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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
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. |
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