Text Book of Biology Part 6

Section 140. In the male we find, in the position of the female uterus, a uterus masculinus (u.m.). The essential s.e.xual organ is the testis (T.), a compact ma.s.s of coiling tubuli, which opens by a number of ducts, the vasa efferentia, into a looser and softer epididymis (ep.), which sends the s.e.xual product onward through a vas deferens (v.d.), to open at the base of the uterus masculinus. The urinary bladder and ureters correspond with those of the female, and the common urogenital duct (= vestibule), the urethra, is prolonged into an erectile p.e.n.i.s (P.) surrounded by a fold of skin, the prepuce. A prostate gland (pr.), contributes to the male s.e.xual fluid. The character of the essential male element, the spermatozoon, the general nature of the reproductive process, will be conveniently deferred until the chapters upon development are reached.

9. _Cla.s.sificatory Points_

Section 141. The following facts of cla.s.sificatory importance may now be considered, but their full force will be better appreciated after the study of other vertebrate types. They are such as come prominently forward in the comparison of the rabbit with other organisms.

Section 142. In the first place, the rabbit is a metazoon, one of the metazoa, i.e., a multicellular organism, as compared with the amoeba, which belongs to the protozoa or one-cell animals (Section 55). In the next place, it is externally bilaterally symmetrical, its parts balance, and where, in its internal anatomy, it departs from this symmetry (as in the case of the aorta, the stomach and intestines, and the kidneys), the departure has an appearance of being the results of partial reductions and distortions of an originally quite symmetrical plan. And the facts of development strengthen this idea; in the very earliest stages we have paired aortic arches, of which, the left only remains, a straight alimentary ca.n.a.l, and less asymmetrical kidneys. In the vast majority of animals the same bilateral symmetry is to be seen, but in the star-fish and sea-urchins, and in the jelly-fish, corals, sea anemones, and hydra, the general form of the animal is, instead, arranged round a centre, like a star and its rays, and the symmetry is called radial.

Section 143. We also see in various organs of the rabbit, and especially in the case of the limbs and vertebral column, what is called metameric segmentation, that is, a repet.i.tion of parts, one behind the other, along the axis of the body. Thus the bodies and arches of the vertebrae repeat each other, and so do the spinal nerves. The renal organ of the rabbit, some time before birth, displays a metameric arrangement of its parts; but this disappears, as development proceeds, into the compact kidney of the adult. But the metameric segmentation in the rabbit's organism is not nearly so marked as that of an earthworm, for instance, which is visibly a chain of rings. If the student wants a perfect figure of metameric segmentation he should think of a train of precisely similar carriages, or a string of beads. One bead, one carriage, one vertebra, would be a metamere.

Section 144. In contrast to metameric segmentation is the antimeric repet.i.tion of radial symmetry (Section 142), in which each ray of the star is called an antimere. It is possible to have bilateral symmetry without a metameric arrangement of parts, as in the mussel and the cuttle-fish; but metameric segmentation without complete or reduced bilateral symmetry does not occur.

Section 145. We are now in a position to appreciate the fact that the old and more popularly know division of animals into vertebrata and invertebrata scarcely represents the facts of the case, that the primary division should be into protozoa and metazoa, and that the vertebrata are one of several groups of metazoa with a fundamental bilateral symmetry and imperfect metameric segmentation.

The rabbit is one of the vertebrata, and, in common with all the other animals collected under this head, it has--

(a) A skeletal axis (the vertebral column) between its central nervous system and its body cavity. In the adult rabbit this consists of a chain of vertebrae, but in the embryo (i.e., the young rabbit before birth) it is represented by a continuous chord, the notochord, and it remains as such in some of the lowest vertebrata throughout life. In other words, in these lower vertebrata, the vertebral axis is not metameric.

(b) A dorsal and -Tubular_ nervous axis. (Section 131, the central ca.n.a.l)

(c) It has, though in the embryo only, certain slits between the throat and the exterior, like the gill slits of a fish. Such slits are-- with one or two remarkable exceptions outside the sub-kingdom-- distinctly vertebrate features, and remain, of course, in fishes throughout life.

The presence of true cartilage and bone mark a vertebrate, but vertebrata occur in which -these tissues- [bone] -are- [is] absent.

Section 146. The rabbit shares the following features with all the vertebrata, except the true fishes, which do not possess any of them--

(a) Lungs (but many fish have a swimming bladder which answers to the lungs in its anatomical relations.)

(b) Limbs which consist of a proximal joint of one bone an intermediate part of two, and a distal portion which has five digits, or is evidently a reduced form of the five-digit limb.*

(c) The absence of a median fin supported by fin rays.**

* The frog shows indications of a sixth digit.

** The frog's tadpole has a median fin, but no fin rays.

Section 147. The rabbit shares the following features with all the vertebrata above the fishes and amphibia (= frogs, toads, newts, and etc.)--

(a) Absence of gills (not gill slits, note) at any stage in development.

(b) An amnion, and

(c) An allantois in development.

The meaning of (b) and (c) we shall explain to the student in the chapters on embryology. We simply mention them here to render our table complete.

Section 148. The rabbit shares with all mammals, and differs from all other vertebrata (i.e., birds, reptiles, amphibia, and fishes), in having--

(a) Hair.

(b) A diaphragm.

(c) Only one aortic arch, and that on the left side of the body.

(d) Its young born alive. (But two very reptile-like mammals of Australia, the duck-billed platypus and the echidna, lay eggs, and certain fish and reptiles bear living young.)

(e) Epiphyses to its vertebral -centre- [centra].*

(f) The cerebral hemispheres covering the mid-brain.

(g) Corpora quadrigemina instead of bigemina.

[(h) A corpus callosum.]

[(i) A spirally coiled cochlea to the internal ear.]

[(In respect to h and i also, the echidna and platypus are scarcely mammalinan.)]

* But certain mammals have no such epiphyses.

Section 149. The rabbit, together with the hares and conies, rats and mice, voles, squirrels, beavers, cavies, guineapigs is included in that order of the cla.s.s of mammals which is called the rodentia, and is distinguished by the character of the incisor teeth from other orders of the cla.s.s.

10. _Questions and Exercises_

1. Describe the venous circulation of the rabbit (with diagrams).

Compare a vein and artery. Compare the distribution of the great venous trunks with that of the arterial system.

2. Construct a general diagram of the circulation of the rabbit, to show especially the relation of the portal system, the lymphatics and lacteals, and the renal circulation to the main blood current.

3. Draw the alimentary ca.n.a.l of the rabbit from memory.

4. What is a villus? Describe its epithelium, and the vessels within it.

Write as explicit an account as you can of the absorbent action of a villus.

5. Tabulate the alimentary secretions, and their action on the food.

6. What is botryoidal tissue? Where does it occur? What is known of its functions?