic cranium and so reach and affect the nerves. But in all +air-frequenting animals this original plan of an ear has to be added to, +to fit it to the much fainter sound vibrations of the compressible and +far less elastic air. A "receiving apparatus" is needed, and is supplied +by the ear-drum, middle ear, or tympanic cavity (T.). In the mammal +there is also a collecting ear trumpet (the ear commonly so-called), +the external ear, and external auditory meatus (e.a.m.). A tightly +stretched membrane, the tympanic membrane, separates this from +the drum. A chain of small bones, the malleus (m.), the incus (i.), the +os orbiculare (o.or.), a very small bone, and a stirrup-shaped stapes, +swing across the tympanum, from the tympanic membrane to the +internal ear. At two points the bony investment of this last is +incomplete-- at the fenestra rotunda (f.r.), and at the fenestra ovalis, +(f.o.), into which latter the end of the stapes fits, and so +communicates the sound vibrations of the tympanic membrane to the +endolymph. A passage, the Eustachian tube, communicates between +the tympanic cavity and the pharynx (Ph.), and serves to equalize the +pressure on either side of the drum-head. A comparative study of the +ears of the vertebrata brings to light the fact that, as we descend in +the animal scale, the four ear ossicles are replaced by large bones +and cartilages connected with the jaw, and the drum and Eustachian +tube by a gill slit. We have, in fact, in the ear, as the student will +perceive in the sequel, an essentially aquatic auditory organ, added to +and patched up to fit the new needs of a life out of water. + + +Section 117. The impressions of smell are conducted through the first +nerve to the brain, and are first received by special hair-bearing cells +in the olfactory mucous membrane of the upper part of the nasal +passage. The sense of taste has a special nerve in the ninth, the +fibres of which terminate in special cells and cell aggregates in the +little papillae (velvet pile-like processes) that cover the tongue. + + +Section 118. At an early stage in development, the brain of a +mammal consists of a linear arrangement of three hollow vesicles +(Figure 5, Sheet VIII., 1, 2, and 3), which are the fore-, mid-, and +hind-brain respectively. The cavities in these in these vesicles are +continuous with a hollow running through the spinal cord. On the +dorsal side of the fore-brain is a structure to be dealt with more fully +later, the pineal gland (p.g.), while on its under surface is the +pituitary body (pt.). + + +Section 119. The lower figure of (5) shows, in a diagrammatic manner, +the derivation of the adult brain from this primitive state. From the +fore-brain vesicle, a hollow outgrowth on either side gives rises to the +(paired) cerebral hemisphere (c.h.), which is prolonged forward as the +olfactory lobe (o.l.). From the fore-brain the retina of the eye and the +optic nerve also originate as an, at first, hollow outgrowth (op.). The +roof of the mid-brain is also thickened, and bulges up to form two +pairs of thickenings, the corpora quadrigemina, (c.q.). The hind-brain +sends up in front a median outgrowth, which develops lateral wings, +the cerebellum (cbm.), behind which the remainder of the hind-brain is +called the medulla oblongata, and passes without any very definite +demarcation into the spinal cord. + + +Section 120. Figure 1 is a corresponding figure of the actual state of +affairs in the adult. The brain is seen in median vertical section. (ch.) +is the right cerebral hemisphere, an inflated vesicle, which, in the +mammal-- but not in our lower types-- reaches back over the rest of +the fore-brain, and also over the mid-brain, and hides these and the +pineal gland in the dorsal view of the brain (Figure 2). The hollow of the +hemisphere on either side communicates with the third ventricle, the +original cavity of the fore-brain (1 in Figure 5), by an aperture (the +foramen of Monro), indicated by a black arrow (f.M.). Besides their +original communication through the intermediation of the fore-brain, +the hemispheres are also united above its roof by a broad bridge of +fibre, the corpus callosum (c.c.), which is distinctive of the +mammalian animals. The original fore-brain vesicle has its lateral +walls thickened to form the optic thalami (o.th.), between which a +middle commissure, (m.c.), absent in lower types, stretches like a +great beam across the third ventricle. The original fore-brain is often +called the thalamencephalon, the hemisphere, the prosencephalon, +the olfactory lobes, the rhinencephalon. + + +Section 121. The parts of mid-brain (mesencephalon) will be easily +recognised. Its cavity is in the adult mammal called the iter; its floor is +differentiated into bundles of fibres, the crura cerebri (c.cb.), figured +also in Figure 4. + + +Section 122. The cerebellum (metencephalon) consists of a central +mass, the vermis (v.cbm.), and it also has lateral lobes (l.l.), +prolonged into flocculi (f.cbm.), which lastare -em-bedded in pits, [in] +the periotic bone, and on that account render the extraction of the +brain from the cranium far more difficult than it would otherwise be. +The roof of the hind-brain, before and behind the cerebellum, +consists of extremely thin plates of nervous matter. Its floor is greatly +thickened to form the mass of the medulla, and in front a great +transverse track of fibres is specialized, the pons Varolii (p.V.). Its +cavity is called, the fourth ventricle. + + +Section 123. Figure 2 gives a dorsal view of the rabbit's brain; a +horizontal slice has been taken at the level of the corpus callosum. +The lateral ventricle (i.e., the hollows of the hemisphere) is not yet +opened. A lower cut (Figure 3) exposes this (V.L.). The level of these +slices is approximately indicated in Figure 1 by the lines A and B. +This latter figure will repay careful examination. The arrow, ar., +plunges into the third ventricle, behind the great middle commissure +(m.c.), and the barb is supposed to lie under the roof of the +mid-brain, the corpora quadrigemina (c.q.). The position of ar. is also +indicated in Figure 1. Before reading on, the beginner should stop a +while here; he should carefully copy or trace our figures and, putting +the book aside, name the parts, and he should then recopy, on an +enlarged scale, and finally draw from memory, correct, and again +draw. By doing this before the brain is dissected a considerable +saving of time is possible. + + +Section 124. Proceeding from the brain are twelve pairs of cranial +nerves. From the fore-brain spring two pairs, which differ from the +rest of the cranial nerves in being, first of all, hollow outgrowths of +the brain-- the others are from the beginning solid. The first nerve is +the olfactory lobe, which sends numerous filaments through the +ethmoid bone to the olfactory organ. The second is the optic nerve, +the visual sensory nerve. + + +Section 125. The mid-brain gives rise to only one nerve, the third, +which supplies all the small muscles of the eye (see Section 114), +except the superior oblique and external rectus. + + +Section 126. The remainder of the nerves spring from the hind-brain. +The fourth pair supply the superior obliques, and the sixth the external +recti; so that III., IV., and VI. are alike purely motor nerves, small and +distributed, to the orbit. The fifth nerve, the trigeminal, is a much +larger and more important one; it is a mixed nerve, having three main +branches, of which the first two are chiefly sensory, the third almost +entirely motor; it lies deeply in the orbit. V1 (see Sheet 9) runs up +over the recti behind the eyeball, it is the ophthalmic branch; V2, the +maxillary branch, runs deeply under the eyeball and emerges in front +of the malar, and V3, the mandibular branch, runs down on the inner +side of the jaw-bone to the jaw muscles and tongue. + + +Section 127. If the student will now recur to the figures of the dog's +skull (Sheet 6), he will see certain apertures indicated in the cranial +wall. Of these, o.f. is the optic foramen for the exit of nerve II., +perforating the orbito-sphenoid. Behind this there comes an irregular +aperture, (f.l.a.), the foramen lacerum anterius, giving exit to III., IV., +VI., and V1. V2 emerges from the foramen rotundum, and V3 from the +foramen ovale, two apertures uniting behind a bony screen.* Just in +front of the bulla is a foramen lacerum medium (f.l.M.), through which +no nerve passes. + +* In the rabbit's skull f.l. anterius, the foramen rotundum, and +foramen ovale are not distinct, and there are two condylar foramina +instead of one, through each of which, a moiety of XII. passes. + + +Section 128. The eighth nerve (auditory) is purely sensory, the nerve +of the special sense of hearing; it runs into the periotic bone, and +breaks up on the labyrinth. The seventh nerve (facial) is almost +entirely motor; it passes through the periotic anterior to VIII., and +emerges by the stylo-mastoid foramen (s.m.f.) behind the bulla, to run +outside the great jaw muscle across the cheek immediately under the +skin (Figure 1). + + +Section 129. The ninth (glossopharyngeal) nerve is chiefly sensory; +it is the special nerve of taste, and is distributed to the tongue. The +tenth nerve (vagus) arises by a number of roots, and passes out of the +skull, together with IX and XI, by the foramen lacerum -posterium- +[posterius] (f.l.p.). It is a conspicuous white nerve, and runs down the +neck by the side of the common carotid artery. It sends a superior +laryngeal branch (Xa) to the larynx. The left vagus passes ventral to +the aortic arch, and sends a branch (l.x.b.) under this along the +trachea to the larynx-- the recurrent laryngeal nerve. The +corresponding nerve on the right (r.x.b.) loops under the subclavian +artery. The main vagus, after this branching, passes behind the heart +to the oesophagus and along it to the stomach. XI., the spinal +accessory, supplies certain of the neck nerves. XII., the hypoglossal, +runs out of the skull by the condylar foramen (c.f.), is motor, crosses +the roots of XI., X., and IX., passes ventral to the carotid, and breaks +up among the muscles of the tongue and neck. + + +Section 130. Of the functions of the several parts of the brain there is +still very considerable doubt. With disease or willful destruction of +the cerebral tissue the personal initiative is affected-- the animal +becomes more distinctly a mechanism; the cerebellum is probably +concerned in the coordination of muscular movements; and the +medulla is a centre for the higher and more complicated respiratory +reflexes, yawning, coughing, and so on. The great majority of reflex +actions centre, however, in the spinal cord, and do not affect the +brain. + + +Section 131. A cross section of the spinal cord is shown in Figure 6, +Sheet 8. It is a cylinder, almost bisected by a dorsal (d.f.) and a +ventral (v.f.) fissure. Through its centre runs a central canal (c.c.), +continuous with the brain ventricles, and lined by ciliated epithelium. +The spinal cord consists of an outer portion, mainly of nervous fibres, +the white matter, and of inner, ganglionated, and more highly +vascular grey matter. (In the cerebrum the grey matter is external, and +the white internal.) The cord, like the brain, is surrounded by a +vascular fibrous investment, and protected from concussion by a +serous fluid. The nerves which emerge from the vertebral column +between the vertebrae, arise, unlike the cranial nerves, by two roots. +The dorsal of these, the sensory root (d.n.), has a swelling upon it, the +dorsal ganglion, and-- by experiments upon living animals-- has been +shown to contain only afferent fibres; the ventral, the motor +root, is without a ganglion, and entirely or mainly motor. The two unite +outside the cord, and thereafter the spinal nerves are both sensory +and motor. + + +Section 132. Besides the great mass of brain and spinal cord +(cerebro-spinal axis), there is, on either side of the dorsal wall of the +body cavity, a sympathetic nervous chain. The nerve fibres of this +system, like the nerve fibres of invertebrates, are non-medullated. It +may be seen as a greyish thread running close by the common +carotid in the neck (sym., Figure 1); it then runs over the heads of +the ribs in the thorax and close beside the dorsal aorta in the +abdominal region. In the anterior region of the neck it dilates to form +a superior cervical ganglion, and opposite the first rib it forms an +inferior cervical ganglion. Thence, backwards, there is a ganglion on +each sympathetic chain opposite each spinal nerve, and the two +exchange fibres through a thread, the ramus communicans. To the +sympathetic chain is delegated much of the routine work of reflex +control of the bloodvessels and other viscera, which would otherwise +fall upon the spinal cord. + + +Section 133. There are eight cervical (spinal) nerves, one in front of +the atlas, and one behind each of the cervical vertebrae. The last four +and the first thoracic (spinal) contribute to a leash of nerves running +out to the fore limb, the brachial plexus (plexus, literally network, but +here meaning a plaited cord). The fourth cervical also sends down a +phrenic nerve (p.n., Figure 1), along by the external jugular vein and +the superior caval vein to the diaphragm. The last three lumbar and +the sacral nerves form a sacral plexus, supplying the hind limb. + + +Section 134. From the sympathetic in the hinder region of the thorax +a nerve, the great splanchnic nerve, arises, and runs, back to a +ganglionated nervous network, just behind the coeliac artery, into +which the vagus also enters; this is the coeliac ganglion, and together +with a similar superior mesenteric ganglion around the corresponding +artery, makes up a subsidiary visceral nervous network, the solar +plexus. A similar and smaller nervous tangle, bearing an inferior +mesenteric ganglion, lies near the inferior mesenteric artery. + + +Section 135. Finally, we may note the pineal gland and the pituitary +body, as remarkable appendages above and below the +thalamencephalon. Their function, if they have a function, is altogether +unknown. Probably, they are inherited from ancestors to whom they +were of value. Such structures are called reduced or vestigial +structures, and among other instances are the clavicles of the rabbit, +the hair on human limbs, the little pulpy nodule in the corner of the +human eye, representing the rabbit's third eyelid, and the caudal +vertebrae at the end of the human spinal column. In certain lowly +reptiles, in the lampreys, and especially in a peculiar New Zealand +lizard, the pineal gland has the most convincing resemblance to an +eye, both in its general build and in the microscopic structure of its +elements; 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 vertebrata. The pituitary body is probably +equivalent to a ciliated pit we shall describe in the lacelet +(Amphioxus). + + + +8. _Renal and Reproductive Organs_ + +Section 136. We have now really completed our survey of the +individual animal's mechanism. But no animal that was merely +complete in itself would be long sanctioned by nature. For an animal +species to survive, there must evidently, also, be proper provision for +the production of young, and the preservation of the species as well +as of the individual. Hence in an animal's physiology and psychology +we meet with a vast amount of unselfish provision, and its structure +and happiness are more essentially dependent on the good of its kind +than on its narrow personal advantage. The mammalia probably owe +their present dominant position in the animal kingdom to the +exceptional sacrifices made by them for their young. Instead of +laying eggs and abandoning them before or soon after hatching, the +females retain the eggs within their bodies until the development of +the young is complete, and thereafter associate with them for the +purposes of nourishment, protection, and education. In the matter of +the tail, for instance, already noted, the individual rabbit incurs the +disadvantage of conspicuousness for the rear, in order to further the +safety of the young. + + +Section 137. The female organs of reproduction are shown in +Sheet 10. The essential organ is the ovary (ov.), in which the ova +(eggs) are formed. Figure 3 gives an enlarged and still more +diagrammatic rendering of the ovary. There is a supporting ground +mass, or stroma, into which numerous bloodvessels and nerves enter +and break up. The ova appear first as small cells in the external +substance of the ovary (as at 1), and move inward (2 and 3), +surrounded by a number of sister cells, which afford them +nourishment. At (4) an ovum with its surrounding group of cells is +more distinct and near the centre of the ovary; a fluid is appearing +within the ovisac as the development proceeds. (5) is a much more +mature ovisac or Graafian follicle. + +Section 138. The ovum (ov.), is now large, and its nucleus and +nucleolus (the germinal vesicle and spot) are very distinct. The wall +of the follicle consists, in the mammal, of several layers of cells, the +membrana granulosa (or "granulosa" simply); the ovum lies on its +outer side embedded in a mass of cells, discus proligerus, +separated from actual contact with the ovum by a zona pellucida. +The ripening follicle moves to the surface of the ovary and bursts, the +ovum falls into the body cavity. In Figure 2, a ripe Graafian follicle +(G.F.), projects upon the ovary. + + +Section 139. The liberated ovum is caught up by the funnel-shaped +opening of the Fallopian tube, which passes without any very +conspicuous demarcation into the cornu uteri (c.ut.) of its side; the +two uterine cornua meeting together in the middle line form the vagina +(V.), which runs out into a vestibule (vb.) opening between tumid lips +to the exterior. The urinary bladder (ur.b.) also opens into the +vestibule, and receives the two ureters from the kidney. + + +Section 140. In the male we find, in the position of the female uterus, +a uterus masculinus (u.m.). The essential sexual organ is the testis +(T.), a compact mass of coiling tubuli, which opens by a number of +ducts, the vasa efferentia, into a looser and softer epididymis (ep.), +which sends the sexual product onward through a vas deferens (v.d.), +to open at the base of the uterus masculinus. The urinary bladder and +ureters correspond with those of the female, and the common +urogenital duct (= vestibule), the urethra, is prolonged into an erectile +penis (P.) surrounded by a fold of skin, the prepuce. A prostate gland +(pr.), contributes to the male sexual fluid. The character of the +essential male element, the spermatozoon, the general nature of the +reproductive process, will be conveniently deferred until the chapters +upon development are reached. + + + +9. _Classificatory Points_ + +Section 141. The following facts of classificatory importance may now +be considered, but their full force will be better appreciated after the +study of other vertebrate types. They are such as come prominently +forward in the comparison of the rabbit with other organisms. + + +Section 142. In the first place, the rabbit is a metazoon, one of the +metazoa, i.e., a multicellular organism, as compared with the +amoeba, which belongs to the protozoa or one-cell animals (Section +55). In the next place, it is externally bilaterally symmetrical, its +parts balance, and where, in its internal anatomy, it departs from +this symmetry (as in the case of the aorta, the stomach and +intestines, and the kidneys), the departure has an appearance of +being the results of partial reductions and distortions of an originally +quite symmetrical plan. And the facts of development strengthen this +idea; in the very earliest stages we have paired aortic arches, of +which, the left only remains, a straight alimentary canal, and less +asymmetrical kidneys. In the vast majority of animals the same +bilateral symmetry is to be seen, but in the star-fish and sea-urchins, +and in the jelly-fish, corals, sea anemones, and hydra, the general +form of the animal is, instead, arranged round a centre, like a star and +its rays, and the symmetry is called radial. + + +Section 143. We also see in various organs of the rabbit, and +especially in the case of the limbs and vertebral column, what is +called metameric segmentation, that is, a repetition of parts, one +behind the other, along the axis of the body. Thus the bodies and +arches of the vertebrae repeat each other, and so do the spinal +nerves. The renal organ of the rabbit, some time before birth, +displays a metameric arrangement of its parts; but this disappears, +as development proceeds, into the compact kidney of the adult. But +the metameric segmentation in the rabbit's organism is not nearly so +marked as that of an earthworm, for instance, which is visibly a +chain of rings. If the student wants a perfect figure of metameric +segmentation he should think of a train of precisely similar +carriages, or a string of beads. One bead, one carriage, one +vertebra, would be a metamere. + + +Section 144. In contrast to metameric segmentation is the antimeric +repetition of radial symmetry (Section 142), in which each ray of the +star is called an antimere. It is possible to have bilateral symmetry +without a metameric arrangement of parts, as in the mussel and the +cuttle-fish; but metameric segmentation without complete or reduced +bilateral symmetry does not occur. + + +Section 145. 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 case, that the +primary division should be into protozoa and metazoa, and that the +vertebrata are one of several groups of metazoa with a fundamental +bilateral symmetry and imperfect metameric segmentation. + +The rabbit is one of the vertebrata, and, in common with all the other +animals collected under this head, it has-- + + (a) A skeletal axis (the vertebral column) between its central + nervous system and its body cavity. In the adult rabbit this + consists of a chain of vertebrae, but in the embryo (i.e., the + young rabbit before birth) it is represented by a continuous chord, + the notochord, and it remains as such in some of the lowest + vertebrata throughout life. In other words, in these lower + vertebrata, the vertebral axis is not metameric. + + (b) A dorsal and -Tubular_ nervous axis. (Section 131, the central + canal) + + (c) It has, though in the embryo only, certain slits between the + throat and the exterior, like the gill slits of a fish. Such + slits are-- with one or two remarkable exceptions outside the + sub-kingdom-- distinctly vertebrate features, and remain, of course, + in fishes throughout life. + +The presence of true cartilage and bone mark a vertebrate, but +vertebrata occur in which -these tissues- [bone] -are- [is] absent. + + +Section 146. The rabbit shares the following features with all the +vertebrata, except the true fishes, which do not possess any of them-- + + (a) Lungs (but many fish have a swimming bladder which answers + to the lungs in its anatomical relations.) + + (b) Limbs which consist of a proximal joint of one bone an + intermediate part of two, and a distal portion which has five + digits, or is evidently a reduced form of the five-digit limb.* + + (c) The absence of a median fin supported by fin rays.** + + * The frog shows indications of a sixth digit. + ** The frog's tadpole has a median fin, but no fin rays. + + +Section 147. The rabbit shares the following features with all the +vertebrata above the fishes and amphibia (= frogs, toads, newts, +and etc.)-- + + (a) Absence of gills (not gill slits, note) at any stage in + development. + + (b) An amnion, and + + (c) An allantois in development. + +The meaning of (b) and (c) we shall explain to the student in the +chapters on embryology. We simply mention them here to render +our table complete. + + +Section 148. The rabbit shares with all mammals, and differs from all +other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having-- + + (a) Hair. + + (b) A diaphragm. + + (c) Only one aortic arch, and that on the left side of the body. + + (d) Its young born alive. (But two very reptile-like mammals of + Australia, the duck-billed platypus and the echidna, lay eggs, and + certain fish and reptiles bear living young.) + + (e) Epiphyses to its vertebral -centre- [centra].* + + (f) The cerebral hemispheres covering the mid-brain. + + (g) Corpora quadrigemina instead of bigemina. + + [(h) A corpus callosum.] + + [(i) A spirally coiled cochlea to the internal ear.] + + [(In respect to h and i also, the echidna and platypus are scarcely + mammalinan.)] + + * But certain mammals have no such epiphyses. + + +Section 149. The rabbit, together with the hares and conies, rats and +mice, voles, squirrels, beavers, cavies, guineapigs is included in that +order of the class of mammals which is called the rodentia, and is +distinguished by the character of the incisor teeth from other orders of +the class. + + + +10. _Questions and Exercises_ + +1. Describe the venous circulation of the rabbit (with diagrams). +Compare a vein and artery. Compare the distribution of the great +venous trunks with that of the arterial system. + +2. Construct a general diagram of the circulation of the rabbit, to show +especially the relation of the portal system, the lymphatics and +lacteals, and the renal circulation to the main blood current. + +3. Draw the alimentary canal of the rabbit from memory. + +4. What is a villus? Describe its epithelium, and the vessels within it. +Write as explicit an account as you can of the absorbent action of a +villus. + +5. Tabulate the alimentary secretions, and their action on the food. + +6. What is botryoidal tissue? Where does it occur? What is known +of its functions? + +7. Copy Diagram I. (enlarged), and insert upon it the visceral nerves +as far as you can. + +8. What are the most characteristic points in the mammalian vertebral +column? + +9. Describe cartilage and bone, and compare them with one another. + +10. Give an account of the amoeba, and compare it with a typical +tissue cell in a metazoon (e.g., the rabbit). + +11. Give a general account of connective tissue. What is tendon? + +12. Trace, briefly, the increased modification of tissues in the +vertebrata. + +13. Describe, with diagrams, the structure of blood. State the function +of each factor you describe. + +14. Compare the pectoral with the pelvic limb and girdle. What other +structures of the adult rabbit display a similar repetition of similar +parts? + +15. Draw from memory typical vertebrae from each region of the +vertebral column. + +16. What are bilateral symmetry and metameric segmentation? + +17. Give a schedule of distinctive mammalian features. + +18. Describe the rabbit's brain (with diagrams). + +19. Give a list of the cranial nerves of the rabbit, and note their origin +in the brain. + +20. Give a list of the nerve apertures of the dog's skull. + +21. What are the chief anatomical differences between a typical +cranial, a spinal, and a sympathetic nerve? + +22. Describe and figure the distribution of nerves V., VII., IX., and X. + +23. Describe the muscles, glands, and nerves of the orbit of the +rabbit. + +24. Describe, with figures, the eye of the rabbit. + +25. Give a diagram of the rabbit's internal ear. + +26. Draw and describe the ear ossicles. What is their function? + +27. Draw and state the precise position of the hyoid bone, the +clavicle, the calcaneum, and the olecranon process. + +28. Describe, as accurately as possible, the position of palatine +bones, pterygoids, the ethmoid bone, the pre- and basi-sphenoids, +in the dog's skull. + +29. What is membrane bone? What is cartilage bone? Discuss their +mutual relationship. + +30. What is an excretion? What are the chief excretory products of an +animal? How are they removed? + +31. Describe the minute anatomy of the liver. Give a general account +of its functions. + +32. Describe the minute anatomy of the kidney, and the functions of +the several parts. + +33. What is ciliated epithelium? Where does it occur in the rabbit? + +34. Describe the mechanism of respiration. What is the relation of +respiration to the general life of the animal? + +35. What are the functions of the skin? Describe its structure. + +36. What is a secretion? Tabulate and classify secretary organs. +What is a goblet cell? + +37. Draw, from memory, the dorsal and ventral aspects of, and a +median section through, a dog's skull. + +38. Name any structures that appear to you to be vestiges or +rudiments, i.e., structures without adequate physiological reason, in +the rabbit's anatomy. + +39. How are such structures interpreted? + +40. Describe the structure of striated muscular fibre. Describe its +functions, and the various means by which they may be called into +activity. + +41. Describe the characters and structure of the blood of the rabbit. +What is the lymphatic system? Describe its relation to the blood +system in a mammal. + +42. Describe the structure of (a) blood, (b) hyaline cartilage, (c) +bone, in the rabbit; (d) point out the most important resemblances and +differences between these tissues; (e) state what you know of the +development of the same tissues. + +43. Draw diagrams, with the parts named, of the male and female +generative organs of the rabbit. + +44. In the rabbit provided dissect on one side and demonstrate by +means of flag-labels the main trunk of the vagus nerve, the phrenic +nerve, and the recurrent laryngeal nerve. + +45. Dissect the rabbit provided so as to expose the abdominal +viscera. Mark with flag-labels the duct of the pancreas, the ureters, +and the oviducts or the sperm ducts (as the case may be). + + [Many of the above questions were actually set at London + University Examinations in Biology.] {In Both Editions.