19,99 €
Mehr erfahren.
- Herausgeber: Science & Technology Publishing
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
Text Book of Biology, Part 1: Vertebrata is a comprehensive and foundational work designed to introduce students and enthusiasts to the study of vertebrate biology. Authored by H. G. Wells, with contributions from G. P. Bidder and others, this textbook was originally published in the late 19th century and remains a valuable resource for understanding the structure, function, and classification of vertebrate animals. The book systematically explores the major groups within the vertebrate subphylum, including fishes, amphibians, reptiles, birds, and mammals, providing detailed descriptions of their anatomy, physiology, and evolutionary relationships. The text is organized to facilitate learning, beginning with general principles of biology and the characteristics that define vertebrates. It delves into the comparative anatomy of different vertebrate classes, highlighting both their unique adaptations and shared features. The book includes clear explanations of complex biological concepts, supported by numerous illustrations and diagrams that enhance comprehension. Topics such as the skeletal, muscular, circulatory, respiratory, and nervous systems are thoroughly covered, offering readers a holistic view of vertebrate biology. In addition to anatomical and physiological details, Text Book of Biology, Part 1: Vertebrata addresses the ecological roles and life histories of vertebrates, emphasizing their importance in natural ecosystems. The text also discusses methods of scientific observation and dissection, making it a practical guide for laboratory work and field studies. Throughout, the authors maintain a clear and engaging style, making the material accessible to both beginners and more advanced students. This classic textbook not only serves as an introduction to vertebrate biology but also reflects the scientific knowledge and educational approaches of its time. It is an essential reference for anyone interested in zoology, comparative anatomy, or the history of biological sciences. Whether used for formal study or personal enrichment, Text Book of Biology, Part 1: Vertebrata offers a thorough and insightful exploration of the fascinating world of vertebrate animals.
Das E-Book können Sie in Legimi-Apps oder einer beliebigen App lesen, die das folgende Format unterstützen:
Seitenzahl: 271
Veröffentlichungsjahr: 2025
Ähnliche
E-text Prepared by "Teary Eyes" Anderson
University Correspondence College Tutorial Series.
-Text-Book Of Biology._
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)_
2. _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._
1. _The Development of the Frog._
2. _The Development of the Fowl_
2. _The Development of the Rabbit_
2. _The Theory of Evolution_
2. _Questions on Embryology_
-Miscellaneous Questions._
-Note on Making Comparisons._
-Syllabus Of Practical Work._
-The Rabbit_
-The Frog_
-The Dog-Fish-
-Amphioxus_
-Development_
-{Key for Dissection Sheets, and Abbreviations.}_
Sheet 1
Sheet 2
Sheet 3 Histological Figures, 1.
Sheet 4 Histological Diagrams, 2.
Sheet 5. Diagram of the Rabbit's Bones.
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.
Sheet 7.
Sheet 8. The Brain of the Rabbit.--
Sheet 9. -The Nerves of the Rabbit_.
Sheet 10. -Reproductive Organs of the Rabbit_.
Sheet 11.
Sheet 12.
Sheet 13. -Urogenital Organs of the Frog_.
Sheet 14. -Skull Structure and Development of the Frog_.
Sheet 15.
Sheet 16.
Sheet 17. -The Uro-genital Organs of the Dog-Fish_.
Sheet 18.
Sheet 19.
Sheet 20. -Sections of Amphioxus_.
Sheet 21. -Phases in the Development of Amphioxus_.
Sheet 22. -The Development of the Frog_.
Sheet 23. -The Development of the Fowl_.
Sheet 24. -The Development of the Fowl_.
E-text Prepared by "Teary Eyes" Anderson
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 Considerations2. The Alimentary Canal of the Rabbit3. The Circulation4. The Amoeba, Cells and Tissue5. The Skeleton6. Muscle and Nerve7. The Nervous System8. Renal and Reproductive Organs9. Classificatory Points10. Questions and Exercises The Frog-- 1. General Anatomy2. The Skull of the Frog (and the Vertebrate Skull generally)3. Questions on the Frog The Dog-Fish-- 1. General Anatomy2. Questions on the Dog-Fish Amphioxus-- 1. Anatomy2. The Development of Amphioxus3. 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. HowesRoyal 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.
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.
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. WellsNovember, 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 spinalcolumn 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; anteriorly 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 posteriorly 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 partialoxydation 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
