Text Book of Biology, Part 1: Vertebrata

by H. G. Wells

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.