} + + + +-The Frog_ + +1. _General Anatomy_ + +Section 1. We will now study the adult anatomy of the frog, and +throughout we shall make constant comparisons with that of the +rabbit. In the rabbit we have a distinctly land-loving, burrowing animal; +it eats purely vegetable food, and drinks but little. In the frog we have +a mainly insectivorous type, living much in the water. This involves the +moister skin, the shorter alimentary canal, and the abbreviated neck +(Rabbit, Section 2) of the frog; 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 rabbit, run together into one dorsal opening, +the cloaca. There is, of course (Rabbit, Section 4), no hair the skin is +smooth, and an external ear is also absent. The remarkable +looseness of the frog's skin is due to great lymph spaces between it +and the body wall. + + +Section 2. If we now compare the general anatomy of the frog (vide +Sheet 11) with that of the rabbit, we notice that the diaphragm is +absent (Rabbit, Section 4), and the body cavity, or coelom, is, with +the exception of the small bag of the pericardium round the heart, one +continuous space. The forked tongue is attached in front of the lower +jaw, and can be flicked out and back with great rapidity in the capture +of the small insects upon which the frog lives. The posterior nares +open into the front of the mouth-- there is no long nasal chamber, and +no palate, and there is no long trachea between the epiglottis and the +lungs. The oesophagus is less distinct, and passes gradually, so far +as external appearances go, into the bag-like stomach, which is +much less inflated and transverse than that of the rabbit. The +duodenum is not a U-shaped loop, but makes one together with the +stomach; the pancreas lies between it and the stomach, and is more +compact than the rabbit's. There is no separate pancreatic duct, but +the bile duct runs through the pancreas, and receives a series of +ducts from that gland as it does so. The ileum is shorter, there is no +sacculus rotundus, and the large intestine has no caecum, none of +the characteristic sacculations of the rabbit's colon, and does not loop +back to the stomach before the rectum section commences. The +anus opens not upon the exterior, but into a cloacal chamber. The +urinary and genital ducts open separately into this cloaca, and +dorsally and posteriorly to the anus. The so-called urinary bladder is +ventral to the intestine, in a position answering to that of the rabbit, +but it has no connection with the ureters, and it is two-horned. + + +Section 3. The spleen is a small, round body, not so intimately +bound to the stomach as in the rabbit, but in essentially the same +position. + + +Section 4. Much that we knew of the physiology of the frog is arrived +at mainly by inferences from our mammalian knowledge. Its histology +is essentially similar. Ciliated epithelium is commoner and occurs +more abundantly than in the rabbit, in the roof of the mouth for +instance, and its red blood corpuscles are much larger, oval, and +nucleated. + + +Section 5. The lungs of the frog are bag-like; shelves and spongy +partitions project into their cavities, but this structure is much simpler +than that of the rabbit's lung, in which the branching bronchi, the +imperfect cartilaginous rings supporting them, alveoli, arteries and +veins, form together a quasi-solid mass. + + +Section 6. The mechanism of respiration is fundamentally different +from that of the mammal. The method is as follows:-- The frog opens +its anterior nares, and depresses the floor of the mouth, which +therefore fills with air. The anterior nares are then closed, and the floor +mouth rises and forces the air into the lungs-- the frog, therefore, +swallows its air rather than inhales it. The respiratory instrument of +the rabbit is a suction pump, while that of the frog is a "buccal force +pump." + + +Section 7. The heart is not quadrilocular (i.e., of four chambers), but +trilocular (of three), and two structures, not seen in Lepus, the +truncus arteriosus and the sinus venosus, into the latter of which +the venous blood runs before entering the right auricle, are to be +noted. The single ventricle is blocked with bars of tissue that render +its interior, not an open cavity, but a spongy mass. Figure 2, Sheet +11, shows the heart opened; l.au. and r.au. are the left and right +auricles respectively; the truncus arteriosus is seen to be imperfectly +divided by a great longitudino-spiral valve (l.s.v.); p.c. is the +pulmo-cutaneous artery -going to the lungs- [supplying skin and +lungs]; d.ao., the dorsal aorta [furnishing the supply of the body and +limbs]; and c.a. the carotid artery going to the head; all of which +vessels (compare Figure 1) are paired. + + +Section 8. It might be inferred from this that pure and impure blood +mix in the ventricle, and that a blood of uniform quality flows to lungs, +head, and extremities; but this is not so. The spongy nature of the +ventricle sufficiently retards this mixing. It will be noted that the +opening of pulmonary arteries lies nearest to the heart, next come the +aortic and carotid arches, which have a common opening at A. +Furthermore, at c.g.l. [the carotid artery, repeatedly divides to form a +close meshwork of arterioles, the carotid gland, forming a sponge-like +plug in this vessel.] is a spongy mass of matter, the carotid gland +inserted upon the carotid. Hence the pulmonary arteries yawn nearest +for the blood, and, being short, wide vessels, present the least +resistance to the first rush of blood-- mainly venous blood for the right +auricle. As they fill up, the back resistance in them becomes equal, +and then greater, than the resistance at A, and the rush of blood, now +of a mixed quality passes through that aperture. It selects the dorsal +aorta, because the carotid arch, plugged by the carotid gland, offers +the greater resistance. Presently, however, the back resistance of the +filled dorsal aorta rises above this, and the last flow of blood, from the +ventricular systole-- almost purely oxygenated blood for the left +auricle-- goes on towards the head. + + +Section 9. At the carotid gland the carotid artery splits into -an- [a] +-external carotid- [lingual] (e.c.), and a deeper internal carotid. The +dorsal aorta passes round on each side of the oesophagus, as +indicated by the dotted lines in Figure 2, Sheet 11, and meets its +fellow dorsal to the liver. Each arch gives off subclavian arteries to the +limbs, and the left, immediately before meeting the right, gives off the +coeliaco-mesenteric artery [to the alimentary canal]. This origin of the +coeliaco-mesenteric artery a little to the left, is the only asymmetry +(want of balance) in the arterial system of the frog, as contrasted with +the very extensive asymmetry of the great vessels near the heart of +the rabbit. [Posteriorly the dorsal aorta forks into two common iliac +arteries (right and left) supplying the hind limbs.] + + +Section 10. Figure 3 gives a side view of the frog, to display the +circulation. + + {Lines from Second Edition only.} + [The venous return to the heart, as in the rabbit, is by paired venae + cavae anteriores and by a single vena cava inferior. The factors of the + anterior cava on either side are an external jugular (ex.j.) an + innominate vein (in.v.) and subclavian (scl.v.). The last receives not + only the brachial vein (b.v.) from the fore limb, but also a large vein + bringing blood for the skin, the cutaneous (p.v.). The innominate vein + has also two chief factors, the internal jugular (l.i.j.v.) and the + subscapular (s.s.v.). The blood returns from each hind limb by a + sciatic (l.sc.) or femoral (f.m.) vein, and either passes to a renal portal + vein (l.r.p.), which breaks into capillaries in the kidney, or by a paired + pelvic vein (l.p.v. in Figures 1 and 3) which meets its fellow in the + middle line to form the anterior abdominal vein (a.ab.v.) going forward + and uniting with the (median) portal vein (p.v.) to enter the liver.] + +-The vessels are named in the references to the figure, which should +be carefully copied and mastered. Here we need only- [Comparing +with the rabbit, we would especially] call attention to the fact that the +vena cava inferior extends posteriorly only to the kidney, and that +there is a renal portal system. The blood from the hind limbs either +flows by the anterior abdominal vein to the portal vein and liver, or it +passes by the renal portal vein to the kidney. There the vein breaks +up, and we find in the frog's kidney, just as we find in the frog's and +rabbit's liver, a triple system of (a) nutritive arterial, (b) afferent* +venous and (c) efferent** venous vessels. + +* a, ad = to; +** e, ex = out of. + + +{This Section missing from Second Edition.} +-Section 11. It is not very improbable that the kidney of the frog +shares, or performs, some of the functions of the rabbit's liver, or +parallel duties, in addition to the simply excretory function. Since +specialization of cells must be mainly the relatively excessive +exaggeration of some one of the general properties of the +undifferentiated cell, it is not a difficult thing to imagine a gradual +transition, as we move from one organism to another, of the functions +of glands and other cellular organs. It is probable that the mammalian +kidney is, physiologically, a much less important (though still quite +essential) organ than the structures which correspond to it in position +and development in the lower vertebrate types.- + + +Section 12. The lymphatic system is extensively developed in the +frog, but, in the place of a complete system of distinctly organized +vessels, there are great lymph sinuses (compare Section 1). In Figure +5, Sheet 12, the position of two lymph hearts (l.h., l.h.) which pump +lymph into the adjacent veins, is shown. + + +Section 13. The skull of the frog will repay a full treatment, and will +be dealt with by itself later. The vertebral column (Sheet 12) +consists of nine vertebrae, the centra of which have faces, not flat, but +hollow in front (pro-coelous), and evidently without epiphyses +(compare the Rabbit). The anterior is sometimes called the atlas, but +it is evidently not the homologue of the atlas of the rabbit, since the +first spinal nerve has a corresponding distribution to the twelfth cranial +of the mammal, and since, therefore, it is probable that the +mammalian skull = the frog's skull + one (or more) vertebrae +incorporated with it. Posteriorly the vertebral column terminates in the +urostyle, a calcified unsegmented rod. The vertebrae have transverse +processes, but no ribs. + + +Section 14. The fore-limb (Figure 6, Sheet 12) consists of an upper +segment of one bone, the humerus, as in the rabbit; a middle section, +the radius and ulna, fused here into one bone, and not, as in the +mammalian type, separable; of a carpus, and of five digits, of which +the fourth is the longest. The shoulder girdle is more important and +complete than that of the higher type. There is a scapula (sc.) with an +unossified cartilaginous supra-scapula (s.sc.); the anterior border of +the scapula answers to the acromion. On the ventral side a +cartilaginous rod, embraced by the clavicle (cl.) (a membrane bone in +this type), runs to the sternum, and answers to the clavicle of the +rabbit. In the place of the rabbit's coracoid process, is a coracoid +bone (co.), which reaches from the glenoid cavity to the sternum; it is +hidden on the right side of Figure 6, which is a dorsal view of the +shoulder girdle. There is a pre-omosternum (o.st.) and a +post-omosternum, sometimes termed a xiphisternum (x.). + + +Section 15. Figure 7 shows the pelvic girdle and limb of the frog. +There is a femur (f.); tibia and fibula (t. and f.) are completely fused; +the proximal bones of the tarsus, the astragalus (as.), and calcaneum +(cal.) are elongated, there are five long digits, and in the calcar (c.) an +indication of a sixth. With considerable modifications of form, the +three leading constituents of the rabbit's pelvic girdle occur in +relatively identical positions. The greatly elongated ilium (il.) +articulates with the single (compare Rabbit) sacral vertebra (s.v. in +Figure 5). The ischium (is.) is relatively smaller than in the rabbit, and +the pubis (pu.) is a ventral wedge of unossified cartilage. The shape of +the pelvic girdle of the frog is a wide departure from that found among +related forms. In connection with the leaping habit, the ilia are greatly +elongated, and the pubes and ischia much reduced. Generally +throughout the air-frequenting vertebrata, we find the same +arrangement of these three bones, usually in the form of an inverted. +Y-- the ilium above, the ischium and pubis below, and the acetabulum +at the junction of the three. + + +Section 16. The uro-genital organs of the frog, and especially those +of the male, correspond with embryonic stages of the rabbit. In this +sex the testes (T., Sheet 13) lie in the body cavity, and are white +bodies usually dappled with black pigment. Vasa efferentia (v.e.) run +to the internal border of the anterior part of the kidney, which answers, +therefore, to the rabbit's epididymis. The hinder part of the kidney is +the predominant renal organ. There is a common uro-genital duct, into +which a seminal vesicle, which is especially large in early spring, +opens. This is the permanent condition of the frog. In the rabbit, for +urogenital duct, we have ureter and vas deferens; the testes and that +anterior part of the primitive kidney, the epididymis, shift back into the +scrotal sacs, and the ureters shift round the rectum and establish a +direct connection with the bladder, carrying the genital ducts looped +over them. The oviducts of the female do not fuse distally to form a +median vagina as they do in the rabbit. In front of the genital organ in +both sexes is a corpus adiposum (c.ad.), which acts as a fat store, +and is peculiar to the frogs and toads. The distal end of the oviduct of +the female is in the breeding season (early March) enormously +distended with ova, and the ovaries become then the mere vestiges of +their former selves. The distal end of the oviduct is, therefore, not +unfrequently styled the uterus. There is no penis in the male, +fertilisation of the ova occurring as they are squeezed out of the +female by the embracing fore limbs of the male. The male has a pad, +black in winter, shown in Figure 1, which is closely pressed against +the ventral surface of the female in copulation, and which serves as a +ready means of distinguishing the sex. + + +Section 17. The spinal cord has a general similarity to that of the +rabbit; the ratio of its size to that of the brain is larger, and the +nerves number ten pairs altogether. The first of these (sp. 1, in +Figure 2, Sheet -12- ) {First Edition error.} [13] corresponds in +distribution with the rabbit's hypoglossal nerve, a point we shall refer to +again when we speak of the skull. The second and third constitute the +brachial plexus. The last three form the sciatic plexus going to the +hind limb. + + +Section 18. The same essential parts are to be found in the brain of +both frog and rabbit, but in the former the adult is not so widely +modified from the primitive condition as in the latter. The fore-brain +consists of a thalamencephalon (th.c. and 1), which is exposed in the +dorsal view of the brain, and which has no middle commissure. The +cerebral hemispheres (c.h.) are not convoluted, do not extend back to +cover parts behind them, as they do in the rabbit, and are not +connected above the roof of the thalamencephalon by a corpus +callosum. Moreover, the parts usually regarded, as the olfactory lobes +(rh.) fuse in the middle line. The mid-brain gives rise to the third +nerve, and has the optic lobes on its dorsal side, but these are hollow, +and they are not subdivided by a transverse groove into corpora +quadrigemina, as in the rabbit. In the hind-brain the cerebellum is a +mere band of tissue without lateral lobes or flocculi, and the medulla +gives origin only to nerves four to ten; there is no eleventh nerve, and +the hypoglossal is the first spinal-- from which it has been assumed +that the rabbit's medulla equals that of the frog, plus a portion of the +spinal cord incorporated with it. The hypoglossal is very distinctly +seen on opening the skin beneath the hyoid plate. + + +Section 19. The first, second, third, and fourth cranial nerves of the +frog correspond with those of the rabbit in origin and distribution. So +do five, six and eight. The seventh nerve forks over the ear-drum-- the +larger branch emerging behind it and running superficially, as shown +in Figure 4. There is also a deeper palatine branch of VII. (P.) running +under V2 and V3 below the orbit, and to be seen together with V1 and +V2 after removal of the eyeball. The ninth nerve similarly forks over the +first branchial slit of the tadpole, and evidence of the fork remains in +the frog. It is seen curving round anterior to the hypoglossal nerve, and +lying rather deeper in dissection. The vagus (tenth) nerve is distributed +to heart, lungs, and viscera-- in the tadpole it also sends for forking +branches over the second, third, and fourth branchial slits. It lies +deeper than IX., and internal to the veins, and runs close beside the +cutaneous artery. Most of these nerves are easily dissected and no +student should rest satisfied until he has actually seen them. + + +Section 20. The sympathetic chain is closely connected with the +aorta. It is, of course, paired, and is easily found in dissection by +lifting the dorsal aorta and looking at its mesentery. In the presence of +ganglia corresponding to the spinal nerves, and of rami +communicantes, it resembles that of the rabbit. + + +Section 21. The whole of this chapter is simply a concise +comparison, of frog and rabbit. In addition to reading it, the student +should very carefully follow the annotations to the figures, and should +copy and recopy these side by side with the corresponding diagrams +of the other types. + + + +2. _The Skull of the Frog (and the vertebrate skull +generally)_ + +Section 22. We have already given a description of the mammalian +skull, and we have stated where the origin of the several bones was in +membrane, and where in cartilage; but a more complete +comprehension of the mammalian skull becomes possible with the +handling of a lower type. We propose now, first to give some short +account of the development and structure of the skull of the frog, +and then to show briefly how its development and adult arrangement +demonstrate the mammalian skull to be a fundamentally similar +structure, complicated and disguised by further development and +re-adjustment. + + +Section 23. Figure 1,I. Sheet 14, shows a dorsal view of a young +tadpole cranium; the brain has been removed, and it is seen that it +was supported simply upon two cartilaginous rods, the trabeculae +cranii (tr.c.). Behind these trabeculae comes the notochord (n.c.), and +around its anterior extremity is a paired tract of cartilage, the +parachordals (p.c.). These structures, underlying the skull, are all that +appear[s] at first of the brain box. In front, and separate from the +cranium, are the nasal organs (n.c.); the eyes lie laterally to the +trabeculae, and laterally to the parachordals are two tracts of cartilage +enclosing the internal ear, the otic capsules. + + +Section 24. Figure 1, II., is a more advanced, phase of the same +structures. The trabeculae have met in front and sent forward a +median (c.t.) and lateral parts (a.o.) to support the nasal organs. They +have also flattened, out very considerably, and have sent up walls on +either side of the brain to meet above it and form an incomplete roof +(t.) over it. The parachordals have similarly grown up round, the +hind-brain and formed a complete ring, the roof of which +is indicated, by b. Further, the otic capsules are fusing with the +brain-case. With certain differences of form these elements-- the +trabeculae, the parachordals, and the otic capsules, are also the first +formed structures of the mammalian cranium. + + +Section 25. In Figures 1,I. and II., there appears beneath the eye a +bar of cartilage (p.p.), the palato-pterygoid cartilage, which is also to +be seen from the side in Figures 8,I. and III. It will be learnt from these +latter that this bar is joined in front to the cranium behind the nasal +organ, and behind to the otic capsule. The cartilaginous bar from the +palato-pterygoid to the otic capsule is called the quadrate, and at the +point of junction, at the postero-ventral angle of the palato-pterygoid, +articulates with the cartilaginous bar which is destined to form the +substratum of the lower jaw-- Meckel's cartilage (M.c. in Figure 8,I.). + + +Section 26. Figure 2 shows a dorsal view of these structures in a +young frog. The parts corresponding to these in 1,II. will be easily +made out, but now ossification has set in at various points of this +cartilaginous cranium. In front of the otic capsule is the paired +pro-otic bone (p.o.); behind it at the sides of the parachordal ring is +the paired ex-occipital (e.o.); in front of the cranium box, and behind +the nasal capsules, is a ring of bone, the (median, but originally +paired) sphenethmoid (s.e.). -A paired ossification appears in the +palato-pterygoid cartilage the pterygoid bone (pt.), while- A splint of +bone, the quadrato-jugal, appears at the angle of articulation with the +lower jaw. These are all the cartilage bones that appear in the cranium +and upper jaw of the frog. + + +Section 27. But another series of bones, developed first chiefly in +dermal connective tissue, and coming to plate over the cranium of +cartilage, are not shown in Figure 2. They are, however, in Figure 3. +These membrane bones are: along the dorsal middle line, the +parieto-frontals (p.f.), originally two pairs of bones which fuse in +development, and the nasals (na.). Round the edge of the jaw, and +bearing the teeth, are pre-maxillae (p.m.), and maxillae (mx.), and +overlying the quadrate cartilage and lateral to the otic capsules are +the T-shaped squamosal bones (sq.). In the ventral view of the skull +(Figure 4) we see a pair of vomers (vo.) bearing teeth, a pair of +palatines (pal.), [and a pair of pterygoids (pt.)] (which [palatines and +pterygoids, we may note,] unlike those of the rabbit, are -stated to be- +membrane bones), and a great median dagger-shaped para-sphenoid +(p.sp.). These two Figures, and 5, which shows the same bones in +side view, should be carefully mastered before the student proceeds +with this chapter. The cartilage bones are distinguished from +membrane bones by cross-shading. + + +Section 28. Turning now to Figure 8,I., we have a side view of a +tadpole's skull. On the ventral side of the head is a series of vertical +cartilaginous bars, the visceral arches supporting the walls of the +tadpole's gill slits. The first of these is called the hyoid arch (c.h.), +and the four following this, the first (br.1), second, third, and fourth +(br.4), branchial arches. Altogether there are four gill slits and between +the hyoid arch and the jaw arch, as it is called (= Meckel's cartilage + +the palato-pterygoid), is "an imperforate slit," which becomes the +ear-drum.* The frog no longer breathes by gills, but by lungs, and the +gills are lost, the gill slits closed, and the branchial arches +consequently much reduced. Figures 8, II., and 8, III., show stages in +this reduction. The hyoid arch becomes attached, to the otic capsule, +and its median ventral plate, including also the vestiges of the first, +second, and fourth branchial arches, is called the hyoid apparatus. In +Figure 5, the apparatus is seen from the side; c.h. is called the (right) +anterior cornu** of the hyoid. The function of the hyoid apparatus in +the frog is to furnish, a basis of attachment to the tongue muscles; it +remains cartilaginous, with the exception of the relic of one branchial +arch, which ossifies as the thyro-hyal (Figure 7 th.h.). It will be noted +that, as development proceeds, the angle of the jaw swings backward, +and the hyoid apparatus, shifts relatively forward. These changes of +position are indicated in Figure 8, III., by little arrow-heads. + +* We may note here that, comparing the ear of the frog with that of the +rabbit, there is no external ear. There is, moreover, no bulla supporting +the middle ear, and the tympanic membrane stretches between the +squamosal in front and the anterior cornu of the hyoid behind. A +rod-like columella auris replaces the chain of ear ossicles, and may, +or may not, answer to the stapes alone, or even possibly to the entire +series. In the internal ear there is no cochlea, and the otic mass is +largely cartilaginous instead of entirely bony. + +** Plural cornua. + + +Section 29. Before proceeding to the comparison of the mammalian +skull with this, we would strongly recommend the student thoroughly +to master this portion of the work, and in no way can he do this more +thoroughly and quickly than by taking a parboiled frog, picking off the +skin, muscle, and connective tissue from its skull, and making out the +various bones with the help of our diagrams. + + +Section 30. Figure 9 represents, in the most diagrammatic way, the +main changes in form of the essential constituents of the cranio-facial +apparatus, as we pass from the amphibian to the mammalian skull. F. +is the frog from the side and behind; b.c. is the brain-case, o.c. the +otic capsule, e. the eye, n.c. the nasal capsule, p.p. the +palato-pterygoid cartilage, mx. the maxillary membrane bones, sq. +the squamosal, and mb. the mandible. The student should compare +with Figure 5, and convince himself that he appreciates the +diagrammatic rendering of these parts. Now all the distinctive +differences in form, from this of the dog's skull (D.), are reducible to +two primary causes-- + + (1) The brain is enormously larger, and the brain-case is vastly + inflated, so that-- + + (a) the otic capsule becomes embedded in the brain-case wall; + + (b) the palato-pterygoid rod lies completely underneath the + brain-case instead of laterally to it; + + (c) the squamosal tilts down and in, instead of down and out, + and the lower jaw articulates with its outer surface instead + of below its inner, and, moreover, with the enormous distention + of the brain-case it comes about that the squamosal is + incorporated with its wall. + + (2) The maxilla anteriorly and the palatine posteriorly send down + palatine plates that grow in to form the bony palate, cutting off + a nasal passage (n.p.) from the mouth cavity (m.p.), and carrying the + posterior nares from the front part of the mouth, as they are in the + frog, to the pharynx. Hence the vomers of the dog lie, not in the + ceiling of the mouth, but in the floor of this nasal passage. + + +Section 31. The quadrate cartilage of the frog is superseded by the +squamosal as the suspensorium of the lower jaw. It is greatly +reduced, therefore; but it is not entirely absent. In the young mammal, +a quadrate cartilage can be traced, connected with the +palato-pterygoid cartilage, and articulating with Meckel's cartilage. Its +position is, of course, beneath the squamosal, and just outside the +otic capsule. As development proceeds, the increase in size of the +quadrate, does not keep pace with that of the skull structures. It loses +its connection with the palato-pterygoid, and apparently ossifies as a +small ossicle-- the incus of the middle ear. A small nodule of +cartilage, cut off from the proximal end of Meckel's cartilage, becomes +the malleus. The stapes would appear to be derived from the hyoid +arch. Hence these small bones seem to be the relics of the discarded +jaw suspensorium of the frog utilized in a new function. Considerable +doubt, however, attaches to this interpretation-- doubt that, if anything, +is gaining ground. + + +Section 32. The tympanic bulla of the dog is not indicated in Diagram +9, and it would appear to be a new structure (neomorph), not +represented in the frog. + + +Section 33. Besides these great differences in form, there are +important differences in the amount and distribution of centres of +ossification of the skull of frog and mammal. There is no +parasphenoid in the mammal*; and, instead, a complete series of +ossifications, the median-, basi-, and pre-sphenoids, and the lateral +ali- and orbito-sphenoids occur. The points can be rendered much +more luminously in a diagram than in the text, and we would counsel +the student to compare this very carefully with that of the Rabbit. + +* Faint vestigeal indications occur in the developing skulls of some +insectivora. + + +Section 34. + + -Cranium_ + + -Nasal_ (paired), -Vomer_ (paired) + -Fronto-Parietal_, Sphenethmoid Bone (median), Eye, Pro-otic Bone, + Otic Cartilage, Ex-occipital (paired) + -Para-sphenoid Bone_ + + -Upper Jaw_ + + -Pre-Maxilla_ (paired), -Palatine_ (paired), Pterygoid (paired), + -Squamosal_, Quadrate Cartilage {To 1.