External Form and General Considerations

Text Book of Biology, Part 1: Vertebrata by H. G. Wells, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. External Form and General Considerations

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; 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 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.

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