} + -Maxilla_ + 1. Quadrato-Jugal + + -Lower Jaw_ + + Mento-meckelian, -Dentary_, -Articulare- [-Angulo Splenial_] + + +Section 35. -Points especially- [Additional points] to be noticed are: + + (1) The otic capsule (= periotic bone) of the dog ossifies from a + number of centres, one of which is equivalent to the frog's prootic. + + (2) The several constituents of the lower jaw are not to be + distinguished in the adult mammal. + + (3) The frog has no lachrymal bone. + + +Section 36. We are now in a position to notice, without any danger of +misconception, what is called the segmental theory of the skull. Older +anatomists, working from adult structure only, conceived the idea that +the brain-case of the mammal represented three inflated vertebrae. +The most anterior had the pre-sphenoid for its body, the +orbito-sphenoids for its neural processes, and the arch was +completed above by the frontals (frontal segment). Similarly, the +basi-sphenoids, ali-sphenoids, and parietals formed a second arch +(parietal segment), and the ex-, basi-, and supra-occipitals a third +(occipital segment). If this were correct, in the frog, which is a more +primitive rendering of the vertebrate plan, we should find the vertebral +characters more distinct. But, as a matter of fact, as the student will +perceive, frontal segment, parietal segment, and occipital segment, +can no longer be traced; and the mode of origin from trabeculae and +para-chordals show very clearly the falsity of this view. The vertebrate +cranium is entirely different in nature from vertebrae. The origin of the +parietals and frontals as paired bones in membrane reinforces this +conclusion. + + +Section 37. But as certainly as we have no such metameric +segmentation, as this older view implies, in the brain-case of the frog, +so quite as certainly is metameric segmentation evident in its +branchial arches. We have the four gill slits of the tadpole and their +bars repeating one another; the hyoid bar in front of these is evidently +of a similar nature; and that the ear drum is derived from an +imperforate gill slit is enforced by the presence of an open slit (the +spiracle) in the rays and dog-fish in an entirely equivalent position. +Does the mouth answer to a further pair of gill slits, and is the jaw +arch (palato-pterygoid + Meckel's cartilage) equivalent to the arches +that come behind it? This question has been asked, and answered in +the affirmative, by many morphologists, but not by any means by all. +The cranial nerves have a curious similarity of arrangement with regard +to the gill slits and the mouth; the fifth nerve forks over the mouth, the +seventh forks over the ear drum, the ninth, in the tadpole and fish, +forks over the first branchial slit, and the tenth is, as it were, a leash +of nerves, each forking over one of the remaining gill slits. But this +matter will be more intelligible when the student has worked over a +fish type, and need not detain us any further now. + + +Section 38. See also Section 13 again, in which is the suggestion +that the occipital part of the skull is possibly a fusion of vertebrae, a +new view with much in its favour, and obviously an entirely different +one from the old "segmental" view of the entire skull, discussed in +Section 36. + + +_Questions on the Frog_ + + [All these questions were actually set at London University + Examinations.] {In Both Editions.} + +1. Give an account, with illustrative sketches, of the digestive organs +of the common frog, specifying particularly the different forms of +epithelium met with in the several regions thereof. + +2. Describe the heart of a frog, and compare it with that of a fish and +of a mammal, mentioning in each case the great vessels which open +into each cavity. + +3. Compare with one another the breathing organs and the +mechanism of respiration in a frog and in a rabbit. Give figures +showing the condition of the heart and great arteries in these animals, +and indicate in each case the nature of the blood in the several +cavities of the heart. + +4. Draw diagrams, with the parts named, illustrating the arrangement +of the chief arteries of (a) the frog, (b) the rabbit. (c) Compare briefly +the arrangements thus described. (d) In what important respects does +the vascular mechanism of the frog differ from that of the fish, in +correlation with the presence of lungs? + +5. In the frog provided, free the heart, both aortic arches, dorsal aorta +as far as its terminal bifurcation, and both chains of sympathetic +ganglia from surrounding structures; and remove them, in their +natural connection, from the animal into a watch-glass. + +6. Describe the male and female reproductive organs of the common +frog, and give some account of their development. + +7. Describe, with figures, the bones of the limbs and limb-girdles of a +frog. + +8. Remove the brain from the frog provided, and place it in spirit. Make +a lettered drawing of its ventral and dorsal surfaces. + +9. Point out the corresponding regions in the brain of a frog and a +mammal, and state what are the relations of the three primary +brain-vesicles to these regions. + +10. (a) Give an account, with diagrams, of the brain of the frog; (b) +point out the most important differences between it and the brain of +the rabbit. (c) Describe the superficial origin and the distribution of the +third, (d) of the fifth, (e) of the seventh., (f) of the ninth, and +(g) of the tenth cranial nerves of the frog. + +11. Describe, with figures, the brain of a frog, and compare it with that +of a rabbit. What do you know concerning the functions of the several +parts of the brain in the frog? + +12. Describe briefly the fundamental properties of the spinal cord in +the frog. By what means would you determine whether a given nerve +is motor or sensory? + +13. Prepare the skull of the frog provided. Remove from it and place in +glycerine on a glass slip the fronto-parietal and parasphenoid bones. +Label them. Mark on the skull with long needles and flag-labels the +sphenethmoid and the pro-otic bones. + +14. Compare the skull of the rabbit and the frog; especially in regard +to the attachment of the jaw apparatus to the cranium, and other +points which distinctly characterize the higher as contrasted with the +lower vertebrata. + +15. Describe the skeleton of the upper and lower jaw (a) in the frog, +(b) in the rabbit. Point out exactly what parts correspond with one +another in the two animals compared. (c) What bone in the rabbit is +generally regarded as corresponding to the quadrate cartilage of the +frog? + + + +-The Dog-Fish_ + +1. _General Anatomy_ + +Section 1. In the dog-fish we have a far more antique type of +structure than in any of the forms we have hitherto considered. +Forms closely related to it occur among the earliest remains of +vertebrata that are to be found in the geological record. Since the +immeasurably remote Silurian period, sharks and dog-fish have +probably remained without any essential changes of condition, and +consequently without any essential changes of structure, down to +the present day. Then, as now, they dominated the seas. They +probably branched off from the other vertebrata before bone had +become abundant in the inner skeleton, which is consequently in their +case cartilaginous, with occasional "calcification" and no distinct +bones at all. Unlike the majority of fish, they possess no swimming +bladder-- the precursor of the lungs; but in many other respects, +notably in the uro-genital organs, they have, in common with the +higher vertebrata, preserved features which may have been disguised +or lost in the perfecting of such modern and specialized fish as, for +instance, the cod, salmon, or herring. + + +Section 2. Comparing the general build, of a dog-fish with that of a +rabbit, we notice the absence of a distinct neck, and the general +conical form; the presence of a large tail, as considerable, at first, in +diameter as the hind portion of the body, and of the first importance in +progression, in which function the four paddle-shaped limbs, the +lateral fins, simply co-operate with the median fin along the back for +the purpose of steering; and, as a consequence of the size of the +tail, we note also the ventral position of the apertures of the body. The +anus, and urinary and genital ducts unite in one common chamber, +the cloaca. Behind the head, and in front of the fore fin (pectoral fin), +are five gill slits (g.s.) leading from the pharynx to the exterior. Just +behind the eye is a smaller and more dorsal opening of the same +kind, the spiracle (sp.). On the under side of the head, in front of the +mouth, is the nasal aperture (olf.), the opening of the nasal sac, +which, unlike the corresponding organ of the air-frequenting vertebrata, +has no internal narial opening. There is, however, a groove running +from olf. to the corner of the mouth, and this, closing, in the vertebrate +types that live in air and are exposed to incessant evaporation of their +lubricating secretions, constitutes the primitive nasal passage. The +limbs are undifferentiated into upper, lower, and digital portions, and +are simply jointed, flattened expansions. + + +Section 3. The skin of the dog-fish is closely set with pointed +tooth-like scales, the placoid scales, and these are continued over +the lips into the mouth as teeth. Each scale consists of a base of true +bone, with a little tubercle of a harder substance, dentine, capped by +a still denser covering, the enamel. The enamel is derived from the +outer layer of the embryonic dog-fish, the epiblast, which also gives +rise to the epidermis; while the dentine and bony base arise in the +underlying mesoblast, the dermis. A mammalian tooth has +essentially the same structure: an outer coat of enamel, derived from +epiblast, overlies a mass of dentine, resting on bone, but the dentine +is excavated internally, to form a pulpcavity containing blood-vessels +and nerves. Most land animals, however, have teeth only in their +mouths, and have lost altogether the external teeth which constitute +the armour of the dog-fish. Besides the teeth there perhaps remain +relics of the placoid scales in the anatomy of the higher vertebrata, in +the membrane bones. How placoid scales may have given rise to +these structures will be understood by considering such a bone as +the vomer of the frog. This bone lies on the roof of the frog's mouth, +and bears a number of denticles, and altogether there is a very strong +resemblance in it to a number of placoid scales the bony bases of +which have become confluent. In the salamander, behind the +teeth-bearing vomers comes a similar toothed parasphenoid bone. +The same bone occurs in a corresponding position in the frog, but +without teeth. In some tailed amphibians the vomers and splenials are +known to arise by the fusion of small denticles. These facts seem to +point to stages in the fusion of placoid bases, and their withdrawal +from the surface to become incorporated with the cranial apparatus as +membrane bones, a process entirely completed in the mammalian +type. + + +Section 4. The alimentary canal of the dog-fish, is a simple tube +thrown into a Z shape. The mouth is rough with denticles, and has a +fleshy immovable tongue on its floor. In the position of the +Eustachian tube there is a passage, the spiracle (sp.), running out to +the exterior just external to the cartilage containing the ear. The +pharynx communicates with the exterior through five gill slits (g.s.), +and has, of course, no glottis or other lung opening. There is a wide +oesophagus passing into a U-shaped stomach (st.), having, like the +rabbit's, the spleen (sp.) on its outer curvature. There is no coiling +small intestine, but the short portion, receiving the bile duct (b.d.) +and duct of the pancreas (pan.), is called the duodenum (d'dum.). The +liver has large left (L.lv.) and right lobes, and a median lobe (M.lv.), in +which the gall bladder (g.bl.) is embedded. The next segment of the +intestine is fusiform, containing a spiral valve (Figure 4), the shelf of +which points steeply forward; it is sometimes called the colon (co.). It +is absorptive in function and probably represents morphologically, as +it does physiologically, the greater portion of the small intestine. A +rectal gland (r.g.) opens from the dorsal side into the final portion of +the canal (rectum). + + +Section 5. The circulation presents, in many respects, an +approximation to the state of affairs in the developing embryos of the +higher types. The heart (Figure 3, Sheet -14- {Error in First Edition} +[16]) is roughly, Z shape, and transmits only venous blood. It lies in a +cavity, the pericardial cavity (P.c.c.), cut off by a partition from the +general coelome. At one point this partition is imperfect, and the two +spaces communicate through a pericardio-peritoneal canal (p.p.c.), +which is also indicated by an arrow (p.p.) in the position and direction +in which the student, when dissecting, should thrust his "seeker," in +Figure 1 Sheet 15. A sinus venosus (s.v. in Figure 3, Sheet 16) +receives the venous trunks, and carries the blood through a valve into +the baggy and transversely extended -auricle- [atrium] (au.), whence it +passes into the muscular ventricle (Vn.), and thence into the truncus +arteriosus. This truncus consists of two parts: the first, the conus or +pylangium (c.a.), muscular, contractile, and containing a series of +valves; the second, the bulbus or synangium (b.a.), without valves and +pulsatile. In the rabbit both sinus and truncus are absent, or merged in +the adjacent parts of the heart. + + +Section 6. From the bulbus there branch, on either side, four arterial +trunks, the first of which forks, so that altogether there are five +afferent branchials (a.br.) taking blood to be aerated in the gills, here +highly vascular filamentary outgrowths of the internal walls of the gill +slits. + + {Lines from Second Edition only.} + [There are altogether nine vascular outgrowths (demi-branchs), one + on each wall of each gill slit except the last, on the hind wall of + which there is none. (In the spiracle is a miniature demibranch, the + pseudo-branch. This suggests that the spiracle is really a somewhat + modified gill slit.)] + +Four efferent branchials (e.br.) carry the aerated blood on to the +dorsal aorta (d.ao.). A carotid artery runs forward to the head, and a +hypo-branchial artery supplies the ventral side of the pharyngeal +region. There are sub-clavian, coeliac, mesenteric, and pelvic +arteries, and the dorsal aorta is continued through the length of the +tail as the caudal artery (Cd.A.). + + +Section 7. A caudal vein (Cd.V.), bringing blood back from the tail, +splits behind the kidneys (K.), and forms the paired renal portal +veins (r.p.v.), breaking up into a capillary system in the renal organ. A +portal vein brings blood from the intestines to the liver. + + +Section 8. Instead of being tubular vessels, the chief veins of the +dog-fish are, in many cases, irregular baggy sinuses. Three main +venous trunks flow into the sinus venosus. In the median line from +behind comes the hepatic sinus (H.S.); and laterally, from a dorsal +direction, the Cuvierian sinuses (C.S.) enter it. These, as the student +will presently perceive, are the equivalents of the rabbit's superior +cavae. They receive, near their confluence with the sinus venosus, the +inferior jugular vein (I.J.V.). At their dorsal origin, they are formed by +the meeting of the anterior (A.C.S.) and posterior (P.C.S.) cardinal +sinuses. The anterior cardinal sinus -is, roughly, the equivalent of the +internal jugular vein-, lies along dorsal to the gill slits (g.s.), and +receives an orbital sinus from the eye. The posterior cardinal sinus +receives a sub-clavian vein (s.c.v.) and a lateral vein (L.V.), and fuses +posteriorly with its fellow in the middle line. This median fusion is a +departure from the normal fish type. It must not be confused with the +inferior cava, which is not found in the dog-fish, the [right] posterior +cardinals representing the rabbit's azygos vein. A simplified diagram of +the circulation of a fish is given in Figure 2, Sheet 16, and this should +be carefully compared with the corresponding small figure given of the +vascular system of our other types. + + {Lines from Second Edition only.} + [The blood of the dog-fish resembles that of the frog.] + + +Section 9. The internal skeleton, as we have said, is entirely +cartilaginous, and only those parts which are pre-formed in cartilage in +the skeletons of the higher types are represented here. The spinal +column consists of two types of vertebrae, the trunk, bearing short, +distinct, horizontally-projecting ribs (r.), and the caudal. The +diagrams of Figure 5 [(Sheet 18)] are to illustrate the structure of the +centrum of a dog-fish vertebra; C is a side view, D a horizontal median +section, A and B are transverse sections at the points indicated by -B +and A- [A and B] respectively in Figure C. -(By an unfortunate slip of +the pen in the figure, A was substituted for B; section A corresponds +to line B, and vice versa.)- The vertebrae are hollowed out both +anteriorly and posteriorly (amphi-coelous), and a jelly-like notochord +runs through the entire length of the vertebral column, being +constricted at the centres of the centra, and dilated between them. +The neural arch above the centrum, and containing the spinal cord, is +made up of neural plates (n.p.), and interneural plates (i.n.p.), +completed above by a median neural spine (n.s.). In the caudal region, +instead of ribs projecting outwardly, there are haemal processes, +inclined downwards and meeting below, forming an arch, the haemal +arch, containing the caudal artery and vein-- the vein ventral to the +artery-- and resembling the neural arch, which contains the spinal +cord above, in shape and size. + + +Section 10. The pectoral limb and girdle (Figure 4, Sheet 16) have +only a very vague resemblance to the corresponding structures in the +rabbit. The girdle (g.) is a transverse bar lying ventral to the +pericardial wall, and sending up a portion (sc.), dorsal to the attachment +of the limb, which answers to the scapula and supra-scapula of the forms +above the fish. Three main cartilages, named respectively the +pro- (p.p.), meso- (m.p.), and meta-pterygium, form the base of the +limb. With these, smaller cartilaginous plates, rods, and nodules +articulate, and form a flattened skeletal support for the fin. + + +Section 11. The pelvic girdle and limb (Figure 2, Sheet 15) are +similar in structure, but the pro-pterygium and meso-pterygium are +absent, and the cartilage answering to the meta-pterygium goes by +the name of the basi-pterygium. In the male, but not in the female, the +pelvic fins are united behind the cloaca, and there are two stiff grooved +copulatory organs, the claspers (cl. in Figure 1), which have a +cartilaginous support (cl.c.). These claspers form the readiest means +of determining the sex of a specimen before dissection. + + +Section 12. The skull consists of a cartilaginous cranium, and of jaw +and visceral arches. The cranium persists throughout life, in what +closely resembles a transitory embryonic condition of the higher +types. There is a nasal capsule (na.c.), a brain case proper, and +lateral otic (auditory) capsules (ot.c.) containing the internal ear. +(This should be compared with the frog's embryonic skull.) The upper jaw +has a great bar of cartilage, the palato-pterygoid, as its sole support; +the arch of premaxilla, maxilla, jugal, and squamosal-- all membrane +bones-- is, of course, not represented. In the frog this bar of cartilage +is joined directly to the otic capsule by a quadrate portion, but this is +only doubtfully represented in the dog-fish by a nodule of cartilage in +the pre-spiracular ligament (p.s.). The lower jaw is supported, by +Meckel's cartilage (M.C.). The hyoid arch consists of two main +masses of cartilage, the hyomandibular (h.m.), and the ceratohyal +(c.h.); the former of these is tilted slightly forward, so that the gill +slit between it and the jaw arch is obliterated below, and the cartilage +comes to serve as the intermediary in the suspension of the jaw from +the otic mass. There are five branchia[l] arches, made up pharyngo-, +epi- and cerato-branchials, and the ventral elements fuse in the +middle line to form a common plate of cartilage. Outside these arches +are certain small cartilages, the extra branchials (ex.b.) which, +together with certain small labials by the nostrils and at the sides of +the gape, probably represent structures of considerably greater +importance in that still more primitive fish, the lamprey. The deep +groove figured lateral to the otic capsule is the connecting line of the +orbital and anterior cardinal sinuses; the outline of the anterior +cardinal sinus in this figure and in Figure 1 is roughly indicated by a +dotted line. + + +Section 13. Figure 3a is a rough diagram of the internal ear-- the only +auditory structure of our type (compare Rabbit, Sheet 7). To dissect +out the auditory labyrinth without injury is a difficult performance, but +its structure may be made out very satisfactorily by paring away +successive slices of the otic mass. Such a section is shown by +Figure 3b; through the translucent hyaline cartilage the utriculus and +horizontal canal can be darkly seen. The ductus endolymphaticus +(vide Rabbit) is indicated by a dotted line in our figure. It is situated +internal to the right-angle between the two vertical canals, and +reaches to the surface of the otic capsule. + + +Section 14. The brain shows the three primary vesicles much more +distinctly than do our higher types. The fore-brain has large laterally +separated olfactory lobes (rh.), there are relatively small +"hemispheres" (pr.c.), the stalk of the pineal gland tilts forward, and +the gland itself is much nearer the surface, being embedded in the +cartilage of the brain case, and the pituitary body is relatively very +large, and has lateral vascular lobes on either side. Following the +usual interpretation of the parts, we find optic lobes (op.l.) as the roof +of the mid-brain, and behind a very large, median, hollow, +tongue-shaped cerebellum (c.b.). The medulla is large, and certain +lateral restiform tracts (r.t.) therein, which also occur in the higher +types, are here exceptionally conspicuous. + + +Section 15. The dog-fish has ten pairs of cranial nerves, +corresponding to the anterior ten of the rabbit very closely, when we +allow for the modification the latter has suffered through the +conversion of some part of the spiracular cleft to an eardrum, and the +obliteration of the post-hyoid branchial slits. + +The first and second nerves are really brain lobes, and nerves of the +special senses of smell and sight respectively. + +The third (oculomotor), the fourth (patheticus), and the sixth +(abducens) are distributed to exactly the same muscles of the eyeball +as they are in the rabbit. + +The fifth nerve, has, in the dog-fish, as in the rabbit, three chief +branches. V.2 and V.3 fork over the mouth just as they do in the +mammal; V.1 passes out of the cranium by a separate and more +dorsal opening, and runs along a groove along the dorsal internal wall +of the orbit, immediately beneath a similar branch of VII., which is not +distinct in the rabbit. The grooves are shown in the figure of the +cranium, Sheet 18; the joint nerve thus compounded of V. and VII. is +called the ophthalmic (oph.). It is distributed to the skin above the +nose and orbit. 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 substance, and on removing the tough skin +over this region and over the centre of the skull he finds, lying beneath +it, a quantity of coiling simple tubuli full of such yellowish matter. +These tubuli open on the surface by small pores, and the nerves +terminate in hair-like extremities in their lining. These sense tubes +are peculiar to aquatic forms; allied structures are found over the head +and along a lateral line (see below) in the tadpole, but when the frog +emerges from the water they are lost. They, doubtless, indicate some +unknown sense entirely beyond our experience, and either only +possible or only necessary when the animal is submerged. + +In addition to the ophthalmic moiety mentioned above, the seventh +nerve has a vidian branch (vid.) running over the roof of the mouth, and +besides this its main branches fork over the spiracle, just as V. forks +over the mouth, and as IX. and X. fork over gill clefts. This nerve in the +rabbit is evidently considerably modified from this more primitive +condition. + +The eighth is the auditory nerve, as in the rabbit. + +The ninth nerve forks over the first branchial cleft. + +The tenth nerve is easily exposed by cutting down through the body +wall muscles over the gill clefts, into the anterior cardinal sinus +(A.C.S.). It gives off (a) branches forking over the posterior four gill +slits, (b) a great lateral nerve running inward, and back through the +body-wall muscle, and connected with a line of sense organs similar +to those in the head, the lateral line, and (c) a visceral nerve curving +round to the oesophagus and stomach. In dissection it becomes very +evident that the tenth nerve is really a leash of nerves, each one +equivalent to the ninth. + +We may here call the attention of the reader to the fact of the singular +resemblance of V., VII., IX., and the factors of X. That each has a +ventral fork, we have already noticed. Each also (?IX.) has a dorsal +constituent connected with the sense organs of the skin. The vidian +branch of VII., however, is not evidently represented in the others. + + +Section 16. The coelom of the dog-fish is peculiar-- among the types +we treat of-- in the possession of two direct communications with the +exterior, in addition to the customary indirect way through the oviduct. +These are the abdominal pores (a.p.) on either side of the cloaca in +either sex. They can always be readily demonstrated by probing out +from the body cavity, in the direction indicated by the arrow (a.p.) in +Figure 1, Sheet 15. They probably serve to equalize the internal and +external pressure of the fish as it changes its depth in the water, just +as the Eustachian tubes equalize the pressure on either side of the +mammal's tympanic membrane. + + +Section 17. The musculature of the dog-fish body is cut into +V-shaped segments, the point of the V being directed forward. The +segments alternate with the vertebrae, and are called myomeres. +Such a segmentation is evident, though less marked, in the body wall +muscles of the frog, and in the abdominal musculature of the rabbit +and other mammals it is still to be traced. + + +Section 18. The uro-genital organs of the female dog-fish (Figure 1, +Sheet 17) consist of an unpaired ovary (ov.), paired oviducts (o.d.), +enlarged at one point to form an oviducal gland (o.d.g.), kidneys (k.), +with ureters (ur.) uniting to form a urinary sinus (u.s.) opening into the +cloaca by a median urinary papilla separate from the oviducal +openings. The eggs contain much yolk, and, like those of the fowl, are +very large; like the fowl, too, one of the ovaries is suppressed, and it +is the right ovary that alone remains. The two oviducts meet in front of +the liver ventral to the oesophagus, and have there a common opening +by which the ova are received after being shed into the body cavity. +The eggs receive an oblong horny case in the oviduct; in the figure +such a case is figured as distending the duct at e. The testes of the +male (T. in Figure 2) are partially confluent in the middle line. They +communicate through vasa efferentia (v.e.) with the modified anterior +part of the kidney, the epididymis (ep.), from which the vas deferens +(v.) runs to the median uro-genital sinus (u.g.s.), into which the +ureters (ur.) also open. The silvery peritoneum (lining of the body +cavity) covers over the reddish kidneys, and hides them in +dissection. + + +Section 19. Figure 3, Sheet 17, is a generalized diagram of the +uro-genital organs in the vertebrata; M.L. is the middle line of the +body, G. is the genital organ, Pr. is the pronephros, or fore kidney, +a structure which is never developed in the dog-fish, but which has +functional importance in the tadpole and cod, and appears as a +transitory rudiment in the chick. A duct, which is often spoken of as +the pronephric duct (p.d.), and which we have figured under that +name, is always developed. Anteriorly it opens into the body cavity. It +is also called the Mullerian duct, and in the great majority of +vertebrata it becomes the oviduct, uniting with its fellow, in the case of +the dog-fish, ventral to the oesophagus. In the male it usually +disappears; the uterus masculinus of the rabbit is still very generally +regarded as a vestige of it. Kolliker has shown, however, that this +interpretation is improbable. Ms. is the mesonephros, some or all of +which becomes the epididymis in the male of types possessing that +organ, and is connected with G. by the vasa efferentia. Mt., the +metanephros, is, in -actual fact- [the frog], indistinguishably +continuous with Ms., and is the functional kidney, its duct +(metanephric duct) being either undifferentiated from the mesonephric +(as is the case with the frog) or largely split off from it, as in the +dog-fish, to form the ureter. + + +Section 20. The correspondence of the male organs of the dog-fish +with those of the rabbit, will be more evident if the student imagine-- + + (a) the testes, vasa efferentia, and epididymis of each side to + shift posteriorly until they reach a position on either side of + the cloaca; and + + (b) The uro-genital apertures, instead of meeting dorsally and + posteriorly to the anus, to shift round that opening and meet + anteriorly and ventrically to it. + + +Section 21. This completes our survey of this type. Except where we +have specified differences, the general plan of its anatomy follows the +lines of the other vertebrate types described. + + + +_Questions on the Dog-Fish_ + +1. Describe the alimentary canal of the dog-fish, and compare it with +that of the rabbit in detail. + +2. Compare the coelom of the dog-fish and rabbit. + +3. Draw diagrams to illustrate the course of the circulation in the +dog-fish. + +4. (a) Describe fully the heart of a dog-fish. (b) Compare it with that of +a rabbit. + +5. Give an account of the respiratory apparatus of the dog-fish. + +6. Draw diagrams of a dog-fish vertebra, and compare the centrum +with that of a rabbit. + +7. Compare the vertebral column of the dog-fish and rabbit. + +8. Draw diagrams of the limbs and limb-girdles of the dog-fish. +Compare the pectoral with the pelvic fin. + +9. Draw diagrams of (a) the male and (b) the female urogenital organs +of the dog-fish. (c) Compare them carefully with those of the rabbit. + +10. Compare the circulation in the kidney of dog-fish and rabbit. + +11. Give an account of the cranio-facial apparatus of the dog-fish. +State clearly what representation of this occurs in the frog and in the +rabbit. + +12. Give drawing (a) from above, (b) from the side, of the dog-fish +brain. + +13. State the origin and the distribution of the fifth, seventh, ninth, and +tenth cranial nerves in the dog-fish. + +14. Compare, one by one, the cranial nerves of the dog-fish with those +of any higher vertebrate, as regards their origin and their distribution. + +15. Describe the auditory organ of the dog-fish. What parts are added +to this in the higher type? + +16. Draw the cloaca (a) of a male, (b) a female dog-fish. + +17. (Practical.) Demonstrate in a dog-fish the pathetic nerve, the +opening between pericardium and coelom. the abdominal pores, and +the ureter. + + + +-Amphioxus_ + +1. _Anatomy_ + +Section 1. We find in Amphioxus the essential vertebrate features +reduced to their simplest expression and, in addition, somewhat +distorted. There are wide differences from that vertebrate plan with +which the reader may now be considered familiar. There are no +limbs. There is an unbroken fin along the median dorsal line and +coming round along the ventral middle line for about half the animal's +length. But two lowly vertebrates, the hag-fish and lamprey, have no +limbs and a continuous fin. There is, as we shall see more clearly, a +structure, the respiratory atrium, not apparently represented in the +true vertebrate types, at least in their adult stages. There is no +distinct heart, only a debateable brain, quite without the typical +division into three primary vesicles, no skull, no structures whatever of +cartilage or bone, no genital ducts, no kidneys at all resembling those +of the vertebrata, no pancreas, no spleen; apparently no sympathetic +chain, no paired sense organs, eyes, ears, or nasal sacs, in all of +which points we have striking differences from all true vertebrata; and +such a characteristic vertebrate peculiarity as the pineal gland we can +only say is represented very doubtfully by the eye spot. + + +Section 2. The vertebral column is devoid of vertebrae; it is +throughout life a rod of gelatinous tissue, the notochord (Figure 1, +n.c.), surrounded by a cellular sheath. Such a rod is precursor to the +vertebral column in the true vertebrates, but, except in such lowly +forms as the lamprey, is usually replaced, partially (e.g., dog-fish) +or wholly (as in the rabbit) by at first cartilaginous vertebrae whose +bodies are derived from its sheath. Further, while in all true vertebrata +the notochord of the developing young reaches anteriorly at most to +the mid-brain, and is there at its termination enclosed by the middle +portion of the skull, in Amphioxus it reaches far in front of the anterior +extremity of the nervous system, to the end of the animal's body.* +On this account the following classification is sometimes made of +those animals which have a notochord:-- + + -Chordata_ (= Vertebrata, as used by Lankester). + + 1. Having the notochord reaching in front of the brain. + Cephalochorda = Amphioxus. + + 2. Having the notochord reaching anteriorly to the mid-brain, a + brain of three primary vesicles and a skull. + Craniata = all "true vertebrata": fishes, amphibia, reptiles, + birds, and mammals (Vertebrata of Balfour). + + 3. Having the notochord confined to the tail. + Urochorda = the ascidians, or sea-squirts, certain forms of life + only recently recognised as relatives of the vertebrata. + +* The anterior end of the notochord in the developing rabbit or dog lies +where the middle of the basisphenoid bone is destined to be. + + +Section 3. Figure 1, Sheet 19, shows the general anatomy of +Amphioxus. We recognise four important points of resemblance to the +earlier phases of the higher and the permanent structure of the lower +members of the vertebrata, and it is these that justify the inclusion of +amphioxus in this volume. In the first place there is the-- + + -Notochord_. + In the next, just above it (at s.c.) we find-- + + -A Dorsal Tubular Nervous Axis_, + the spinal cord. Thirdly, the pharynx (ph.) is perforated by-- + + -Respiratory Slits_, + though these, instead of being straight slashes, are modified from + a U-shape [slant very much forward and are much more numerous than + in any true vertebrate.]. -And-, Fourthly, there is, as we shall + see, a-- + + -Vertebrate Type of Circulation_. + [And finally the body-wall muscles are divided into--] + + [-Myomers_.] + + +Section 4. The alimentary canal of Amphioxus commences with an +"oral cavity," not represented in our vertebrata, surrounded by a +number of cirri, or tentacles, supported by a horny substance which +seems to be chitin, a common skeletal material among invertebrates. +A velum (v.) forms a curtain, perforated by the mouth and by two +smaller hyoidean apertures, between the oral cavity and the pharynx +(ph.). "Pharyux" is here used in a wider sense than in the true +vertebrata; it reaches back close to the liver, and is therefore +equivalent to pharynx + oesophagus + a portion or all of the stomach. +The [so-called] hyoidean apertures are not equivalent to the +similarly-named parts of the vertebrata. Behind the pharynx the +intestine (int.) runs straight out to the anus (an.), which opens not in +the middle line, as one might expect, but in the left side! The liver lies +usually on the creature's right, and instead of being a compact gland, +is simply bag-like. + + +Section 5. The circulation is peculiarly reduced (Figure 2). The +cardiac aorta (c.ao.) lies along the ventral side of the pharynx, and +sends branches up along the complete bars between the gill slits. +There is no -distinct- heart, but the whole of the cardiac aorta is +contractile, and at the bases of the aortic arches that run up the bars +there are contractile dilatations that assist in the propulsion of the +blood. Dorsal to the pharynx, as in fishes, there is a pair of dorsal +aorta (d.ao.) that unite above the liver (compare the frog, for instance), +and thence run backward as a median dorsal aorta (d.ao.'). A portal +vein (p.v.) bring blood back from the intestine (and apparently from the +whole posterior portion of the animal) to the liver. Thence hepatic veins +(hep.) take it to the cardiac aorta. + + {Lines from First Edition only.} + -When we remember that in the embryonic vertebrate the heart is at + first a straight tube, this circulation appears even more strikingly + vertebrate in its character than before.- + + +Section 6. The coelom, or body cavity, of Amphioxus lies, of course, +as in the vertebrata, between the intestinal wall and the body walls, +and, just as in the vertebrata, it is largely reduced where gill slits +occur. But matters are rather complicated by the presence of an +atrial cavity round the pharynx, which is not certainly represented in +the vertebrata, and which the student is at first apt to call the body +cavity, although it is entirely distinct and different from that space. The +mutual relation of the two will become apparent after a study of +Figures 10, 11, 12 (Sheet 21). Figure 10 gives diagrammatically a +section of a very young stage of Amphioxus; P is the pharynx portion +of the alimentary canal, coe. is the coelom surrounding it at this stage +here as elsewhere; mt.c. are certain lymph spaces, the metapleural +canals, between which a small invagination (i.e., a pushing-in), at., of +the outer epidermis occurs; n.c. is the notochord, and s.c. the spinal +cord. The gill slits, by which P. communicates with the exterior, are +not shown. Next Figure 11 shows the invagination (at.) pushing its +way in, and cut off from the exterior by a meeting of the body wall +below. Note that at. is a portion of the animal's exterior thus +embraced by its body, and that its lining is therefore of the same +material as the external integument. In Figure 12, at. is developing +upward, so that the true body hangs into it. Now imagine the gill slits +perforated, as shown by the double-headed arrow in Figure 12. Figure +3, on Sheet 20, is a less diagrammatic representation of a +cross-section of the pharyngeal region (vide Figure 1, Sheet 19). The +student should compare Figure 3, Sheet 20, and Figure 12, Sheet 21. +The atrium and metapleural canals are easily recognised in both. In +Figure 3 the coelom is much cut up by the gill slits, and we have +remaining of it (a) the dorsal coelomic canals (d.c.c.) and (b) the +branchial canals (br.c.) in the bars between the slits. The atrial cavity +remains open to the exterior at one point, the atrial pore (at.p.). + + +Section 7. The method of examining cross-sections is an extremely +convenient one in the study of such a type as Amphioxus. The +student should very carefully go over and copy the six sections on +Sheet 20, comparing Figure 1 as he goes. He should do this before +reading what follows. One little matter must be borne in mind. These +figures are merely intended to convey the great structural ideas, and +they are considerably simplified; they must not be regarded as a +substitute for the examination of microscopic sections. [He will notice +a number of rounded masses from the body wall. The] -For instance, +the body-wall- muscles of Amphioxus are arranged in bundles bent +sharply in an arrow shape, the point forward. -A number of these +bundles are cut in any one section, and so the even shading of our +diagrams, if they professed to be anything more than diagrams, +should be broken up into masses.- These -bundles, we may mention-, +are called myomeres, and they are indicated in Figure 1 by lines +pointing acutely forward. [Several are consequently cut in any +transverse section (Sheet 20), and these are the rounded masses he +sees.] Similar myomeres, similarly situated, are found in fish, behind +the head, and, less obviously, they occur with diminishing importance +as the scale of the vertebrata is ascended. + + +Section 8. If we compare the nervous system of amphioxus with that +of any vertebrate, we find at once a number of striking differences. In +the first place, the skeletal covering of it, the cranium and the neural +arches of vertebrae, are represented only by a greatly simplified +connective tissue. In the next, a simple and slight anterior dilatation +alone represents the brain. A patch of black pigment anterior to this +(e.s.) may or may not be what its name implies an eye-spot. There is +a ciliated funnel, c.f. (Figure 1, Sheet 19), opening on the left side, +which has been assumed to be olfactory in its functions, and in the +mouth chamber a ciliated pit (c.p.), which may, or may not, be an +organ of taste. The ventral fissure of the spinal cord is absent. The +dorsal nerves are without ganglia, and do not come off in pairs, but +alternately, one to the left, then one to the right, one to the left, one +to the right, and so on. The ventral nerves are very short, more numerous +than the dorsal, and never unite with these latter to form mixed +nerves. + +The student will observe that here, just as in the case of the ciliated +funnel and anus, the Amphioxus is not strictly symmetrical, but +twisted, as it were, and so departs from the general rule of at least +external bilateral symmetry obtaining among the vertebrates. It +habitually lies on one side in the mud of the sea bottom, and it is +probable that this external asymmetry is due to this habit, so that +too much classificatory importance must not be attached to it. The +soles and other related fish, for instance, are twisted and +asymmetrical, through a similar specific habit, to such an extent that +both eyes lie on one side of the animal. + + +Section 9. No kidney on the vertebrate pattern is found, but the +following structures have, among others, been suggested as renal +organs:-- + + (a) Certain canals, the brown tubes of Lankester (b.t.L., Figure 2, + Sheet 19), a pair of pigmented tubes opening into the atrium at the + hind end of the pharynx, lying forward along by the dorsal coelomic + canals, and having an internal opening also. + + (b) Certain tubuli described by Weiss as situated in a series along + the upper corners of the atrial cavity, and communicating, after the + fashion, of the "nephridia" of the earthworm, with the coelom and with + the exterior (or, rather, with that portion of the animal's exterior + enclosed in by the atrial wall; compare Section 6). + + (c) The general epithelial lining of the atrium. + +The reproductive organs (Figure 4, Sheet 20, g.) are masses of +cells situated in an isolated part of the coelom in the atrial folds, and, +having no ducts, their contents must escape into the atrium by +rupture of the body-wall. Thence they escape either by gill-slits, +pharynx and mouth, or, more generally, through the atrial pore. The +animals, like all the vertebrata, are dioecious, i.e., male or female. + + +Section 10. The endostyle (end.), in Figures 3 and 4, is a ciliated +path or groove on the under side of the pharynx, which is generally +supposed to represent the thyroid gland of vertebrates. The vertebrate +thyroid, early in development, is certainly an open and long narrow +groove in the ventral side of the pharynx. The hyper-pharyngeal +groove (h.p.) has been in the past compared to the pituitary body, +but there is little doubt now that this structure is represented by the +ciliated pit. + + +Section 11. The student is advised to revise this chapter before +proceeding, and to schedule carefully the anatomical features under +the headings of (1.) distinctly vertebrate characters, (2.) characters +contrasting with the normal vertebrate structure, (3.) facts of doubtful +import, with the suggestions given in the text written against them. + + + +2. _The Development of Amphioxus_ + +Section 12. The development of amphioxus, studied completely, is at +once one of the most alluring and difficult tasks in the way of the +zoologist; but certain of its earlier and most obvious fasts may very +conveniently be taken into consideration now. + +Section 13. The phenomena of the extrusion of polar bodies and +fertilization are treated of later, and will, therefore, not be considered +now. We will start our description with an egg-cell, which has +escaped, of course, since there are no genital ducts, by rupture of the +parent, has been fertilized by the male element, and is about to +develop into a young amphioxus. It is simply a single cell, with some +power of amoeboid motion, a single nucleus and nucleolus; and in +amphioxus its protoplasm is clear and transparent. Frequently ova are +loaded with granules of food store (yolk), which enable the young +animal to go far with its development before it is hatched and has to +begin fending for itself. Such an ovum as that of our present type, +however being devoid of such yolk (alecithal = without yolk), +necessitates a very early start in life, and, for reasons too +complicated to state fully here, the development in such a case is +considered particularly instructive and primitive by zoologists. + + +Section 14. The first thing to be seen in the developing cell is a +deepening circular groove (Figure 1, Sheet 21), which divides the +ovum into two parts. Another groove then cuts at right angles to this +subdividing the two into four (Figure 2). Another groove, at right angles +to both the former, follows, making the four eight (Figure 3). And so +subdivision goes on. The whole process is called segmentation or +cleavage. + + +Section 15. At the end of segmentation we get a hollow sphere of +small cells, the cells separating from one another centrally and +enclosing a cavity as the process proceeds. This is the +blastosphere, shown diagrammatically in Figure 4, and of which an +internal view, rather truer to the facts of the case as regards shape, +is given as Figure 5. The central cavity is the segmentation cavity +(s.c.). + + +Section 16. Invagination follows (Figure 6). In this process a portion +of the blastosphere wall is the tucked into the rest, as indicated by +the arrow, so that a two-layered sack is formed. The space ar. is the +archenteron, the primordial intestine, and its mouth is called, the +blastopore (bp.). The outer layer of this double-walled sac is called +the epiblast. For the present we will give the inner lining no special +term. The young amphioxus has, at this stage, which is called the +gastrula stage, a curious parallelism with such a lowly form as the +Hydra of our ditches. This latter creature, like the gastrula, consists +essentially of two layers of cells, an outer protective and sensory +layer, and an inner digestive one; it has a primordial intestine, or +archenteron, and its mouth is sometimes regarded as being a +blastopore. All animals that have little yolk, and start early in life for +themselves, pass through a gastrula stage, substantially the same +as this of amphioxus. + + +Section 17. The anus is perforated later near the region occupied at +this stage by the blastopore. Hence the anterior end of the future +amphioxus, the head end, is pointing towards the Figure 6, and the +letters ep. are marked on the side which will be dorsal. + + +Section 18. Figure 7 i. is a dorsal view of the gastrula at a somewhat +later stage, and here indications of distinctly vertebrate relationships +already appear. Figure 7 ii. is a cross-section, its position, being +shown by cross-lines in 7 i. and 6. Note first that the epiblast along +the mid-dorsal line is sinking in to form what is called the neural +plate (n.p.), and simultaneously on either side of it rise the neural +folds (n.f.). Now, at Figure 8, a slightly later stage is represented, and +at 9 i. the inturned part is separated from the general external epiblast +as the spinal cord. The remainder of the epiblast constitutes the +epidermis. + + +Section 19. Reverting to Figure 7 ii., along the dorsal side of the +archenteron a thickening of its wall appears, and is gradually pinched +off from it to form a cellular rod, lying along under the nervous axis and +above the intestine. This is the notochord (compare Figures 8 and 9). + + +Section 20. Finally, we note two series of buds of cells, one on either +side of the archenteron in Figure 7 ii. In 8 these buds have become +hollow vesicles, growing out from it, the coelomic pouches. They are +further developed in 9; and in 9 ii., which is a diagrammatic figure, +they are indicated by dotted lines. They finally appear to (? entirely) +obliterate the segmentation cavity-- they certainly do so throughout +the body; and their cavities are in time cut off from the mesenteron, by +the gradual constriction of their openings. In this way the coelom +(body cavity) arises as a series of hollow "archenteric" outgrowths, +and ms. becomes the alimentary canal. mt.c., the metapleural +canals, probably arise subsequently to, and independently of, the +general coelomic space, by a splitting in the body-wall substance. + + +Section 21. Hence, in considering the structure of amphioxus, we +have three series of cells from which its tissues are developed:-- + + 1. The epiblast. + + 2. Walls of the coelomic pouches, which form (a) an inner lining + to the epiblast, (b) an outer coating to the hypoblast, and (c) the + mesentery (m.), by which the intestine is supported. This is the + mesoblast. + + 3. The lining of the mesenteron, or hypoblast. + +From the epiblast the epidermis (not the dermis), the nervous system +(including the nerves), and the sensory part of all sense organs are +derived. From the mesoblast the muscles, the dermis genital and +excretory organs, circulatory fluid and apparatus, any skeletal +structures; and all connective tissue are derived. The mass of the +body is thus evidently made of mesoblast. The hypoblast is the lining +of the intestine and of the glands which open into it; and +the material of the notochord is also regarded, as hypoblast. + + +Section 22. Figure 9 ii. shows all the essential points of the structure +of amphioxus. Epiblast is indicated by a line of dashes, mesoblast +by dots, and hypoblast, dark or black. The true mouth is formed late +by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and +fuses with the hypoblast, and is then perforated. The position of this +mouth is at the velum. The formation of the atrium has been +described. The metapleural folds run forward in front of the velum, as +the epipleurs (ep. in Sections 1 and 2), and form an oral hood (b.c.), +around which the tentacles appear, and which is evidently not +equivalent to the vertebrate mouth cavity, but in front of and outside +it. The anus is formed by a tucking in, the proctodaeum, similar to the +stomodaeum. + + +Section 23. The formation of the respiratory slits is complicated, and +difficult to describe, but, since investigators have still to render its +meaning apparent, it need not detain the elementary student.* + +* See Balfour's Embryology, Volume 2, and Quarterly Journal of +Microscopical Science March, 1891. + + + +_Questions on Amphioxus_ + +1. Draw diagrams, with the parts named, of the alimentary canal of (a) +amphioxus, (b) any craniate; (c) indicate very shortly the principal +structural differences between the two. + +2. Describe, with a diagram, the circulation of amphioxus. Compare it +with that of the craniata. + +3. Draw from memory transverse sections, of amphioxus (a) in the +oral region, (b) through the pharynx, (c) just anterior, and (d) just +posterior to atrial pore. + +4. Describe fully the coelom of amphioxus, and compare it with that of +the frog in regard to (a) development, (b) its relation to other organs in +the adult. + +5. Compare the atrial cavity and coelom of amphioxus. To what series +of cavities in the frog are the metapleural canals to be compared? + +6. Describe the notochord of amphioxus, and point out its differences +from the vertebrate notochord. + +7. Describe, with diagrams, the nervous system of amphioxus, and +compare its nervous axis, in detail, with that of a vertebrate. + +8. Compare the genital organs of amphioxus with those of a higher +vertebrate. + +9. What structures have been regarded, as renal organs in +amphioxus? + +10. What is a gastrula? With what lower type has the gastrula been +compared? Discuss the comparison. + + + +-Development_ + +_The Development of the Frog_ + +Section 1. We have now to consider how the body of the frog is built +up out of the egg cell, but previously to doing so we must revert to the +reproductive organs of our type. + + +Section 2. In the testes of the male is found an intricate network of +tubuli, the lining of which is, of course, an epithelium. The cells of this +epithelium have their internal borders differentiated into spermatozoa, +which, at a subsequent stage, are liberated. A spermatozoon (Figure +3, Sheet 13, sp.) is a rod-shaped cell containing a nucleus; in fact, +consisting chiefly of nucleus, with a tail, the flagellum, which is +vibratile, and forces the spermatozoon, forward by its lashing. The +spermatozoa float in a fluid which is the joint product of the testes, +anterior part of the kidney, and perhaps the prostate glands. + + +Section 3. In the ovary, the ova are formed, and grow to a +considerable size. They are nucleated cells, the nucleus going by +the special name of the germinal vesicle and the nucleolus the +germinal spot. The ova prey upon the adjacent cells as they develop. +The protoplasm of the ovum, except at that part of the surface where +the germinal vesicle lies, is packed with a great amount of food +material, the yolk granules. This yolk is non-living inert matter. An +ovum such as this, in which the protoplasm is concentrated towards +one pole, is called telolecithal. + + +Section 4. After the ovum has finished its growth, and elaborated the +yolk within itself, a peculiar change occurs in the small area free from +yolk-- the animal pole, in which the germinal vesicle lies. This +germinal vesicle divides, and one moiety is budded off from the ovum. +The ovum has, in fact, undergone cell division into a very large cell +containing most of its substance, and a small protoplasmic pimple +surrounding half of its nucleus. The disproportion is so great between +the two cells, that the phenomenon does not at first suggest the idea +of cell division, and it is usually described as the extrusion of the +first polar body. There follows a second and similar small cell, +behind the first, the second polar body. Since the nucleus of the +ovum has divided twice, it is evident that the nucleus remaining now in +the ovum is a quarter of the original nucleus. Very little protoplasm is +given off with the polar bodies; they play no further part in +development, but simply drop off and disappear. Not only in the frog's +ovum, but in all vertebrata, two polar bodies are given off in this way +before the sexual process occurs. Their exact meaning has been +widely discussed. It is fairly evident that some material is removed +from the nucleus, which would be detrimental to further developments, +and the point debated is what is the precise nature of this excreted +material. This burning question we can scarcely deal with here. + + +Section 5. But here we may point out that in all cells the function of +the nucleus appears to be to determine growth and division. It is the +centre of directive energy in the cell. + + +Section 6. Fertilization is effected by a spermatozoon meeting with +the ovum. It fuses with it, its nucleus becoming the male pro-nucleus. +This and the female pro-nucleus, left after the extrusion of the polar +cells, move towards each other, and unite to form the first +segmentation nucleus. + + +Section 7. The ovum next begins to divide. A furrow cutting deeper +and deeper divides it into two; another follows at right angles to this, +making the two four, and another equatorial furrow cuts off the animal +pole from the yolk or vegetative pole. (See Sheet 22, Figures 1, 2, +and 3.) And so segmentation (= cleavage) proceeds, and, at last, a +hollow sphere, the blastosphere (Figure 4) is formed, with a +segmentation cavity (s.c.). But, because of the presence of the yolk +at the vegetative pole of ovum, and of the mechanical resistance it +offers to the force of segmentation, the protoplasm there is not nearly +so finely divided-- the cells, that is to say, are much larger than at +the animal pole. The blastosphere of the frog is like what the +blastosphere of amphioxus would be, if the future hypoblast cells +were enormously larger through their protoplasm being diluted with +yolk. + + +Section 8. 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 yolk. In such types (e.g., amphioxus) +a part of the blastosphere wall is tucked into the rest, and a gastrula +formed by this process of invagination. In the frog (Figure 5) there is a +tucking-in, but the part that should lie within the gastrula, the +yolk-containing cells, are far larger than the epiblast (ep.) which +should, form the outer layer of cells. Hence the epiblast can only by +continual growth accommodate what it must embrace, and the +process of tucking-in is accompanied by one of growth of the epiblast, +as shown by the unbarbed arrow, over the yolk. This stage is called +the gastrula stage; ar. is the cavity of the gastrula, the archenteron; +b.p. is its opening or blastopore. Such a gastrula, formed mainly by +overgrowth of the epiblast, is called an epibolic gastrula, as +distinguished from the invaginate gastrula of amphioxus. The +difference is evidently entirely due to the presence of yolk, and the +consequent modification of invagination in the former case. + + +Section 9. Comparing the two gastrulas, 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 +protoplasm, we should have a gastrula essentially like that of the frog. + + +Section 10. Figure 6 shows a slightly later ovum than Figure 5, seen +from the dorsal side. b.p. is the blastopore. In front of that appears a +groove, the neural groove, bordered on either side by a ridge, the +neural fold (n.f.). This is seen in section in Figure 7; s.c. is the neural +groove; n.f., as before, the neural fold. The neural folds ultimately bend +over and meet above, so that s.c. becomes a canal, and is finally +separated from the epiblast to form the spinal cord. Below the neural +groove a thickening of the dorsal wall of the archenteron appears, and +is pinched off to form a longitudinal rod, the precursor of the vertebral +column, the notochord, shown in Figure 7 (n.c.), as imperfectly +pinched off. + + +Section 11. Simultaneously, on either side of the notochord appear a +series of solid masses of cells, derived mainly by cell division from the +cells of the wall of the archenteron, and filling up and obliterating the +segmentation cavity. These masses increase in number by the +addition of fresh ones behind, during development, and are visible in +the dorsal view as brick-like masses, the mesoblastic somites or +proto-vertebrae (Figure 6, i., ii., iii.). In Figure 7, these masses are +indicated by dotting. In such a primitive type as amphioxus these +mesoblastic -somites- [masses] contain a cavity, destined to be the +future body cavity, from the first. In the frog, the cavity is not at first +apparent; the mesoblast at first seems quite solid, but subsequently +what is called the splitting of the mesoblast occurs, and the body +cavity (b.c. in Figure 7) appears. The outer mesoblast, lying +immediately under the epiblast, constitutes the substance of the +somatopleur, and from it will be formed the dermis, the muscles of the +body wall, almost all the cartilage and bone of the skeleton, the +substance of the limbs, the kidneys, genital organs, heart and +bloodvessels, and, in short, everything between the dermis and the +coelom, except the nervous system and nerves, and the notochord. +The inner mesoblast, the mass of the splanchnopleur, will form the +muscle and connective tissue of the wall of the alimentary canal, and +the binding substance of the liver and other glands that open into the +canal. + + +Section 12. Figure 8 is one which we reproduce, with the necessary +changes in each plate of embryological figures given in this book, so +that the student will find it a convenient, one for the purpose of +comparison. The lines of dashes, in all cases, signify -epiblast- +[hypoblast] , the unbroken black line is -hypoblast-, [epiblast] dotting +shows mesoblast, and the shaded rod (n.c.) is the notochord. c.s. is +the spinal cord; br.1, br.2, br.3 are the three primary vesicles which +constitute the brain, and which form fore, mid, and hind brain +respectively. I. is the intestine and Y. the yolk cells that at this early +stage constitute its ventral wall. + + +Section 13. Figure 9 gives a similar diagram of a later stage, but here +the blastopore is closed. An epiblastic tucking-in at st., the +stomodaeum pre-figures the mouth; pr., the proctodaeum, is a +similar posterior invagination which will become the anus. Y., the +yolk, is evidently much absorbed. Figure 10 is a young tadpole, seen +from the side. The still unabsorbed yolk in the ventral wall of the +mesentery gives the creature a big belly. Its mouth is suctorial at this +stage, and behind it is a sucker (s.) by which the larvae attach +themselves to floating reeds and wood, as shown in the three black +figures below. + + +Section 14. We may now consider the development of the different +organs slightly more in detail, though much of this has already been +approached. The nervous system, before the closure of the neural +groove, has three anterior dilatations, the fore-, mid-, and hind-brains, +the first of which gives rise by hollow outgrowths to two pairs of lateral +structures, the hemispheres and the optic vesicles. The latter give rise +to the retina and optic nerve as described in {Development} Section +40. + + +Section 15. The hypoblastic notochord is early embraced by a +mesoblastic sheath derived from the protovertebrae. This becomes +truly cartilaginous, and at regular intervals is alternately thicker and +thinner, compressing the notochord at the thicker parts. Hence the +notochord has a beaded form within this, at first, continuous +cartilaginous sheath. 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 constricted. Hence what +remains of the notochord lies within the vertebral bodies in the frog; +while in a cartilaginous fish, such as the dog-fish, or in the embryonic +rabbit, the lines of separation appear where the notochord is thickest, +and it comes to lie between hollow-faced vertebrae. Cartilaginous +neural arches and spines, formed outside the notochordal sheath, +enclose the spinal cord in an arcade. The final phase is ossification. +As the tadpole approaches the frog stage the vertebral column in the +tail is rapidly absorbed, and its vestiges appear in the adult as the +urostyle. + + +Section 16. The development of the skull is entirely dissimilar to that +of the vertebral column. It is shown on Figures 1 and 8, Sheet 14; and +in the section devoted to the frog's skull a very complete account of +the process is given. The process of ossification is described under +the histology of the Rabbit. + + +Section 17. The origin of the circulatory and respiratory organs is +of especial interest in the frog. In the tadpole we have essentially the +necessities and organization of the fish; in the adult frog we have a +clear exposition of the structure of pigeon and rabbit. The tadpole has, +at first, a straight tubular heart, burrowed out in somatic mesoblast, +and produced forward into a truncus arteriosus. From this arise four +afferent branchial arteries, running up along the sides of the four +branchial arches, and supplying gills. They unite above on either side +in paired hyper-branchial arteries, which meet behind dorsal to the +liver, to form a median dorsal aorta. Internal and external carotid +arteries supply the head. These four afferent branchial arches are +equivalent to the first four of the five vessels of the dog-fish. At first, +the paired gills are three in number, external, and tree-like, covered by +epiblast (Figures 10 and 11, e.g.), and not to be compared to fish gills +in structure, or in fact -with- [to] any other gills within the limits of +the vertebrata. Subsequently (hypoblastic) internal gills (int.g., Figure +12), strictly homologous with the gills of a fish, appear. Then +a flap of skin outside the hyoid arch grows back to cover over the gills; +this is the operculum (op. in Figures 11 and 12, Sheet 22), and it +finally encloses them in a gill chamber, open only by a pore on the +left, which resembles in structure and physiological meaning, but +differs evidently very widely in development, from the amphioxus +atrium. At this time, the lungs are developing as paired hollow +outgrowths on the ventral side of the throat (Figure 12, L.). As the +limbs develop, and the tail dwindles, the gill chamber is obliterated. +The capillary interruptions of the gills on the branchial arches (aortic +arches) are also obliterated. The carotid gland occupies the position +of the first of these in the adult. The front branchial arch here, as in +all higher vertebrata, becomes the carotid arch; the lingual represents +the base of a pre-branchial vessel; the second branchial becomes the +aortic arch. The fourth loses its connection with the dorsal aorta, and +sends a branch to the developing lung, which becomes the pulmonary +artery. The third disappears. A somewhat different account to this is +still found in some text-books of the fate of this third branchial arch. +Balfour would appear to have been of opinion that it gave rise to the +cutaneous artery, and that the third and fourth vessels coalesced to +form the pulmocutaneous, the fourth arch moving forward so as to +arise from the base of the third; and most elementary works follow +him. This opinion was strengthened by the fact that in the higher +types (reptiles, birds, and mammals) no fourth branchial arch was +observed, and the apparent third, becomes the pulmonary. But it has +since been shown that a transitory third arch appears and disappears +in these types. + + +Section 18. The origin of the renal organ and duct has very +considerable controversial interest.* In Figure 13, Sheet 22, a +diagrammatic cross-section, of an embryo is shown. I. is the +intestine, coe. the coelom, s.c. the spinal cord; n.c. the notochord, +surrounded by n.s., the notochordal sheath, ao. is the dorsal aorta. +In the masses of somatic mesoblast on either side, a longitudinal +canal appears, which, in the torpedo, a fish related to the dog-fish, +and in the rabbit, and possibly in all other cases, is epiblastic in +origin. This is the segmental duct, which persists, apparently, as the +Wolffian duct (W.D.). Ventral to this appears a parallel canal, the +Mullerian duct (M.D.), which is often described as being split off from +the segmental duct, but which is, very probably, an independent +structure in the frog. A number of tubuli, at first metamerically +arranged, now appear, each opening, on the one hand, into the +coelom by a ciliated mouth, the nephrostome (n.s.), and on the other +into the segmental duct. These tubuli are the segmental tubes or +nephridia. There grows out from the aorta, towards each, a bunch, of +bloodvessels, the glomerulus (compare Section 62, Rabbit). These +tubuli ultimately become, in part, the renal tubuli, so that the primitive +kidney stretches, at first, along the length of the body cavity from the +region, of the gill-slits backward. The anterior part of the kidney, called +the pronephros, disappears in the later larval stages. Internal to the +kidney on either side there has appeared a longitudinal ridge, the +genital ridge (g.r.), which gives rise to testes or ovary, as the case +may be. + +* In the discussion whether the vertebrata have arisen from some +ancestral type, like the earthworm, metamerically segmented, and of +fairly high organization, or from a much lower form, possibly even from +a coelenterate. Such a discussion is entirely outside the scope of the +book, though its mention is necessary to explain the importance given +to these organs. + + +Section 19. The student should now compare the figures on Sheet 17. +In the male, tubular connections are established between the testes +and the middle part of the primitive kidney (mesonephros). These +connections are the vasa efferentia (v.e.), and the mesonephros is +now equivalent to the epididymis of the rabbit. The Wolffian duct is the +urogenital duct of the adult, and the Mullerian duct is entirely +absorbed, or remains, more or less, in exceptional cases. + +In the female, 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 oviduct; the Wolffian duct is the +ureter, and the mesonephros is not perverted in function from its +primary renal duty. + + +Section 20. Tabulating these facts-- + + In the adult male: + Pronephros disappears. + The Mullerian duct (? = pronephric duct) disappears. + Mesonephros = Epididymis; its duct, the urogenital. + Metanephros and duct, not clearly marked off from + Mesonephros. + (Compare Dog-fish, Section 19.) + + In the adult female: + Pronephros disappears. + The Mullerian duct, the oviduct. + Mesonephros and Metanephros, the kidney, and their unseparated + ducts, the ureters. + + +Section 21. Hermaphrodism (i.e., cases of common sex) is +occasionally found among frogs; the testis produces ova in places, +and the Mullerian duct is retained and functional. The ciliated +nephrostomata remain open to a late stage of development in the frog, +and in many amphibia throughout life. Their connection with the renal +tubuli is, however, lost. + + +Section 22. The alimentary canal is, at first, a straight tube. Its +disproportionate increase in length throws it into a spiral in the tadpole +(int. Figure 11), and accounts for its coiling in the frog. The liver and +other digestive glands are first formed, like the lungs, as hollow +outgrowths, and their lining is therefore hypoblastic. The greatest +relative length of intestine is found in the tadpole, which, being a +purely vegetable feeder, must needs effect the maximum amount of +preparatory change in its food. + + + +_The Development of the Fowl_ + +Section 23. The frog has an ovum with a moderate allowance of yolk, +but the quantity is only sufficient to start the little animal a part +of its way towards the adult state. The fowl, on the contrary, has an +enormous ovum, gorged excessively, with yolk, and as a +consequence the chick is almost perfected when it is hatched. The +so-called yolk, the yellow of an egg, is the ovum proper; around that is +a coating of white albumen, in a shell membrane and a shell. At either +end of the yolk (Figure 1, y.) twisted strands of albuminous matter, +the chalazae (ch.) keep the yolk in place. The animal pole is a small +grey protoplasmic area, the germinal area (g.a.), on the yolk. + + +Section 24. 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 poles. The late F.M. Balfour, borrowing a mathematical +technicality, suggested that the rate of segmentation in any part of an +ovum varies inversely with the amount of yolk. In the fowl's egg, +except just at the germinal area, the active protoplasm is at a +minimum, the inert yolk at a maximum; the ratio of yolk to protoplasm +is practically infinity, and the yolk therefore does not segment at all. +The yolk has diluted the active protoplasm so much as to render its +influence inappreciable. The germinal area segments, and lies upon +the yolk which has defeated the efforts of its small mingling of +protoplasm to divide. Such a type of segmentation in which only part +of the ovum segments is called meroblastic. If we compare this with +the typical blastosphere of the lower type, we see that it is, as it +were, flattened out on the yolk. This stage is shown in section in the +lower figure of Figure 1. b.d., the blastoderm, is from this point of +view, a part of the ripped and flattened blastosphere, spread out on +the yolk; s.c. is the segmentation cavity, and y. the yolk. + + +Section 25. There is no open invagination of an archenteron in the +fowl, as in the frog--, the gastrula, like the blastosphere, stage is +also masked. But, in the hinder region of the germinal area, a thick +mass of cells, grows inward and forward, and, appearing in the +dorsal view of the egg as a white streak, is called the primitive +streak (p.s.). 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 frog. 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 out. The tension of the protoplasm, +straining over the enormous yolk, answers to the pressure. The +gastrula in the fowl is solid. The primitive streak is, in fact, the scar +of a closed blastopore. As we should expect from this view of its +homology, at the primitive streak, the three embryonic layers are +continuous and indistinguishable (Figure 2). Elsewhere in the +blastoderm they are distinctly separate. Just as the yolk cells of the +frog form the ventral wall of the intestine, so nuclei appear along the +upper side of the yolk of the fowl, where some protoplasm still +exists, and give rise to the ventral hypoblastic cells. By conceiving a +gradually increasing amount of yolk in the hypoblastic cells in the +ventral side of the archenteron, the substantial identity of the +gastrula stage in the three types, which at first appear so strikingly +different, will be perceived. Carry Figures 4 and 5 of the frog one +step further by increasing the size of the shaded yolk and leaving it +unsegmented, and instead of ar. in 5 show a solid mass of cells, +and the condition of things in the fowl would at once be rendered. + + +Section 26. Figure 3a of the fowl will conveniently serve for +comparison with Figure 7 of the frog. The inturning of the medullary +groove is entirely similar in the two cases. The mesoblast appears as +solid mesoblastic somites. In the section above Figure 4 this layer is +shown as having split into somatopleur (so.) and splanchnopleur +(spch.). Figure 3 answers to Figure 6 of the frog, and Figure 4 is a +later stage, in which the medullary groove is beginning to close at its +middle part. The clear club-shaped area around the embryo (a.p.) is +the area pellucida; the larger area without this is the area opaca +(a.o.), in which the first bloodvessels arise by a running together and +a specialization of cells. The entire germinal area grows steadily at its +edges to creep over and enclose the yolk. + + +Section 27. So far, the essential differences between the development +of fowl and frog, the meroblastic segmentation, absence of a typical +gastrula, and the primitive streak, seem comprehensible on the theory +that such differences are due to the presence of an enormous amount +of yolk. Another difference that appears later is that, while the tadpole +has an efficient pronephros, the fowl, which has no larval (free +imperfect) stages in its life history, has the merest indication of such +a structure. + + +Section 28. Another striking contrast, due to, or connected with, this +plethora of yolk, is the differentiation of a yolk sac (= umbilical +vesicle) and the development of two new structures, the amnion and +allantois, in the fowl. If the student will compare Figure 10 of the frog, +he will see that the developing tadpole encloses in its abdomen all the +yolk provided for it. This is a physical impossibility in the fowl. In the +fowl (Figure 2, Sheet 24) the enormous yolk (Y.) lies outside of the +embryo, and, as the cells of the germinal area grow slowly over +it, umbilical bloodvessels are developed to absorb and carry the +material to the embryo. In the case of an embryo sinking in upon, as +it absorbs, this mass of nutritive material, a necessity for some +respiratory structure is evident. From the hinder end of the fowl's +intestine, in a position corresponding to the so-called, urinary bladder +of the frog, a solid outgrowth, the allantois, which speedily becomes +hollow, appears. Early stages are shown in Figures 1 and 2, Sheet +24 (al.); while the same thing is shown more diagrammatically on +Sheet 23, Figure 6 (all.). This becomes at last a great hollow sac, +which is applied closely to the porous shell, and the extent of which +will be appreciated by looking at Figure 5, Sheet 24, where the +allantois is shaded. Allantoic bloodvessels ramify thickly over its +walls, and aeration occurs through the permeable shell. + + +Section 29. The nature of the amnion will be understood by following +Figures 4b, 5, and 6 on Sheet 23. The three embryonic layers are +indicated by broken lines, dots, and black lines, just as they are in +the frog diagrams. Not only is the embryo slowly pinched off from the +yolk sac (y.s.), but, as the yolk is absorbed beneath it, and it grows +in size, it sinks into the space thus made, the extra-embryonic +somatopleur and epiblast rise up round it as two folds, which are +seen closing in 5, and closed in 6, over the dorsal side of the young +chick. In this way a cavity, a., lined by epiblast, and called the +amniotic cavity, is formed. Dorsal to this, in 6, comes a space lined +by somatic mesoblast, and continuous with p.p., the +pleuro-peritoneal cavity, or body cavity of the embryo. Outside this, +again, is a layer, of somatopleur internally and epiblast externally, the +false amnion (f.a.), which is continuous with the serous membrane +(s.m.) enclosing the rest of the egg. The student should, carefully +copy these diagrams, with coloured pencils or inks for the different +layers, and should compare them with the more realistic renderings of +Figures 2, 5, and 8, Sheet 24. + + +Section 30. The heart in the fowl appears first as a pair of vessels, +which unite to form a straight trunk in the median line, as the +flattened-out embryo closes in from the yolk. The way in which this +straight trunk is thrown, first of all, into the S shape of the fish heart, +and then gradually assumes the adult form, is indicated roughly by +Figure 3. In one respect the development of the heart does not follow +the lines one would expect. Since, between the fish and the higher +form comes the condition of such an animal as the frog, in which the +auricles are divided, while there is only one ventricle, we might expect +a stage in which the developing chick's heart would have one ventricle, +and a septum between the auricles. But, as a matter of fact, the +ventricles in fowl and rabbit are separated first, and the separation of +the auricles follows, and is barely complete at birth. + + +Section 31. Two vitelline veins from the yolk sac (v.v.) flow into the +heart from behind, as shown in Figure 1. A later more complete and +more diagrammatic figure of the circulation is seen in Figure 7. At first +there are two anterior cardinal (a.c.), and two posterior cardinal veins +(p.c.) uniting to form Cuvierian sinuses (c.s.) that open into the heart +just as in the dog-fish. But later the inferior cava is developed and +extends backward, the posterior cardinals atrophy, the Cuvierian +sinuses become the superior cavae, and the anterior cardinals the +internal jugular veins. The vitelline veins (v.v.) flow, at first, +uninterruptedly through the liver to the inferior cava, but, as +development proceeds, a capillary system is established in the liver, +and the through communication, the ductus venosus, is reduced-- at +last-- completely. Bearing in mind that the yolk is outside the body in +the fowl and inside it in the frog, the vitelline veins of the former +have a considerable resemblance in position, and in their relation to the +portal vein, to a portion of the single anterior abdominal vein. Blood is +taken out to the allantois, however, by the arteries of the latter type. + + +Section 32. Five aortic arches are generally stated to appear +altogether in the fowl, but not simultaneously. The first two, the +mandibular and the hyoid vascular arches, early disappear, and are +not comparable to any in the frog. The third is the first branchial arch, +and, like the corresponding arch in the frog, forms the carotid artery; +the second branchial is the aortic arch; and what has hitherto been +regarded as the third (the fifth arch, i.e.) the pulmonary artery. A +transitory arch, it is now known, however, appears between the +second branchial and the last, and it is therefore the fourth branchial +arch which is the pulmonary, just as it is in the frog. + + +Section 33. Blood, it may be mentioned, first appears in the area +vasculosa, the outer portion of the area opaca. Embryonic cells send +out processes, and so become multipolar; the processes of adjacent +cells coalesce. The nucleus divides, and empty spaces appear in the +substance of each of the cells. + +In this way, the cavities of the smaller vessels and capillaries are +formed, and the products of the internal divisions of the cells become +the corpuscles within the vessels. The red blood corpuscles of the +rabbit, it may be added, are nucleated for a considerable portion of +embryonic life. Larger vessels and the heart are burrowed, as it were, +out of masses of mesoblast cells. The course of the blood in the +embryo is by the veins to the right auricle, thence through the +imperfection of the auricular septum already alluded to, into the left +auricle. Then the left ventricle, aortic arches (for the future pulmonary +artery is in communication by a part presently blocked, the ductus +arterious, with the systemic aorta), arteries, capillaries, veins. The +liver capillary system and the pulmonary system only become +inserted upon the circulation at a comparatively late stage. + + +Section 34. With the exception of the reduction of the pronephros, +what has been said of the development of the frog's nervous system, +renal and reproductive organs, and skeleton, applies sufficiently to the +fowl for our present purposes. The entire separation of Wolffian and +Mullerian ducts from the very beginning of development is here beyond +all question (vide Section 18). But the notochord in the fowl is not so +distinctly connected with the hypoblast, and so distinct from the +mesoblast, as it is in the lower type, and no gills, internal or external, +are ever developed. The gill slits occur with a modification due to the +slitting and flattening out of the embryo, already insisted upon; for, +whereas in the tadpole they may be described as perforations, in the +fowl they appear as four notches between ingrowing processes that +are endeavouring to meet in the middle line. + + + +_The Development of the Rabbit_ + +Section 35. The early development of the rabbit is apt to puzzle +students a little at first. We have an ovum practically free from yolk +(alecithal), and, therefore, we find it dividing completely and almost +equally. We naturally assume, from what we have learnt, that the next +stages will be the formation of a hollow blastosphere, invagination, a +gastrula forming mesoblast by hollow outgrowths from the +archenteron, and so on. There is no yolk here to substitute epiboly +(Section 9) for invagination, nor to obliterate the archenteron and the +blastopore through its pressure. + +Yet none of these things we have anticipated occur! + +We find solid mesoblastic somites, we find primitive streak, allantois +and amnion, features we have just been explaining as the +consequence of an excess of yolk in the egg. We even find a yolk sac +with no yolk in it. + + +Section 36. A solid mass of cells is formed at the beginning, called a +morula, Figure 1. In this we are able to distinguish rather smaller +outer layer cells (o.l.c.), and rather larger inner layer cells (i.l.c.), +but these cells, in their later development, do not answer at all to the +two primitive layers of the gastrula, and the name of Van Beneden's +blastopore (V.B.b.), for a point where the outer layer of cells is +incomplete over the inner, only commemorates the authorship of a +misnomer. The uniformity, or agreement, in the development of our +other vertebrate types is apparently departed from here. + +{Illustration: Development Section 36.} + + +Section 37. As the egg develops, however, we are astonished to find +an increasing resemblance to that of the fowl. A split occurs at one +point between outer layer and inner layer cells, and the space +resulting (Y in Figure 2) is filled by an increasing amount of fluid, and +rapidly enlarges, so that presently we have the state of affairs shown +in 3, in which the inner layer cells are gathered together at one point +on the surface of the ovum, and constitute the germinal area. If, with +Hubrecht, we regard the outer layer cells as an egg membrane, there +is a curious parallelism between this egg and the fowl's the fluid Y +representing the yolk; and the inner layer cells the cells of the fowl's +germinal area. + +At any rate, the subsequent development goes far to justify such a +view. The inner cells split into epi-, meso-, and hypo-blast, like the +blastoderm in the fowl; there is a primitive streak and no blastopore; +an amnion arises; the yolk sac, small and full of serous fluid, is cut off +just as the enormous yolk of the fowl is cut off; and an allantois arises +in the same way. There is no need to give special diagrams-- Figures +3, 4b, 5, and 6 of the fowl will do in all respects, except proportion, for +the development of the rabbit. The differences are such as we may +account for, not on the supposition that the rabbit's ovum never had +any yolk, but that an abundant yolk has been withdrawn from it. The +nutrition of the embryo by yolk has been superseded by some better +method. The supposition that the rabbit is descended from ancestors +which, like the birds and reptiles, laid eggs with huge quantities of +yolk, meets every circumstance of the case. + + +Section 38. But the allantois and yolk sac of the rabbit, though they +correspond in development, differ entirely in function from the similar +organs of the fowl. The yolk sac is of the very smallest nutritive value; +instead of being the sole source of food, its contents scarcely avail +the young rabbit at all as nourishment. Its presence in development is +difficult to account for except on the supposition, that it was once of +far greater importance. At an early stage, the outgrowing allantois, +pushing in front of it the serous membrane, is closely applied to the +lining of the mother's uterus. The maternal uterus and the embryonic +allantois send out finger-like processes into each other which +interlock, and the tissue between the abundant bloodvessels in them +thins down to such an extent that nutritive material, peptones and +carbohydrates, and oxygen also, diffuse freely through it from +mother to foetus,* and carbon dioxide, water, and urea from the +foetus to the mother. The structure thus formed by the union of the +wall of the maternal uterus, allantois, and the intermediate structures +is called the placenta. Through its intermediation, the young rabbit +becomes, as it were, rooted and parasitic on the mother, and utilizes +her organs for its own alimentation, respiration, and excretion. It +gives off CO2, H2O, and urea, by the placenta, and it receives O and +elaborated food material through the same organ. This is the better +method that has superseded the yolk. + +* The embryo. + + +Section 39. In its later development, the general facts already +enunciated with regard to the organs of frog and fowl hold, and where +frog and fowl are stated to differ, the rabbit follows the fowl. In the +circulation the left fourth vascular arch (second branchial) gives rise to +the aortic arch; in the right the corresponding arch disappears, except +so much of it as remains as the innominate artery. The azygos vein +(Chapter 3) -is a vestige of- [is derived from] the right posterior +cardinal sinus. Both pulmonary arteries in the rabbit are derived from +the left sixth vascular arch (= fourth branchial). Compare Section 32. The +allantois altogether disappears in the adult fowl; in the adult mammal +a portion of its hollow stalk remains as the urinary bladder, and the +point where it left the body is marked by the umbilicus or navel. The +umbilical arteries become the small hypogastric arteries on either side +of the urinary bladder. There is no trace of a pronephros at all in the +rabbit. + + +Section 40. We may note here the development of the eye. This is +shown in Figure 4, Sheet 24. A hollow cup-shaped vesicle from the +brain grows out towards an at first hollow cellular ingrowth from the +epidermis. The cavity within the wall of the cup derived from the brain +is obliterated, [and the stalk withers,] the cup becomes the retina, and +-its stalk- [thence fibres grow back to the brain to form] the optic +nerve. The cellular ingrowth is the lens. The remainder of the +eye-structures are of mesoblastic origin, except the superficial +epithelium of the cornea. The retinal cup is not complete at first +along the ventral line, so that the rim of the cup, viewed as in +Figure 1, r., is horseshoe shaped. -Hence the optic nerve differs from +other nerves in being primitively hollow.- In all other sense organs, +as, for instance, the olfactory sacs and the ears, the percipient +epithelium is derived, from the epiblast directly, and not indirectly +through the nervous system. These remarks apply to all vertebrate +types. + + +Section 41. The supposition, 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 reptiles, is +strengthened by the fact that the two lowest and most reptile-like of +all the mammalia, the duck-billed platypus and the echidna, have +been shown to depart from the distinctive mammalian character, and +to lay eggs. And, in further confirmation of this supposition, we find, in +tracing the mammals and reptiles back through the geological record, +that in the Permian and Triassic rocks there occur central forms +which combine, in a most remarkable way, reptilian and mammalian +characteristics. + + +Section 42. In conclusion, we would earnestly recommend the +student to see more of embryological fact than what is given him here. +It is seeing and thinking, much more than reading, which will enable +him to clothe the bare terms and phrases of embryology with coherent +knowledge. In Howes' Atlas of Biology there is a much fuller series of +figures of the frog's development than can be given here, +and they are drawn by an abler hand than mine can pretend to be. +There is also an Atlas d'Embryologie, by Mathias Duval, that makes +the study of the fowl's development entertaining and altogether +delightful. Such complete series as these are, from the nature of the +case, impossible with the rabbit. Many students who take up the +subject of biology do so only as an accessory to more extended work +in other departments of science. To such, practical work in +embryology is either altogether impossible, or only possibly to a very +limited extent. The time it will consume is much greater, and the +intellectual result is likely to be far less than the study of such plates +as we have named. + + + +_The Theory of Evolution_ + +Section 43. We have now considered our types, both from the +standpoint of adult anatomy and from embryological data; and we +have seen through the vertebrate series a common structure +underlying wide diversity in external appearance and detailed +anatomy. We have seen a certain intermediateness of structure in the +frog, as compared with the rabbit and dog-fish, notably in the skull +and skeleton, in the circulation, in the ear, and in the reduced +myomeres; and we have seen that the rabbit passes in these +respects, and in others, through dog-fish- and frog-like stages in its +development, 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 life. + + +Section 44. It has been suggested by very excellent people that these +resemblances are due to some unexplained necessity of adherence +to type, as though, the power that they assume created these +animals originally, as they are now, coupled creative ability with a +plentiful lack of ideas, and so perforce repeated itself with impotent +variations. On the other hand, we have the supposition that these are +"family likenesses," and the marks of a common ancestry. This is the +opinion now accepted by all zoologists of repute. + + +Section 45. It must not be for a moment imagined that it is implied +that rabbits are descended from frogs, or frogs from dog-fish, but that +these three forms are remote cousins, derived from some ancient +and far simpler progenitor. But since both rabbit and frog pass through +phases like the adult condition of the dog-fish, it seems probable that +the dog-fish has remained more like the primordial form than these +two, and similarly, the frog than the rabbit. + + +Section 46. Hence we may infer that the mammals were the last of +the three groups, of which we have taken types, to appear upon the +earth, and that the fishes preceded, the amphibia. Workers in an +entirely independent province, that of palaeontology, completely +endorse this supposition. The first Vertebrata to appear in the fossil +history of the world are fishes; fish spines and placoid scales +(compare dog-fish) appear in the Ordovician rocks. In the coal +measures come the amphibia; and in the Permo-triassic strata, +reptile-like mammals. In the Devonian rocks, which come between the +Silurian and the coal measures, we find very plentiful remains of +certain fish called the dipnoi, of which group three genera still survive; +they display, in numberless features of their anatomy, transitional +characters between true fish and amphibia. Similarly, in the Permian +come mammal-like reptiles, that point also downward to the amphibia. +We find, therefore, the story told by the ovum written also in the +rocks. + + +Section 47. Now, when this fact of a common ancestry is considered, +it becomes necessary to explain how this gradual change of animal +forms may have been brought about. + + +Section 48. Two subcontrary propositions hold of the young of any +animal. It resembles in many points its parent. It differs in many +points from its parent. The general scheme of structure and the +greater lines of feature are parental, inherited; there are also novel and +unique details that mark the individual. The first fact is the law of +inheritance; the second, of variation. + + +Section 49. Now the parent or parents, since they live and breed, +must be more or less, but sufficiently, adapted to their conditions of +living-- more or less fitted to the needs of life. The variation in the +young animal will be one of three kinds: it will fit the animal still +better to the conditions under which its kind live, or it will be a change +for the worse, or it is possible to imagine that the variation-- as in the +colour variations of domesticated cats-- will affect its prospects in life +very little. In the first case, the probability is that the new animal will +get on in life, and breed, and multiply above the average; in the +second, it is probable that, in the competition for food and other +amenities of life, the disadvantage, whatever it is, under which the +animal suffers will shorten its career, and abbreviate the tale of its +offspring; while, in the third case, an average career may be expected. +Hence, disregarding accidents, which may be eliminated from the +problem by taking many cases, 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 lives. That is, while the conditions remain +unchanged, the animals, considered as one group, are continually +more highly perfected to live under those conditions. And under +changed conditions the specific form will also change. + + +Section 50. The idea of this process of change may be perhaps +rendered more vivid by giving an imaginary concrete instance of its +working. In the jungles of India, which preserve a state of things which +has existed for immemorial years, we find the tiger, his stripes +simulating jungle reeds, his noiseless approach learnt from nature in +countless millions of lessons of success and failure, his perfectly +powerful claws and execution methods; and, living in the same jungle, +and with him as one of the conditions of life, are small deer, alert, +swift, light of build, inconspicuous of colour, sharp of hearing, +keen-eyed, keen-scented-- because any downward variation from +these attributes means swift and certain death. To capture the deer is +a condition, of the tiger's life, to escape the tiger a condition of the +deer's; and they play a great contest under these conditions, with life +as the stake. The most alert deer almost always escape; the least +so, perish. + + +Section 51. But conditions may alter. For instance, while most of +these deer still live in the jungle with tigers, over a considerable area +of their habitat, some change may be at work that thins the jungle, +destroys the tigers in it, and brings in, let us say, wolves, as an +enemy to the deer, instead of tigers. Now, against the wolves, which +do not creep, but hunt noisily, and which do not spring suddenly upon +prey, but follow by scent, and run it down in packs, keen eyes, sharp +ears, acute perceptions, will be far less important than endurance in +running. The deer, under the new conditions, will need coarser and +more powerful limbs, and a larger chest; it will be an advantage to be +rough and big, instead, of frail and inconspicuous, and the ears and +eyes need not be so large. The old refinements will mean weakness +and death; any variation along the line of size and coarseness will be +advantageous. Slight and delicate deer will be continually being killed, +rougher and stronger deer continually escaping. And so gradually, +under the new circumstances, if they are not sufficient to exterminate +the species, the finer characteristics will be eliminated, and a new +variety of our old jungle deer will arise, and, if the separation and +contrast of the conditions is sufficiently great and permanent, we +may, at last, in the course of ages, get a new kind of deer specifically +different in its limbs, body, sense organs, colour, and instincts, from +the deer that live in the jungle. And these latter will, on their side, be +still continually more perfected to the jungle life they are leading. + + +Section 52. Take a wider range of time and vaster changes of +condition than this, and it becomes possible to imagine how the +social cattle-- with their united front against an enemy, fierce +onslaught, and their general adaptation to prairie life-- have +differentiated from the ancestors of the slight and timid deer; how the +patient camel, with his storage hump, water storage, and feet padded +against hot sand, has been moulded by the necessity of desert life +from the same ancestral form. And so we may work back, and link +these forms, and other purely vegetarian feeders, with remoter +cousins, the ancestral hogs. Working in this way, we presently get a +glimpse of a possible yet remoter connection of all these hoofed and +mainly vegetarian animals, with certain "central types" that carry us +across to the omnivorous, and, in some cases, almost entirely +vegetarian bears, and to the great and prosperous family of clawed, +meat-eaters. And thus we elucidate, at last, a thread of blood +relationship between the, at present, strongly contrasted and +antagonistic deer and tiger, and passing thence into still wider +generalizations, it would be possible to connect the rabbit playing in +the sunshine, with the frog in the ditch, the dog-fish in the sea-waters +and the lancelet in the sand. For the transition from dog-fish to rabbit +differs from the transition from one species of deer to another only in +magnitude: it is an affair of vast epochs instead merely of thousands +of years. + + +Section 53. It would, however, be beyond the design of this book to +carry our demonstration of the credibility of a common ancestry of +animals still further back. But we may point out here that it is not a +theory, based merely upon one set of facts, but one singularly rich in +confirmation. We can construct, on purely anatomical grounds, a +theoretical pedigree. Now the independent study of embryology +suggests exactly the same pedigree, and the entirely independent +testimony of palaeontology is precisely in harmony with the already +confirmed theory arrived at in this way. + + +Section 54. It is in the demonstration of this wonderful unity in life, +only the more confirmed the more exhaustive our analysis becomes, +that the educational value and human interest of biology chiefly lies. In +the place of disconnected species of animals, arbitrarily created, and +a belief in the settled inexplicable, the student finds an enlightening +realization of uniform and active causes beneath an apparent diversity. +And the world is not made and dead like a cardboard model or a +child's toy, but a living equilibrium; and every day and every hour, +every living thing is being weighed in the balance and found sufficient +or wanting. + +Our little book is the merest beginning in zoology; we have stated one +or two groups of facts and made one or two suggestions. The great +things of the science of Darwin, Huxley, Wallace, and Balfour remain +mainly untold. In the book of nature there are written, for instance, the +triumphs of survival, the tragedy of death and extinction, the +tragi-comedy of degradation and inheritance, the gruesome lesson of +parasitism, and the political satire of colonial organisms. Zoology is, +indeed, a philosophy and a literature to those who can read its +symbols. In the contemplation of beauty of form and of mechanical +beauty, and in the intellectual delight of tracing and elucidating +relationships and criticising appearances, there is also for many a +great reward in zoological study. With an increasing knowledge of the +facts of the form of life, there gradually appears to the student the +realization of an entire unity shaped out by their countless, and often +beautiful, diversity. And at last, in the place of the manifoldness of a +fair or a marine store, 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 surely. + +To the student who feels inclined to amplify this brief outline of +Vertebrate Anatomy, we may mention the following books: +Wiedersheim's and Parker's Vertebrates, Huxley's Anatomy of the +Vertebrata, Flower's Osteology of the Mammalia, Wallace's +Distribution, Nicholson and Lyddeker's Palaeontology (Volume 2), +the summaries in Rolleston's Forms of Animal Life (where a +bibliography will be found), and Balfour's Embryology. But reading +without practical work is a dull and unprofitable method of study. + + + +_Questions on Embryology_ + + [All these questions were actually set at London University + Examinations.] + {In Both Editions.} + +1. Describe the changes in the egg-cell which precede fertilization; +describe the process of fertilization and the formation of the primary +cell-layers, as exhibited, in three of the animal types known to you. +What is the notochord, and how is it developed in the frog? + +2. Describe the early stages in the development of the egg of the fowl +as far as the closure of the neural groove. How do you account for +the primitive streak? + +3. Describe the cleavage and the surface appearances of the egg of +the frog and of the rabbit, up to the time when the first gill-slits +appear in the embryo. Give illustrative diagrams of what you +describe. + +4. Describe the structure and cleavage of the ovum (a) of the frog, (b) +of the fowl, and (c) of the rabbit. (d) Explain as far as possible the +differences in the cleavage of these three eggs. (e) Point out how the +embryo is nourished in each case, and (f) describe the constitution of +the placenta in the rabbit. + +5. (a) What are the protovertebrae? (b) How does the notochord +originate in the frog? (c) How are the vertebrae laid down in the +tadpole? (d) Describe the vertebral column of the adult frog. (e) In what +important respects do the centra of the vertebrae of the frog, the +dog-fish, and the rabbit differ from one another? + +6. Give an account of the more important features in the development +of the frog. + +7. What temporary organs are developed in the embryo frog which are +absent from the embryo bird and mammal, and what in the two latter +which are absent from the former? + +8. Draw diagrams, with the parts named, of the heart and great +arteries of the frog, giving descriptions only in so far as is necessary +to explain your diagrams; trace the development of these structures in +the tadpole; point out particularly in which of the embryonic visceral +(branchial) arches the great arteries of the adult run. + +9. Trace the history of the post-oral gill-slits and their accompanying +cartilaginous bars and vascular arches in the frog, fowl, and rabbit. + +10. Give a short account, with illustrative figures, of the mode of +formation of the primary germinal layers in amphioxus and in the frog. +What explanation can you give of the differences between the two +cases? + +11. Give a short account, with diagrammatic figures, of the principal +changes which occur in the circulatory and respiratory organs during +the metamorphosis of the tadpole into the frog. + +12. How do protozoa differ from higher animals (metazoa) as regards +(a) structure, (b) reproduction? Compare the process of fission in an +amoeba with the segmentation of the ovum in amphioxus, pointing out +the resemblances and differences between the two cases. + + + +-Miscellaneous Questions_ + + [Most of these questions were actually set at the Biological + Examinations of London University.] {In Both Editions.} + +1. Describe (a) the digestive, (b) the circulatory, (c) the excretory, and +(d) the reproductive organs of the amphioxus. + +2. Describe the stomach and intestines of the dog-fish and rabbit, and +point out in what way their differences are connected with diet. + +3. Describe the mechanism of respiration in the adult frog, and +contrast it with that of the tadpole. + +4. Give an account of the structure of the epidermis and its +outgrowths in the frog and the rabbit. + +5. Describe the organs of circulation (heart and main arteries and +veins) and respiration in the frog in its mature and immature states. + +6. Give a brief account of the physiology of respiration. Describe fully +the means by which respiration is effected in the following animals:-- +frog, amphioxus, rabbit, and dog-fish. + +7. Describe the minute structure of the blood of the rabbit, frog, and +amphioxus. + +8. Describe and illustrate by means of sketches the chief points of +difference between the skeleton of the rabbit as a typical mammal, +and that of the common frog as a typical amphibian. + +9. (a) Explain what is meant by the term "central nervous system." +(b) Describe the tissue elements which enter into its composition. +(c) Explain, as far as you can, the function of each structure +described. (d) How is the central nervous system developed in the +frog, and (e) in the rabbit? (f) What conclusions may be drawn from +the facts stated as to the origin of the central nervous system in +evolution? + +10. Give an account of the structure (including histology) and of the +functions of the spinal cord and spinal nerves of the frog. + +11. Give a description of the minute structure and chemical +characters of the following tissues as seen in the frog:-- cartilage, +bone, muscle. From which of the primary cell-layers of the embryo are +they respectively developed? + +12. What substance is excreted by the renal organ of a frog, and what +relation does this substance bear to the general life of the organism? +Describe the parts by which similar excretion is believed to be +effected in amoeba, hydra, earthworm, mussel, and lobster. + +13. Describe, with illustrative sketches, the structure of the connective +tissue, cartilage, and muscular tissue of a frog. Also describe the +structure of the muscular tissue of the lobster and snail. + +14. Give in account of the more important features in the +development of the frog. + +15. Describe and compare the structure of the renal organs in a frog +and a rabbit. + +16. Give an account of the structure of the genito-urinary organs of the +frog. Compare these organs of the frog with those of the dog-fish and +of the rabbit. Distinguish in each case the conditions of the two +sexes, and describe briefly the microscopic structure and +development of the ova and of the spermatozoa. + +17. Describe, with diagrams, the arrangement of the urinary and +generative organs in the male of (a) the rabbit, (b) the dog-fish, and +(c) the frog; (d) point out the most important differences between +them. + +18. (a) Describe the structure of the ovarian egg of the rabbit, (b) and +of the pigeon, (c) and of the frog; (d) from what part of the embryo do +they originate? (e) What is the structure and origin of the ovarian +follicle in the rabbit, and (f) of the ovarian stroma? (g) What is the +"granulosa" and what the "zona pellucida"? + +19. Describe the pre-segmentation changes, mode of impregnation, +and early stages of development in the ovum of the frog, as far as the +closure of the neural canal. + +20. Illustrate, with diagrams, from the structure of typical organisms, +the principle of repetition of similar parts. + + + +-Note on Making Comparisons_ + +Students preparing for examinations are frequently troubled by +"comparison" questions. Tabulation is often recommended, but we are +inclined to favour a rather more flexible plan of marking off differences +and resemblances. In tabulation a considerable loss of time is +occasioned by writing down the features of both the things compared, +and this is a serious consideration for the examinee. We advise him +therefore, first, if he possibly can, to draw side by side and in +corresponding positions the two things under consideration, and then, +going over them in a methodical way, to state simply the difference +between each homologous part. We append as examples three test +answers actually submitted (with figures) in "Correspondence" work:-- + +1. Compare the brain of the frog with that of the rabbit. + + In the frog's fore-brain-- + + The olfactory lobes are fused in the middle line. + + There is no corpus callosum, nor is there a middle commissure to + the third ventricle. + + The cerebral hemispheres are not convoluted, and, looked at from + the dorsal aspect, do not hide the thalamencephalon and mid-brain. + + The pineal gland lies in the cranial wall and not deeply between + the hemispheres, and its stalk is longer and tilts forward. + + In the mid-brain-- + + The optic lobes are two, instead of being corpora quadrigemina, + and hollow. + + In the hind-brain-- + + The cerebellum is a very small transverse band, and has no + lateral parts. + + The medulla is relatively larger. + + There are no spinal accessory nor hypoglossal nerves to the + brain. + +2. Compare the vertebrae of dog-fish, rabbit, and frog. + + The centra of the dog-fish are -opistho- [amphi]-coelous + (i.e., hollow at either end). + + The centra of the rabbit are flat-faced. + + The centra of the frog are procoelous (hollow in front). + + The notochord persists between the centra in the dog-fish and + rabbit, within the centra in frog. + + The centra of the rabbit have epiphyses, absent in the dogfish + and frog. + + The transverse processes of the rabbit typically bear ribs. + Short ribs occur in the dog-fish, but their homology with those + of the rabbit is doubtful. The frog has no ribs. + + The interneural plates are peculiar to the dog-fish in this + comparison. + +3. Compare the skull of the dog with that of the frog. + + The Brain Case-- + + Of the frog is a cylindrical box, from which the otic capsules + project conspicuously on either side. It contains only two + ossifications in its cartilaginous substance (the sphen-ethmoid + and the ex-occipital), being protected by the membrane bones, + the parieto-frontals above and the parasphenoid below. + + In the mammal it is enormously inflated, and the otic capsules + are imbedded in its wall. There are supra- and basi- as well as + ex-occipital bones; the para-sphenoid is (? entirely) gone, and + its place is taken by the basi- and pre-sphenoids, and the + lateral walls contain fresh paired ossifications, the ali- and + orbito-sphenoids-- all cartilage bones. The sphenethmoid is + perhaps represented in part by the ethmoid. + + As a result of the inflation of the brain-case, the squamosal, + which slopes downward and outward in the frog, and overlies the + cartilaginous suspensorium (quadrate cartilage), has become a + constituent of the brain-case wall, and slopes downwardly and in. + + Jaw Suspension-- + + The point of attachment of the jaw has shifted outward, and the + original suspensorial cartilage (the quadrate) has taken on a new + and minor function as the incus of the middle ear-- the squamosal + superseding it as the suspensory part. + + Lower Jaw-- + + Distinct bones in the frog; one mass in the dog. + + Otic Capsule-- + + Position as specified. One centre of ossification in the frog + forming pro-otic; several fuse together and form periotic of the + dog. + + There is no bulla and no external ear in the frog. + + Palate-- + + In the frog the posterior nares open into the front of the mouth. + In the dog the maxillae and palatines send plates down and in (the + palatine plates) to cut off a nasal passage from the rest of the + buccal chamber, and carry the posterior nares back to the pharynx, + thus cutting the vomers off from the mouth roof. + + The pterygoids in the dog are much reduced, and do not reach back + to the suspensorium. + + The frog has no lachrymal bone. + + + +-Syllabus Of Practical Work_ + +We would impress upon the student at the outset the importance of +some preliminary reading before dissection is undertaken. No one +would dream of attempting to explore a deserted city without some +previous study of maps and guide-books, but we find again and again +students undertaking to explore the complicated anatomy of a +vertebrated animal without the slightest, or only the slightest, +preparatory reading. This is entirely a mistake. A student should be +familiar with the nomenclature of the structures he contemplates +examining, he should have some idea of their mutual relations and +functions, or his attention will inevitably be diverted by the difficulty +of new names and physiological questionings to the neglect of his +dissection, and that careful observation of form and mutual position +which is the essential object of dissection. On the other hand, it is +equally necessary-- perhaps more so-- to warn students against the +bookish fallacy, and to assure them of the absolute impossibility of +realizing biological facts from reading alone. Practical work can alone +confirm and complete the knowledge to which the text-book is the +guide. In scientific teaching it may sometimes be convenient for the +thought to precede the thing, but until the thing has been dealt with +the knowledge gained is an unsatisfactory and unstable possession. + +For such dissection as the subject-matter of this book requires, the +following appliances will be needed:-- + + (a) Two or three scalpels of various sizes. + + (b) Scissors, which must taper gradually, have straight blades, and + be pointed at the ends, and which must bite right up to the tips (or + they are useless). Two pairs, small and large, are advisable. + + (c) Forceps, which must hold firmly, and meet truly at the points. + + (d) Two needles set in wooden handles. + + (e) An ordinary watchmaker's eye-glass is very helpful, but not + indispensable. + + (f) A dissecting dish-- an ordinary pie dish will do-- into which + melted paraffin wax has been poured, to the depth of, say, + three-quarters of an inch, and allowed to solidify. (This wax may + be blackened by mixture with lampblack. If the wax floats up at any + time, it can, of course, be remelted. Or it may be loaded with + lead.) + + (g) A rough table or board (for the rabbit and dog-fish). + + (h) Blanket pins, and ordinary pins. + + (i) A pickle or other wide-mouthed jar, and some common, methylated + spirit. + + (j) A microscope, with low power of 1 inch or 1/2 inch, and high + power 1/6 inch or 1/4 inch. Glass slips and cover glasses, and a + bottle of very weak (1 per cent.) solution of salt. + +Animals for dissection may be obtained from the recognised dealers, +who usually advertise in such scientific periodicals as Nature, +Natural Science, and Knowledge. Sinel (naturalist, Jersey) is the most +satisfactory dealer in dog-fish in our experience; Bolton (Malvern) will +supply Amphioxus through the post; frogs and rabbits may be +obtained anywhere. The tame variety of rabbit is quite satisfactory for +the purpose of dissection. + +The following notes may possibly be of some use to the student; they +follow the lines of work arranged by the author for the evening classes +of the University Tutorial College, classes considerably restricted as +regards time, when compared with ordinary laboratory workers. Most +of the sections below occupied about three hours, but for a student +working alone they are more likely to take four or five, and even then it +is not probable that they will be so satisfactory as if performed under +skilled supervision. There are many points extremely difficult to +convey verbally which are elucidated at once by actual demonstration +upon a specimen. Each of these dissections should be repeated, and +it is well if a different condition of the type is selected for the +repetition-- an old one if the first specimen was immature, a female if +the first was a male. + + +-The Rabbit_ + +May be killed by chloroform, or potassium cyanide, or drowned. It +may also be readily suffocated with house-hold gas. It should be killed +immediately before use, as otherwise the gastric juice attacks the +wall of the stomach, and the dissection is, in consequence, rendered +extremely disagreeable. A very young rabbit is unsatisfactory as +regards the genitalia, but otherwise there is no objection to a little +one, and it has this advantage-- that it may be immersed more +conveniently under water, in a large pie dish, for purposes of fine +dissection. The external features of the animal should be examined: +eyelids, whiskers and teeth, toes, anus, perineal space on either side +of the same, urogenital opening, and position of the ribs, vertebral +column, and limb girdles beneath the skin should be made out. Then +the animal should be pinned out through the legs, the ventral surface +uppermost, the skin opened up along the middle line from pelvic girdle +to symphyses of jaw; separated from the body wall below by means +of the handle of a scalpel, and turned back; and then the abdominal +wall should be cut into and two flaps pinned back to expose its +contents. Note the xiphisternum. The caecum and colon will be +recognised (Section 16); the stomach, the right and left central, and +left lateral lobes of the liver will probably be apparent; and the urinary +bladder (especially if distended) in the middle line behind. Without any +further dissection, but simply by turning the parts over, all the +structures of the abdomen in Figure 1, Sheet 1, will be identified. +Seek especially for and note particularly, the gall bladder, bile duct, +and portal vein, pancreatic duct, sacculus rotundus, vermiform +appendix, ureters (by pulling urinary bladder forward), genital ducts +(looping over ureters), spleen, kidneys, and adrenals. The vena cava +inferior is seen dorsally. The genital duct guides the student to the +genital gland; if the subject is a male, the testes may be exposed by +dissection, or by pulling the vas deferens gently the scrotal sac will be +turned inside out, and the testes brought into view. The ovary lies +exposed without dissection posterior to the kidney. Examine all this +carefully, and make small sketches of points of interest-- the duodenal +loop and the pyloric end of the stomach, for instance; the meeting of +colon, caecum, and sacculus rotundus again; or the urinary bladder +and adjacent parts. Note the dorsal aorta and vena cava and their +connexions behind. (Compare figure of circulation.) Cut through pelvic +girdle, and remove one hind leg, to see bladder and genital ducts +better (compare Sheet 10). Wash away any blood that may flow. Turn +all the intestines over to the animal's right, and see the dorsal aorta +and vena cava inferior of the abdomen, the inferior mesenteric artery, +and the spermatic (or ovarian) artery (compare, of course, with figure +in book). In front, immediately dorsal to the spleen, is a variable +quantity of lymphoidal tissue, which must be very carefully cleared to +see the superior mesenteric and coeliac arteries. Separate Spigelian +lobe from stomach, and look for vagus nerve descending by +oesophagus, solar plexus around the superior mesenteric artery, and +thrown up very distinctly by the purple vena cava inferior beneath, and +the splanchnic nerve. To see the abdominal sympathetic behind, +gently remove the peritoneum that lies on either side of the aorta; +blood-vessels will be seen running in between the vertebral bodies, +and the sympathetic chain, with its ganglia, made out very distinctly, +as it runs across them longitudinally. Now cut oesophagus just in +front of stomach, and cut the rectum, cut through the mesentery +supporting the intestine, and remove and unravel alimentary canal; cut +open, wash out, and examine caecum and stomach. Bleeding to a +considerable extent is inevitable, chiefly from the portal vein. The liver +had better remain if the same rabbit is to serve for the second +dissection. + +Second Dissection.-- Skin front of thorax and neck. Note subclavian +veins running out to fore limbs-- avoid cutting these. Cut through ribs +and remove front of thorax, to expose its contents; cut up middle line +of neck, and clear off small muscle bands, to expose bloodvessels; +pick away carefully whatever is left of thymus gland; make out +structure of heart and blood-vessels, as described, in Chapter 3; note +larynx and trachea. Now proceed to the examination of the nerves of +this region. See phrenic nerve, by vena cava inferior, and between +heart and lungs, and sympathetic, running over the heads of the ribs. +By the common carotids will be found the large white vagus nerve, the +greyish sympathetic, and a small branch of X., the depressor. Make +out branches of X. named in text. The big white cervical spinal nerves +will be evident dorsally. Clear forward into the angle between the jaw +and the bulla tympani, to see XII. and XI.; IX. will be found, lying +deeper, dorsal to the carotid artery and body of the hyoid. Compare +with figure given of this. Skin the cheek, and see VII. running over it. +Cut through malar and remove it; cut through lower jaw-bone and turn +it back, to see the third branch of the fifth nerve on its inner side; +examine the muscles of eyeball, and remove it, to expose the first +and second branches of V.-- the latter is especially deep within orbit. +Remove, open, wash out, and examine the heart. Shave off the dorsal +wall of cranium, to expose hemispheres of brain, and then put the +head in strong spirit for a week or so. With a second rabbit, this +dissection may advantageously be varied by removing the lower jaw, +cutting -up- [through] soft palate, and observing openings of the +Eustachian tubes. [The tonsils (on the ventral side of the soft palate) +must not confused with these.] The heart should also be cut out, +washed out and examined (Compare Sections 38, 44.) + +Third Dissection.-- (Before this is performed the mammalian skull +should have been studied and examined.) Take the head of a rabbit, +the brain of which has been hardened by spirit, and carefully remove +cranium; be particularly careful in picking away the periotic bone, on +account of the flocculi of cerebellum. It is difficult to avoid injury to +the pituitary body embedded in the basisphenoid bone. Examine with +the help of Sheet 8. Make the sections there indicated. + + +-The Frog_ + +May be killed by drowning in dilute methylated spirit, or by chloroform. +Take a recently-killed frog, and examine a drop of its blood, spread +out on a glass slip, under the microscope; compare it with your own. +Before using the high power, put a cover glass over the object, of +course. Scrape the roof of the mouth of the frog gently, to obtain +ciliated epithelium; and mount in very weak salt solution-- the cilia will +still be active. Squamous epithelium may be seen by the student +similarly scraping the interior of his own cheek. Take a piece of +muscle from one of the frog's limbs, tease out with needles upon a +glass slip, and examine. To see the striations clearly, the high power +will be needed. Compare a piece of muscle from the wall of the +alimentary canal. Similarly examine nerve and connective tissue. + +First Dissection.-- Pin out the frog in a dissecting dish, ventral surface +uppermost, and cover with water. Open up the skin along the +mid-ventral line. Note the large sub-cutaneous lymph spaces, the +pelvic and pectoral girdles, and the anterior abdominal vein. Cut into +the body cavity on one side of this latter, cut across in front of where +the vein dips down to liver, and peel the body wall away from it. The +xiphisternum will probably be cut in this operation. In early spring the +females are greatly distended with ova, and the greater portion of the +ovary may, with advantage, be removed. The oviduct is dead white +then, and larger and much more in evidence than the (pinkish) +intestine even. Turn over the viscera, and compare with Sheet 11; +one lung is often found greatly inflated, and then projects back into +the body cavity; the stomach is, in some cases, pushed forward and +hidden behind the shoulder girdle. Observe the allantoic bladder, the +spleen, gall bladder, portal vein, and pancreas. By squeezing the gall +bladder gently, the bile duct will be injected with bile, and will be +apparent if the stomach is turned over. The oesophagus, just in front +of the stomach, should be cut through, and the rectum, and the +mesentery and alimentary canal supported by it, removed. This will +expose the urogenital organs. (Vide Figures given.) These vary +greatly, especially in the females, at different seasons. The condition +figured would be seen in late autumn, or winter. In spring females are +often found copulating with males, and then the ovary itself is +inconspicuous, while the lower part of the oviduct is enormously +distended with ova, 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 (wall of oviduct). The vena cava inferior is +seen between the kidneys and the renal portal vein beside the ureter. +Cutting through the mesentery supporting the kidney laterally, the +dorsal aorta is exposed, and on either side of it the sympathetic chain +and rami communicantes, often tinged with black pigment. +This black pigment is a frequent but variable feature of the frog's +anatomy, and usually dapples or blackens the testes, and also +sometimes darkens the otherwise pale pink arteries. Behind the +kidneys the sciatic plexus also becomes visible. Careful drawings +should be made. Cut off the head of the frog, shave off top of brain +case, and put the head in strong spirit. + +Second Dissection.-- A fresh frog is required. Pin out under water as +before, and open up body cavity. Now carefully remove the muscle +from the ventral portion of the shoulder girdle, to expose the clavicles +and coracoids. Cut away xiphisternum, and then cut through clavicles +and coracoids on either side, and remove ventral part of shoulder +girdle, to expose the heart. Open out the cut portions of body wall and +pin. The veins going towards the heart should now, with a little +examination, be evident. Make out the external jugular, the +innominate, and its two branches, and the pulmo-cutaneous and vena +cava superior. Clear by carefully picking away any shreds of +semi-transparent tissue. Make out, by feeling, the position of the +hyoid body, and of its anterior cornua. Note the hypoglossal nerve +(first spinal) running ventral to this, and the ninth cranial nerve, +running parallel to it but dorsal to the hyoid-- hidden therefore by +the hyoid, and reappearing in front. The vagus may also be made out less +distinctly, running "postero-ventrally" towards the heart. By clearing +the muscle by the rumus of the jaw, VII. may be seen, and the third +branch of V., running across the jaw at about the middle of its length. +Pick off the thin transparent pericardium from the heart very carefully, +and proceed to cut away all the veins made out. The truncus +arteriosus may then be followed up as it branches. Note all the +branches shown in the figures in this book. The precise position of the +vessels will vary to a certain extent with the attitude in which the frog +is pinned. The cutaneous artery will prevent the student following up +the aortic arch until it is cut; then the arch may be followed round until +it meets its fellow to form the dorsal aorta. Note the sympathetic +again. Make careful drawings of all this. Cut off lower jaw, and note +posterior nares and Eustachian openings. If time allows, remove the +heart, and examine by cutting open and washing. (Compare, Section +44) Remove eyeball, to see the first and second branches of the fifth +nerve, and the Vidian (i.e. palatial) branch of the seventh. + +Third Dissection.-- Read the account of the frog's skull carefully. +Take the head of a recently killed frog and drop into boiling water for +a minute. Then pick off, very carefully, muscle, connective tissue, +nerves, and etc., to clear the cranio-facial apparatus; examine the +bones, compare with figures given in this book, and draw. Take the +head, which has been in spirit a fortnight or so, pick away cranium, +and compare brain with figures given. Examine ventricles, by taking +sections, after drawings have been made. + + +-The Dog-Fish- + +First Dissection.-- Examine external characters, nasal grooves-- no +internal nares-- fins, spiracle, scales passing over lips, and cloaca. +Cut off tail below the cloacal opening. The males are distinguished by +the large claspers along the inner edge of the pelvic fin. Open up body +cavity. Usually this is in a terrible mess in the fish supplied by +dealers, through the post-mortem digestion of the stomach. Wash out +all this under a stream of water from a tap or water-bottle. Frequently +the testes are washed out of the male in this operation and ova from +the loose ovaries in the female. Now compare with figure given in this +book, allowing for the collapse of the stomach, if it has occurred. Cut +through the oesophagus and rectum, and remove alimentary canal +from body; cut open and wash out the intestine, and examine spiral +valve. Now make a careful examination of the cloaca and its +apertures, and dissect away the peritoneum hiding the kidney. In the +female find the opening of the oviducts in front of the liver. Remove +liver, and cut off body now behind pectoral fin. Before throwing tail and +hinder part of body away, note the myotomes of body wall, the +notochord and vertebral body, neural canal, and, in the tail, the +haemal canal. [(See {Section 9 the Dog-fish})] + + {Lines from First Edition only.} + -The relation of the vertebral bodies to the notochord may be very + well seen by taking successive slices, about one-tenth of an inch + thick, through the vertebral body. The cartilage is hard and + semi-transparent, the notochord jelly-like, least at the centres + of the centra, and at a maximum intervertebrally.- + +[The notochord is a soft jelly.] Cut away the ventral part of the pectoral +girdle, to open pericardium. With a seeker, make out the pericardio +peritoneal opening. Cut into the sinus venous, and run seekers into +the Cuvierian and hepatic sinuses. [Cut open the Cuvierian and +posterior cardinal sinuses, and run seekers into their affluents.] +Dissect along the truncus arteriosus to afferent branchials. [Cut away +the heart and oesophagus; run a seeker up the dorsal aorta and cut +along it from the ventral side to subclavian and efferent branchial +arteries.] Skin the top of the head. Note, while doing this, the yellow, +jelly-like sense-tubuli beneath the skin. Shave off top of brain-case, +and leave the head in spirit for a week or so. + +Second Dissection.-- Place the head with the ventral side downward, +skin all the dorsal surface as yet unskinned. Refer to book for +precise position of the anterior cardinal sinus, and then cut down +through body wall into this just over gill slits. The tenth nerve will +become visible, with its "slit" branches athwart the floor of the sinus. +Clear to make this more evident, and make out its lateral line and +visceral branches, and the ninth nerve. + + {Lines from Second Edition only.} + [The pharyngo-branchials may be felt beneath the sinus. Run a + seeker from the dorsal aorta to the efferent branchials.] + +Proceed now to orbit, and, without any dissection beyond the removal +of skin, make out recti and oblique muscles of eyeball, and the optic, +third and fourth nerves. Cut through these structures carefully and +remove, exposing nerves seven, and five, as described and figured in +the text. Examine the otic capsule by taking successive slices +through it to show the labyrinth of the ear. -Remove the dorsal wall of +the skull to obtain a dorsal view of brain. If this is sufficiently hard, +examine it; if not, return it to spirit for a more convenient occasion.- +[Examine brain.] + + +-Amphioxus_ + +Two specimens of this type should be obtained. It should be +examined entire by the naked eye and with the low power of the +microscope. Immersion, in glycerine will render it more transparent; or +it may be cleared with oil of cloves, put up temporarily in that, or +permanently in Canada balsam. One specimen should then be pinned +out in the dissecting dish, ventral side uppermost, and the atrium +opened to expose liver and pharynx. A part of the pharynx may be +examined with the low power to see the form of the gill slits. The +second specimen should be soaked in turpentine for some time, and +then dropped into melted paraffin wax. Transverse sections may then +be cut with a razor, the paraffin wax removed from these by solution in +turpentine, the turpentine in its turn dissolved out by alcohol, and the +sections, after immersion in oil of cloves, may be transferred to +Canada balsam for examination and preservation. This work should +not be attempted until some practical histological work has been done +in botany, and it may be altogether avoided by the purchase of +stained and mounted sections. + + +-Development_ + +Laboratory work in this portion of the science is not usually +undertaken by elementary students of biology, but the reader will +probably find it helpful, in the realization of the facts given in this +book, to look out for frog spawn, in February and March, and to catch +and examine tadpoles of various sizes. A small dissecting dish may be +made by pouring melted paraffin wax into one of those shallow china +pots chemists use for cold-cream, and tadpoles may be pinned out +with entemologists' pins and dissected with needles. But this is a +work of supererogation. Partially incubated hen's eggs may be +obtained at a small cost almost anywhere, and the later stages +profitably examined and dissected under warm water. For a clear +understanding of the allantois and amnion, this last is almost +indispensable. A few microscopic slides of sections of embryonic +chicks should also be compared with our rough diagrams. + + + + +-{Key for Dissection Sheets, and Abbreviations.}_ + +Sheet 1 + +Figure 1. Main facts of the Rabbit's Anatomy (diagrammatic). +an., anus. +a.ao., arch of the aorta. +au., auricle. +a.r., ad-renal body. +br., brain. +b.d., bile duct. +brch., bronchus. +cd.st., cardiac end of stomach. +co., colon. +cae., caecum. +ddnm., duodenum. +d.ao., dorsal aorta. +dia., diaphragm. +ep., epiglottis. +g.d., genital duct (either sex). +il., ileum. +in.art., innominate artery. +k., kidney. +lg., lung. +lv., liver. +l., larynx. +l.s.c., [l.c.c.] left common carotid artery. +m., mouth. +na., nasal passage. +oes., oesophagus. +p.v., pyloric valve. +p.d., pancreatic duct. +pt., peritoneal cavity. +r., rectum. +st., stomach. +[stm., sternum.] +s.r., sacculus rotundus. +s.c., spinal cord. +tr., trachea. +ur., ureter. +ur.b., urinary bladder. +v.b., a vertebral body. +v.ap., vermiform appendix. +v.v., [v.p.] velum palatium. +v., ventricle of heart. +v.c.i., vena cava inferior. + +Figure 2. The Liver (diagrammatic). +g.b., the gall bladder. +r.l., r.c., l.l.., l.c., right lateral and central, and left lateral and +central, lobes respectively. +sp., the Spigelian lobe (fits into angle of stomach and oesophagus). + +{Illustration: Diagram Sheet 1.} + + + + +Sheet 2 + +Figure 1. The Rabbit's Circulation (see footnote to Section 45). + +(Throughout l. indicates left, r. right. Vessels without r. or l. prefixed +are median.) + +-[* The figure is inaccurate at one point; l.c.c. should spring from the +base of inn. See Sheet 9.]- {First Edition only text} + +ao.a., aortic arch. +au., auricle. +az.v., (p.c. in Figure 2), azygos vein. +c.c., common carotid. +c.il.a., common iliac artery. +coe.a., coeliac artery. +d.ao., dorsal aorta. +e.il.v., external iliac vein. +e.ju., external jugular vein. +f., femoral artery. +h.v., hepatic vein. +inn., innominate artery. +in.j., internal jugular vein. +i.il.a., internal iliac artery. +i.il.v., internal iliac vein. +k., kidney. +lv., liver. +l.g.v., lienogastric vein (portal). +m.v., mesenteric (portal system). +p.m.a., posterior mesenteric artery. +p.v., main portal vein. +p.a. pulmonary artery. +r., rectum. +r.a., renal artery. +r.v., renal vein. +s.v., and a., spermatic (or ovarian) vein and artery (to genital +organ). +s.mes.a., superior mesenteric artery. +s.-cl.a., subclavian artery. +s.-cl.v., subclavian vein. +v.c.s., vena cava superior. +v.c.i., vena cava inferior. +v. or vn., ventricle. + +Figure 2. Figure of Circulation (simplified) illustrating certain points in +development to be referred to later. + +Figure 3. Respiration. See text, Section 41. + +Figure 4. Blood. See text, Section 35. + +{Illustration: Diagram Sheet 2.} + + + + +Sheet 3 + +Histological Figures, 1. + +{No numbers I., or II.} + +Figure III. An amoeba.-- +n., nucleus. +ns., necleolus. +c.v., contractile vacuole. + +Figure IV. Embryonic tissue from the blastoderm of a chick. + +Figure V. Columnar epithelium.-- +g.c.1, g.c.2, g.c.3, successive phases in the development of a goblet +cell. + +Figure VI. +g.end., is geminating endothelium; the cells divide and apparently +drop off to become white corpuscles in the lymph current. +sq.end., squamous endothelium from the mesentery. +sq.ep., squamous epithelium (from the mucous membrane within the +cheek). +st., are opening (stomata) communicating between the lymphatics in +the mesentery and the peritoneal (coelomic) space. + +Figure VII. Ciliated epithelium from the roof of the frog's mouth. + +Figure VIII. Forms of glands.-- +g.ep., is a gastric gland from the stomach; trs., below, is cross +section. This is one of the simplest types of gland. +s.g., a sweat gland, is also a simple tube, but convoluted below. +r.g., is a racemose gland, such as the pancreas, Brunner's or the +salivary glands. + +The kidney, we shall see later, is simply an aggregate of branching +tubuli (Sheet 7). + +Figure IX. A duodenal villus.-- +lac., the lacteal. +v., the vein. + +Figure X.a. Diagram of liver structure.-- +b.d., the inter-lobular bile duct. +h.a., the hepatic artery, bringing blood to oxygenate and nourish the +liver tissue, and similarly distributed. +h.v., the hepatic vein taking blood from the liver to the heart, its twigs +commencing in the lobuli (intra-lobular). +lb. lb., lobuli. +p.v., the portal vein bringing blood, from which substances are to +be elaborated, into the liver, and breaking up between the lobuli +(inter-lobular). + +Figure X.b. A diagram of the appearance of an injected liver lobule as +seen in section under the microscope. + +{Illustration: Diagram Sheet 3.} + + + + +Sheet 4 + +Histological Diagrams, 2. + +Figure XI. A blood capillary. White corpuscles are migrating through +the walls into the tissues (compare Section 66). + +Figure XII. Hyaline cartilage (Section 64). + +Figure XIII. +c.c., connective tissue corpuscle. +w.i.f., white inelastic fibres. +y.e.f., yellow elastic fibres. + + +Figure XIV. Botryoidal tissue (Section 66). + +Figure XV. Development of a fat drop.-- +f.d., fat drop, in a connective tissue corpuscle; c.c., in the formation of +adipose tissue (Section 67). + +Figure XVI. Diagrammic cross section of a long bone.-- +b.c., bone corpuscle in a lacuna. +H.v., Haversian vessel (in the Haversian canal) surrounded by +concentric lamellae of bone, c.l., and together with these and zones +of bone corpuscles, called a Haversian system. +i.l., inner lamellae. +m.c., medullary canal full of yellow marrow. +o.l., outer lamellae. +p.o., periosteum. + +Figure XVII. To illustrate bone development (Section 71). + +Figure XVIII. Dentition of rabbit, incisors 2/1, canine 0/0, premolar 3/2, +molar 3/3. + +{Illustration: Diagram Sheet 4.} + + + + +Sheet 5. + +Diagram of the Rabbit's Bones. +To be compared with the real things. + +D and D' show the fore and hind limbs, to illustrate their homology. +D is in the embryonic position. The radius and tibia are, at an early +stage in development, on the anterior edge of their respective limbs; +the ulna and fibula, posterior; the former are spoken of as preaxial in +position, the latter as postaxial. But in the adult the humerus is +twisted so that the proximal end of the radius lies at the outer side of +the elbow, whence it crosses the ulna, so that its distal end is +inside, while the femur is also twisted round, so that the entire tibia +is internal. + +Figures 1 and 2. -Limbs.-- +a.c., acetabulum. +acr., acromion. +as., astragulus. +c., carpus. +ca., calcaneum. +co., coracoid. +[coty., cotyloid bone.] +fb., fibula. +fe., femur. +g., glenoid cavity (for head of humerus). +hd., head of femur. +hum., humerus. +i., ilium. +is., ischium. +m.c., meta-carpals. +na., navicular. +o., olecranon process of ulna. +o.f., olfactory fossa. +pb., pubis. +r., radius. +u., ulna. + +Figure 3. -Sternum.-- +Mb., manubrium. +r1., r2., and etc., sternal ribs. +st., sternebrae. +xi., Xiphisternum. + +Figure 4. Vertebrae.-- +At., Atlas. +Ax., axis. +c., [b.] centrum. +C.V., caudal vertebra. +c.v., [Cer.V.] cervical vertebra. +ep., epiphysis. +f.r., fused rib (in cervical vertebrae). +L.V., Lumbar vertebra. +m., metapophysis (of lumbar vertebra). +n.a., neural arch. +n.s., neural spine. +r., rib. +S.V., sacral vertebra. +[T.V., Thoracic.] +tr.p., transverse process. +v.a.c., vertebrarterial canal. +z., zygapophysis. + +{Illustration: Diagram Sheet 5.} + + + + +Sheet 6. + +The Skull of Canis.*-- +1. Dorsal. 2. Ventral. 3. Right Lateral Aspect. 4. Section a little to the +left of the nasal septum. 5. Lower jaw (smaller) 6. Hyoid apparatus. + +{Lines from First Edition only.} +-*A Fox in this case. The skull is quite like that of a Dog, but it has +the advantage of more distinct sutures between the bones.- + +a.n., anterior nares. +a.s., ali-sphenoid. +b.h., body of the hyoid. +b.o., basi-occipital. +b.sp., basi-sphenoid. +c., condyle of the skull. +{c.1, c.4, canines.} +c.f., condylar foramen (for XII.). +c.h., cerato-hyal. +E.f., Eustachian foramen. +e.h., epihal. +-e.n., or a.n., the anterior nares.- +e.o., exoccipital. +eth., ethmoid. +e.t., ethmo-turbinal. +f., frontal. +f.l.a., foramen lacerum anterius. +f.l.m., foramen lacerum medium. +f.l.p., foramen lacerum posterius (for IX., X., XI.). +F.M., or f.m., foramen magnum. +f.o., foramen ovale. +f.r., foramen rotundum. +{i., incisors.} +ju., jugal. +m., molars. +m.t., maxillo-turbinal. +mx., maxilla. +na., nasal. +n.t., nasal turbinal. +o.f., optic foramen. +o.s., orbito-sphenoid. +p., or pal., palatine. +pa., parietal. +p.m., pre-maxilla. +p.m.1, p.m.4, premolars. +p.n., posterior nares. +p.sp., pre-sphenoid. +pt., pterygoid. +s.h., stylo-hyal. +s.m.f., stylo-mastoid foramen (for VII.). +s.o., supra-occipital. +sq., squamosal. +s.t., sectorial tooth. +t.h. thyro-hyal. +vo., -black line indicating position of- vomer. +z.p., zygomatic process of squamosal. + +{Illustration: Diagram Sheet 6.} + + + + +Sheet 7 + +Figure 1. Striated muscle fibre (of the Rabbit), ruptured to show +sarcolemma. +e.p., its end plate. +K.m., membrane of Krause. +n., nucleus. +nv., nerve. +sc., sarcolemma. +s.e., sarcous elements. + +Figure 2. Cardiac muscle. + +Figure 3. Unstriated muscle fibres. + +Figure 4. Diagram of the Skin. +b.v., blood vessel. +d., areolar tissue of the dermis (mesoblastic). +s.c., stratum corneum, and s.m., stratum mucosum of the +epidermis. +s.g., sweat gland. +t.c., tactile corpuscle. + +Figure 5. To illustrate Kidney structure.-- +a.b.v., and e.b.v., afferent and efferent blood-vessels, of which the +latter go to break up upon the tubli. +B.c., one of Bowman's capsules of the cortex; +ur.t., the uriniferous tubule running from it into the medulla, where +it loops and branches; around it branches a blood-vessel, of which the +latter go to break up upon the tubuli. +c., cortex. +g., glomerulus, a knot of blood-vessels in the capsule. +m., medulla. +p., pelvis. +ur., ureter. + +The water of the urine is probably filtered off in the capsule, the urea +and other salts secreted by the tubuli. + +{No Figure 6.} + +Figure 7. The Auditory structures of the Rabbit (diagram). See text, +Section 115. + +Figure 8. The Eye (diagram). See text, Section 111. + +Figure 9. The Retina (diagram). See text, Section 112. + +{Illustration: Diagram Sheet 7.} + + + + +Sheet 8 + +The Brain of the Rabbit.-- + +1. In median section. 2. From above, with the top of the right +hemisphere sliced off horizontally at the level of the corpus callosum. +3. A deeper section through the thalamencephalon, corresponding to +B in (1). 4. Under-view of the brain. 5. Diagram referred to in the text +and for comparison with Sheet 7, 3b., and Sheet 18, 2. + +{Figures 1-5.} +ar., arrow in the iter. +a.c., the anterior commissure, a thickening of the anterior wall of the +third ventricle. +c.c., corpus callosum. +c. cb., crura cerebri. +c.h., cerebral hemispheres. +c.q., corpora quadrigemina. +f.cbm. (right), flocculus of the cerebellum. +l.h., left cerebral hemisphere (=ch.). +l.l., lateral lobe of cerebellum. +m.c., middle commissure. +m.o., medulla oblongata. +op., optic nerve. +o.l., olfactory lobe. +o.th., (right), optic thalamus. +p.c., posterior commissure (thickening of postero-dorsal wall of the +third ventricle). +p.g., pineal gland. +pt., pituitary body. +p.V., pons Varolii. +s.c., thin roof of the fourth ventricle. +v.cbm., vermis of cerebrum. +v.l., lateral ventricle. + +{Figure 4.} Nerves.-- +I., Olfactory. +II., Optic. +III., Oculo-motor. +IV., Patheticus. +V., Trigeminal. +VI., Abducens. +VII., Facial (portio dura). +VIII., Auditory (portio mollis). +IX., Gustatory (glossopharyngeal. +X., Pneumogastric or vagus. +XI., Spinal Accessory. +XII., Hygoglossal. + +Figure 6. The Spinal Cord in section.-- +c.c., the central canal. +d.f., the dorsal fissure. +d.n., the dorsal nerve root; g., its ganglion. +v.f., the ventral fissure. +v.n., the ventral nerve root. + +Note that in Figure 1 the central canal is continuous with the fourth +ventricle. + +Figure 7. Histological elements.-- +g.c., multipolar ganglion cell. +n., nucleus of a medullated nerve. +a.c., its axis fibre. +s.S., (sheath of Schwann), medullary sheath interrupted at intervals by +n.R., the nodes of Ranvier. +n.m.f., a non-medullated fibre. + +{Illustration: Diagram Sheet 8.} + + + + +Sheet 9. + +-The Nerves of the Rabbit_. + +Figure I. Rough sketch of dissection of the neck from the left ventral +aspect.-- The bands of muscle between hyoid, mandible, and +sternum, and the thymus gland carefully cleared. +lr., is the larynx, and b., the balla. +s.m.g., the right sub-maxillary gland (the left has been removed). + +The nerves are numbered. + +l.r.l.n., [r.r.l.n.] is the left recurrent laryngeal looping under that +solid connection between the pulmonary artery (p.a.) and ao., the aortic +arch, which was an open tube in the embryo, the ductus arteriosus. +hy., is the hyoid with its posterior cornua. +ph.n., is the phrenic nerve. +r.r.l.n., [l.r.l.n.] is the right recurrent looping under the sub-clavian. +s.c.g., is the super or cervical ganglion of the sympathetic (sym.); +s.l.n., is the left superior laryngeal, and g. the left depressor +branch of x. +z., is the ramus descendens noni of the twelfth nerve. + +In early development the heart lay just beneath the pharynx in the +position of the larynx (compare Dog-fish and Frog); as the neck +elongated, the heart shifted back with its vessels, and so the long +loop of the recurrent laryngeal comes to be drawn out in this singular +way. + +Figure II. Diagram of orbit to show V.1 orbit-nasal, V.2 the maxillary, +and V.3 the mandibular branch of V. In order to show these in +dissection, the malar must be cut away, and the eye and glands of +the orbit removed. +s.r., e.r. [p.r.], i.r., and a.r., cut ends of the superior, external (or +posterior), inferior, and anterior (or internal) recti muscles. +s.o., and i.o., the superior and inferior obliques. + +Figure III. General diagram of the Rabbit's cranial nerves. + +Figure IV. Rough sketch of dissection of the nerves and +blood-vessels dorsal to stomach.-- + +The stomach turned over to the animal's right, the Spigelian liver lobe +cleared from the oesophagus, the mesentery supporting spleen and +hiding solar plexus picked off, and the mesentery hiding sympathetic +cleared. + +coe.art., coeliac artery, and s.m.a., superior mesenteric artery. +coe.g. coeliac, and s.m.g., superior mesenteric ganglion. The two +together form the solar plexus. +l.abd.sym., left abdominal sympathetic (in the actual dissection, the +right would also be visible). +l.a.r., left adrenal. +l.sp.n., left splanchnic nerve. +r.art., renal artery. +r.v., renal vein. +st., the stomach, and sp., the spleen. +x., the vagus on oes., the oesophagus. + +{Illustration: Diagram Sheet 9.} + + + + +Sheet 10. + +-Reproductive Organs of the Rabbit_. + +Figure 1. The Male. + +Figure 2. The Female Organs. (The symbols below the figures +indicate the sex.) + +pb., is the pubic symphysis [which has been] cut through. +R., the rectum, with r.g., the rectal gland, and a., the anus. +t., the tail. +r.ur., the right ureter. +l.ur., the left ureter. +ur.b., the urinary bladder. + +In the Male +ep., the epididymis. +P., the penis. +pp., the prepuce. +scr., the scrotal sac, containing these; +r.v.d., the right vas deferens. +T., is the testis. +u.m., the uterus masculinus. + +In the Female +c.ut, the left cornu uteri. +F.t., the left Fallopian tube. +ov., is the ovary, with a Graafian follicle, G.F. +V., the vagina. +v.b., the vestibule. + +Figure 3. Diagram of ovary with stages in the development of a +Graafian follicle 1, 2, 3, 4, 5, see text, Section 137. The arrow +indicates the changes in position of the developing follicles. + +{Illustration: Diagram Sheet 10.} + + + + +Sheet 11. + +Figure 1. General dissection of Frog (male). + +Figure 2. The heart and great vessels laid open. + +Figure 3. The circulatory system from the side. + +Figure 4. Blood. +{n., nucleus.} +r.c., red corpuscle (oval and nucleated). +w.c., white corpuscle + +Small figure of Frog in left-hand corner is to show position +of heel, h. + +Reference Letters. +all.b., allantoic bladder (= urinary bladder). +c.ad., corpus adiposum. +cl.c., cut end of the right clavicle. +d., duodenum. +g.b., gall bladder. +il., ileum. +k., kidney. +l.au., left auricle. +l.g., lung. +l.int., large intestine. +l.s.v., longitudino-spiral valve. +L.v., Liv., liver. +pan., pancreas. +r.au., right auricle. +sp., spleen. +st., stomach. +T., testis. +t.a., truncus arteriosus. +ur., urogenital duct. +v., ventricle of heart. + +Arteries (white). +ao., aorta. +c.a., carotid arch. +c.g. [c.gl.], carotid gland. +coe., coeliac. +cu., -and pa.",- cutaneous. +d.ao., dorsal aorta. +e.c., lingual artery. +[i.c., internal carotid.] +l.a.a., left aortic arch. +pa., and p., pulmonary. +p.c. [p.cu.], pulmo-cutaneous. +r.a.a., right aortic arch. +[s.cl., sub-clavian.] +t.a., truncus arteriosus. + +Veins of the Caval System -(black)-. +b.v., brachial (from fore limb). +e.j., external jugular. +h.v., hepatic vein. +i.j., internal jugular. +[in.v., innominate vein.] +l.v.c.s., left vena cava superior. +p.v., cutaneous vein. +[s.cl.v., sub-clavian vein] +s.s.r., sub-scapular vein. +v.c.i., vena cava inferior. + +Veins of the Portal and Renal Portal Systems -(shaded)-. +a.ad., and a.ab.v., anterior abdominal vein. +b.v., and p.v., united are called the sub-clavian vein. +l.fm., left femoral. +l.p., left pelvic. +l.r.p., (and r.p.) left renal portal. +l.sc., left sciatic. +p.v., portal vein. + +-(The anterior abdominal is coloured black in Figure 1.)- + +The cutaneous artery in the above figures is turned back. In dissection +it will be found to lie over and hide the dorsal-ward sweep of the aortic +arch. + +{Illustration: Diagram Sheet 11.} + + + + +Sheet 12. + +Figure 1. Upper view of the Frog's brain. + +Figure 2. Under view of the same. + +Figure 3. The same-- median section. + +Figure 4. The distribution of the Frog's nerves. Compare Sheet 9, +Figure III. + +The shaded part in 4 is the -otic capsule- [tympanum]. The hyoid +apparatus is roughly represented in black to show its relation to IX. +(dorsal to it) and sp. 1 (ventral). Compare {nerves} IX and XII in Sheet +9. The nerves are numbered. + +cb., the cerebellum. +c.h., cerebral hemispheres. +f.t., filum terminale. +g.tr., ganglion on the fifth nerve. +l.t., lamina terminalis. +mb., mid-brain. +md., medulla oblongata. +o.l., optic lobes. +pin., pineal gland. +pit., pituitary body. +r.h., olfactory lobes (rhinencephalon). +th.c., thalamencephalon. +sp.1, first spinal nerve. +sp.2, 3, brachial plexus to fore limb. + +Figure 5. The spinal column (and pelvic girdle) of the Frog. + +Figure 5b. Vertebrae. + +Figure 6. The pectoral girdle and limb, dorsal view. + +Figure 7. The pelvic girdle and right limb from the side. + +(l.h. shows the position of the right lymph hearts-- they are +paired.) + +as., astragalus. +b., body. +c., calcar (?= a sixth digit). +cal., calcaneum. +cl., clavicle overlying a procoracoid cartilage. +co., coracoid. +f., fibula. +[FE., femur.] +h., humerus. +il., ilium. +is., ischium. +o.st., omosternum. +pu., pubis. +r., radius. +sc., scapula. +s.sc., supra-scapula. +s.v., sacral vertebra. +t., tibia. +t.p., transverse process. +ul., ulna. +ur., urostyle. +x., xiphisternum. +z., zygapophysis. + +1, 2, and etc., first, second, and etc., digits. + +D. and D'. are simplified diagrams of the limbs for comparison with the +similar ones of the Rabbit. In each girdle we have a dorsal ossification +(scapula, ilium) and two ventral parts (pubis and procoracoid cartilage, +ischium and coracoid), and at the meeting-place of the three in each +case the proximal bone of the limb (humerus, femur) articulates. + +{Illustration: Diagram Sheet 12.} + + + + +Sheet 13. + +-Urogenital Organs of the Frog_. + +Figure 1. The Male. + +Figure 2. The Female. The oviduct removed on the animal's left, and +the ovary on its right. + +Organs common to both sexes.-- +al.b., allantoic bladder. +c.ad., corpus adiposum. +cl., cloaca. +int., intestine. +K., kidney. +lg., (dotted outline of) lung. +oes., oesophagus. +r.p.v., renal portal vein. +st., stomach. + +In the Male.-- +T., testis. +v.e., vasa efferentia. +u.g.d., urogenital duct. +p., prostate gland. + +In the Female.-- +adr., adrenal. +f.t., fallopian tube (anterior part of oviduct). * its opening. +o.d., oviduct (letters on [the opening] -uterine portion-). +ov., ovary. +ur., ureter. + +(This would be the condition about midwinter.) In March o.d. will be +either enormously distended with eggs, or large, flabby, and empty, +and ov. will be small and brownish, without any large eggs; the ovary +gradually recovers its size through the summer. + +Figure 3. Spermatozoa attached to the parent cell (g.e.) from the +lining epithelium of the testis, and one free. +fl., the flagellum. + +{Illustration: Diagram Sheet 13.} + + + + +Sheet 14 + +-Skull Structure and Development of the Frog_. + +Figure 1. I., II., early and late stages of the Tadpole's chindrocranium. +Diagrammatic. + +Figure 2. Dorsal view of a young Frog's cranium-- the membrane +bones removed. Diagrammatic. + +Figures 3 and 4. Dorsal and ventral views, respectively, of the Frog's +skull-- the lower jaw removed. + +Figure 5. Side view of the Frog's skull. + +Figure 6. Median section of the brain case. + +Figure 7. The hyoid apparatus. + +Figure 8. I., II., III., progressive stages of the Tadpole's skull from the +side. After W. K. Parker. + +Figure 9. F., side and hind views of the Frog's skull. D., the same of +the Dog. Roughly diagrammatic. + +N.B.-- +In all cartilage is dotted, cartilage bone cross-barred, and membrane +bone, white. In Figure 4, pt., should be cross-barred; +and in 5, th.h. plain. + +a.c., anterior cornu of hyoid [(= CH.)] -not lettered, in {Figure} 5-. +a.o., antorbital cartilage. +ar., angulo-splenial -(On Frog Section 34, for Articulare read +-Angulo-Splenial_)-. +-b., parachordal part of brain box-. +b.c., brain case. +b.h., body of hyoid. +b.r., branchial arches. +CH = a.c. +c.t., cornua trabeculi. +d., dentary. +e., eye. +E.N., external nares. +e.o., exoccipital bone. +f., fenestra (membranous part of cranial wall). +-f.p., fronto-parietal.- +h.m., hyomandibular cleft = Eustachian tube and ear drum. +mb., mandible. +[M.C., Meckel's Cartilage.] +m.mk., mento-Meckelian bone. +m.p., mouth passage. +mx., maxilla. +n.c., notochord. +n.o., nasal organ. +n.p., nasal passage. +ot., or o.c., otic (auditory) capsule. +pal., palatine bone. +PAL., hard palate of Mammal. +p.c., parachordal. +p.f., [parieto-frontal] -see f.p.- +p.m., premaxilla. +P.N., internal nares. +p.o., prootic bone. +p.p., palato-pterygoid cartilage. +psph., parasphenoid bone. +pt., pterygoid bone. +q., quadrate cartilage. +q.j., quadrato-jugal. +s.e., sphenethmoid bone. +sq., squamosal. +t., trabecular part of brain box. +t.c., trabecula. +th.h., thyrohyal. + +{Illustration: Diagram Sheet 14.} + + + + +Sheet 15 + +Figure 1. Dissection of -Male- [Female] Dog-Fish to show alimentary +canal, the pericardium also being opened and the cloaca slit up. +[Above is also seen the dorsal view of the head.] + +Figure 2. The pelvic girdle and fin skeleton [of a male]. + +{No Figure 3, in First Edition.} + +Figure 4. The spiral valve in the colon. {Figure 3, in Second Edition.} + +a.p., abdominal pore. +aur., -auricle- [atrium] of heart. +b.d., bile duct. +b.pt., basi-pterygium. +-cl., clasper.- +cl.c., -its- [the] supporting cartilage [of the clasper]. +co., colon. +d'dnm., duodenum. +e., the eye. +g.bl., gall bladder. +g.s., gill slits. +L.Lv., left lobe of liver. +M.Lv., middle lobe of liver. +olf., olfactory opening. +[pan., pancreas.] +pcd., pericardial wall. +pel.g., the pelvic girdle. +p.p., arrow through pericardio-peritoneal canal. +r.g., rectal gland. +[R.Liv., right lobe.] +sp., spiracle. +spl., spleen. +st., the stomach. +s.v., sinus venosus. +u.g.p., uro-genital pore. +v., ventricle. + +{Illustration: Diagram Sheet 15.} + + + + +Sheet 16. +Figure 1. Circulation of the Dog-Fish. + +Figure 2. Simplified and more typical fish circulation, in which the +posterior cardinals have not coalesced in the median line. The +Cuvierian veins = the vena cava superior of the higher type; the +posterior cardinal is represented by the azygos vein in the Rabbit. +Compare Sheet 24, Figure 7, and Sheet 2, Figure 2. + +Figure 3. Side view of the pericardium. + +a.br., afferent branchial artery. +a.c.s., anterior cardinal sinus (= internal jugular vein). +au., atrium (auricle) (= the two auricles of higher forms). +b.a., bulbus arteriosus. +c.a., conus arterious. +cd. a., caudal artery. +cd.v., caudal vein. +c.s., Cuvierian sinus. +d.a., dorsal aorta. +E., eye. +e.br., efferent branchial arteries. +g.s., in position of gill slits. +h.br.a., hypobranchial artery. +H.S., hepatic sinus. +[i.j.s., inferior jugular sinus (= external jugular vein).] +K., kidney. +L.V., lateral vein. +[oe.s., ventral wall of oesophagus.] +P.C.C., pericardial cavity. +P.C.S., posterior cardinal sinus. +p.p.c., pericardio-peritoneal canal. +P.V., portal vein. +r.p.v., reno-portal vein. +s.c.v., subclavian vein. +Vn., ventricle. +-v.s.v., inferior (= external) jugular vein-. + +Figure 4. Skeleton of pectoral limb, and girdle.-- +g., the girdle (also in Figure 3). +m.p., meso-pterygium. +mt.p., meta-pterygium. +p.p., pro-pterygium. +sc., its dorsal portion. + +{Illustration: Diagram Sheet 16.} + + + + +Sheet 17 + +-The Uro-genital Organs of the Dog-Fish_. + +Figure 1. The Female, the oviduct of the left side cut away, -and an +egg case in the oviduct.- + +Figure 2. The Male. + +The rectum is removed in both cases, and the silvery peritoneum +dissected off from the kidneys. + +Figure 3. A generalized diagram of the uro-genital organs.-- +All references in text. +Ms., the mesonephros, is the epididymis in the male, and is reduced +in the female; Ms.d., its duct, is the vas deferens in the male, and +persists only as the urinary receptacle in the female. +Mt. and Mt.d., the metanephros and metanephric duct, become the +functional kidney and ureter in both sexes. G. is the gonad +(reproductive gland), and M.L. the animal's middle line (median +plane). +-Ps.-, [Pr.,] the pronephros, is never developed in the Dog-fish; +P.d., its supposed duct, is the oviduct of the female, and is +suppressed in the male. + +{Illustration: Diagram Sheet 17.} + + + + +Sheet 18. + +Figure 1. The Dog-Fish Brain, dorsal view. + +Figure 2. Median section of the same. To the right a more +diagrammatic figure. The nerves are numbered:-- +[BR1, BR2, BR3, BR4 branches of X forking over the second to the +fifth gillslit.] +cb., cerebellum. +h.s.c., horizontal semi-circular canal of ear, exposed by the slicing +down of the otic mass. +[LAT., lateral-line branch of X.] +m.o., medulla oblongata. +oph., ophthalmic nerve (V.1+VII.1). +op.l., optic lobe. +pit., pituitary body. +pr.c., prosencephalon (cerebral hemisphere). +rh., olfactory lobe (rhinencephalon). +r.t., -its- restiform tracts [of medulla]. +-st-. [S.P.G.], stalk of the pineal gland. +th., thalamencephalon. +th.c., thalamencephalon. +-ut., the utriculus, seen through the semi-transparent cartilage-. +Vid., the Vidian branch of VII. +[Visc., visceral branch of X.] + +Figure 3. Diagram of the ear of a fish. +The structure of this is easily made out by clearing otic capsule and +cutting slices of the cartilage in the Dog-Fish (e.g., Figure 1, h.s.c.). + +amp., their ampullae. +a.v.c., p.v.c., h.c., anterior, posterior, horizontal canal respectively. +[amp., the ampullae.] +d.e., the ductus endo-lymphaticus. +-sac., the sacculus; c., a small outgrowth of the latter, corresponding +to the rabbit's cochlea-. +-ut., the utriculus-. + + +Figure 4. The cranium and branchial bars of a Dog-Fish. +The groove in the otic capsule connects the orbital and anterior +cardinal sinuses. + +A.C.S., position of the anterior cardinal sinus (dotted outline). +c., the vertebral centra. +c.b., the cerato-branchial. +c.h., the cerato-hyal. +e.b., epi-branchial. +ex.b., extra-branchial. +h.M., the hyo-mandibular. +i.n.p., inter-neural plate. +M.C., Meckel's (lower jaw) bar. +Na.C., the nasal capsule. +n.p., neural plate. +n.s., neural spine. +Ot.C., the otic capsule. +ph.b., the pharyngo-branchial. +P.pt., the palato-pterygoid bar (upper jaw bar). +p.s., pre-spiracular ligament, containing a cartilaginous nodule. +r., rib. +sp., the position of the spiracle. + +Figure 5. Diagrams of a vertebral centrum.-- For reference letters, +see text (Section 9). + +{No Figure 6, in First Edition.} +[Figure 6. Diagram for comparison with Figure III., Sheet 9.] + +{Illustration: Diagram Sheet 18.} + + + + +Sheet 19. + +Figure 1. Amphioxus, seen from the right side. a----b shows the +natural size. The animal is supposed to be clarified, and mounted in +some highly refracting medium, so that it is practically transparent; I., +II., III., and etc., refer to the section figured on Sheet 20. + +Figure 2. Amphioxus, General Dissection. (Slightly altered from a +figure by Professor E. R. Lankester.) The ventral atrial wall is +removed. The pharynx cut away from the dorsal body-wall, and with +the true ventral body-wall turned over to the (animal's) right. The arrow +a., a., passes through anus to intestine; b., b., is thrust through the +atrial pore to the atrial cavity. Note coe., the body cavity. + +References to the two figures. +an., anus. +at., atrial cavity. +at.w., atrial wall. +at.p., atrial pore. +a.d., anterior dilatata of nervous system. +b.w., body-wall. +b.t.L., brown tubes of Lankester. +c.f., ciliated funnel. +coe., coelome. +c.ao., cardiac aorta. +d.ao., dorsal aorta (paired). +d.ao'., dorsal aorta median. +g., gonads (male or female genital gland). +hep., hepatic vein. +in., intestine. +i.w., intestine wall. +lv., liver. +m.f., median fin. +n.c., notochord. +p.v., portal vein. +ph., pharynx. +-p.s.-, [e.s.] pigment spot ("eye spot"). +s.c., spinal cord. + +{Illustration: Diagram Sheet 19.} + + + + +Sheet 20 + +-Sections of Amphioxus_. + +The Roman numerals indicate the corresponding region in Figure 1, +Sheet 19. The lettering is identical; but note, in addition; +br.c., branchial canal. +c.f., ciliated funnel. +d.c.c., dorsal coelomic canal. +end., endostyle. +ep., epipleur. +e.s., eye spot. +h.p., hypopharyngeal grove. +h.vn., for hepatic vein. +o.c., oral cavity (or hood). + +{Illustration: Diagram Sheet 20.} + + + + +Sheet 21. + +-Phases in the Development of Amphioxus_. + +Figures 1, 2, 3, 4. Phases in segmentation. + +Figure 5. The blastosphere. + +Figure 6. The gastrula in section, anterior end to the right. + +Figure 7. i. Dorsal view post gastrula stage. + +Figure 7. ii. Diagrammatic section of the same in the position +indicated by the transverse line in 7, i. + +Figure 8. Diagrammatic section of a later stage. +coe.p., the coelomic pouches. +n.c., the notochord. +n.p., the neural plate. + +Figure 9.i. Still later section. + +Figure 9. ii. Diagrammatic view of late embryo. + +Figures 10, 11, 12 illustrate the formation of the atrium as a median +ventral invagination, at. + +{Illustration: Diagram Sheet 21.} + + + + +Sheet 22. + +-The Development of the Frog_. + +These diagrams must be studied with the text. They should be +compared with the corresponding ones of Amphioxus as indicated +below. + +Figures 1, 2, 3. Stages in segmentation (compare 1, 2 ,3 of {Sheet +21} Amphioxus). + +Figure 4. Blastosphere stage (compare 5, Amphioxus). This, on a +smaller scale. The cells on the ventral side are so much larger +because distended with yolk. + +Figure 5. Gastrula stage in section (compare 6, Amphioxus). The +Frog on a smaller scale than Amphioxus. + +Figure 6. Dorsal view of gastrula (compare 7, Amphioxus). + +Figure 7. Part of a transverse section of developing tadpole, +corresponding to Figure 8 of Amphioxus. + +Figures 8 and 9. Diagrammatic longitudinal sections of tadpoles +(compare 9. ii. of Amphioxus). Y. represents a mass of +yolk cells. + +Figure 10. Side view of young tadpole, showing external gills (e.g.) +and suckers (s.). Note the ventral bulging due to the yolk. + +Figure 11. Ventral view of a later tadpole. +op., the operculum. +int., coiling intestine. + +Figure 12. Head of still later tadpole in horizontal section to show +atrial chamber formed by operculum. +int.g., internal gills. +L., developing lungs. + +Figure 13. Diagrammatic cross-section of the mid-dorsal part of an +embryonic vertebrate. + +ao., aorta. +B.C., Bowman's capsule. +coe., coelom. +d.g., ganglion on dorsal root of spinal nerve. +gl., -its branch- [arteriole] to form glomerulus. +g.r., genital ridge. +I., intestine. +M.D., Mullerian duct. +ns. [nst.], nephrostome. +n.c., notochord; -n.s.-, [n.sh.] its sheath. +s.c., neural canal. +W.D., Wolffian duct. + +{Illustration: Diagram Sheet 22.} + + + + +Sheet 23. + +-The Development of the Fowl_. + +Figure 1. Diagram of the early ovum. The section below is a small +portion of the blastodermic area. +b.d., blastoderm. +y., the undivided yolk. +s.c., the segmentation between the blastoderm and yolk. Compare +s.c. in {Sheet} 22, {Figure} 4. + +Figure 2. Area pellucida about the sixteenth hour. The figure below is +the central part of the section indicated by the transverse line, and +showing the primitive streak (p.s.). + +Figure 3. Area pellucida about the twenty-first hour. Two sections +through a and b below. + +Figure 4. About the twenty-fifth hour; surface view; longitudinal section +to right and transverse above. + +Figure 4b. Diagrammatic rendering of same stage (compare Figure 9 +of Frog and 9.ii. Amphioxus). This will be most clearly understood if +the reader look at Sheet 22, {Figure} 9, and imagine Y. enormously +increased, and the embryo sinking into it. Epiblast, ep., -line of +dashes- [black line]. Mesoblast, dotted. Hypoblast, -black- [line of +dashes]. pp., the pleuro-peritoneal cavity. + +Figure 5 and 6 illustrate formation of amnion (a.) and allantois (all.). +6 is about the fourth day. + +{Illustration: Diagram Sheet 23.} + + + + +Sheet 24. + +-The Development of the Fowl_. +Figure 1. Chick about the -fifth- [third] day. At this stage the chick lies +on its left side in the yolk. [For lettering of blood vessels, see (7) +below.] +i., the intestine. +u.v., the yolk sac. +v.v., the vitelline veins. +al., the allantois. + +Figure 2. Chick about sixth day. + +Figure 3. Development of heart. + +Figure 4. Development of the eye. + +Figure 5. Chick about the sixteenth day. +A.M. is the amnion surrounding the embryo. Note particularly how the +allantois (al.) has spread over surface of shell and how the yolk sac is +shrivelled. + +Figure 6. Figures to illustrate the relative function and importance of +allantois and yolk sac in bird and mammal. In the fowl, however, the +blood-vessels of the allantois also probably absorb the albumen of the +egg, and may excrete urea into the egg-space. + +Figure 7. Simplified figure of the embryonic circulation, for comparison +with the similar figures annexed to Dog-Fish and Rabbit. + +{Lines from Second Edition only.} +[A.C., anterior cardinal. +Ao., Aorta. +Br4, sixth aortis arch (fourth branchial). +C.S. Cuvierian sinus. +H., the heart. +I.C., inferior cava. +P.C., posterior cardinal vein. +Tr.A., truncus arteriosus. +v.v., vitelline vein.] + +Figure 8. Chick on the nineteenth day. + +{Illustration: Diagram Sheet 24.} + + + +***END OF THE PROJECT GUTENBERG EBOOK TEXT BOOK OF BIOLOGY, PART 1: +VERTEBRATA*** + + +******* This file should be named 21781.txt or 21781.zip ******* + + +This and all associated files of various formats will be found in: +https://www.gutenberg.org/dirs/2/1/7/8/21781 + + + +Updated editions will replace the previous one--the old editions +will be renamed. + +Creating the works from public domain print editions means that no +one owns a United States copyright in these works, so the Foundation +(and you!) can copy and distribute it in the United States without +permission and without paying copyright royalties. Special rules, +set forth in the General Terms of Use part of this license, apply to +copying and distributing Project Gutenberg-tm electronic works to +protect the PROJECT GUTENBERG-tm concept and trademark. 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