SKELETON OF THE RABBIT.
The head in the Rodents is generally of small or moderate size in proportion to the body, and the skull is usually rather elongated, and flattened on the upper surface. The tympanic bullæ are generally of considerable size; the zygomatic arch is in nearly all cases well developed; but the orbits of the eyes are never closed behind, and only in certain families is there even a small process of the zygomatic arch behind the orbits, as an indication of possible closure. Of the vertebral column we need only say that the lumbar vertebræ are remarkable for possessing large transverse processes directed forwards, and that the tail varies greatly in length, being sometimes longer than the body, sometimes reduced to very small proportions, whilst between these two extremes almost every grade of development may be met with.
The sternum, or breast-bone, is usually long and narrow. Collar-bones are nearly always present, but in a few forms they become rudimentary, or even disappear altogether. The pelvis is long and narrow. The limbs exhibit a very great variety in their development; in many, the two pairs are nearly equal in length, but in the majority the hind limbs are distinctly longer and more powerful than their fellows, and in some groups they attain a most disproportionate length, and serve almost exclusively as the organs of locomotion. On the other hand, in the great majority of the order, the fore limbs serve in a certain degree as hands, and are used for holding the food to the mouth; and in these the radius and ulna, which are always distinct bones, retain the power of rotation. The corresponding bones in the hind limbs (tibia and fibula) are, on the contrary, firmly anchylosed together in two great groups of the order. The feet have usually five toes, but sometimes this number is reduced to four, or even to three, in the hind feet. These toes are armed with claws, which, however, in one family, acquire more or less of the appearance of hoofs.
In point of intelligence the Rodentia do not stand high. The brain is comparatively small, and the cerebral hemispheres show no traces of those convolutions of the surface which are characteristic of most Mammals (see figures). The Capybara alone is known to have a few convolutions. The cerebellum is entirely uncovered by the hemispheres. The organs of the senses are generally well developed, and the eyes and external ears, especially, are often of large size. In the Mole Rats and some other burrowing forms, however, the external ears are entirely wanting, and the eyes are very much reduced in size, and in some instances even concealed beneath the skin. The intestinal canal is long, and in all but one family furnished with a distinct cæcum.
BRAIN OF BEAVER (from above).
BRAIN OF BEAVER. (Profile.)
The body in the Rodents is generally plump and short, and the head is borne upon a short neck. The limbs also are usually short, so that the belly is close to the ground; but in some cases all four legs are of moderate length, or, as already stated, the hind legs are enormously developed, forming powerful leaping organs. In general structure, as to a certain extent in habits, there is, in fact, a most striking parallelism between the Rodentia and the Insectivora (see Vol. I., p. 343); in both we find arboreal and terrestrial forms, and among the latter some specially organised for burrowing in the earth, and others equally adapted for springing lightly over its surface; a few, also, in both orders, are aquatic. But here the parallel ceases. The dentition in the two groups is widely divergent, and, as might be anticipated from this circumstance, the food is very different; for, although some Rodents, such as the common Mouse and Rat, are omnivorous, there is no doubt that, as a whole, the Rodents must be regarded as vegetarians. Grass and the leaves of plants and trees furnish some of them with nourishment; whilst others feed upon fruits, seeds, and nuts, in the consumption of which last the powerful incisor teeth come into play. Many species lay up stores of food for the winter season, of which they pass more or less in a state of torpidity; and some of these are provided with cheek-pouches, often of considerable size, in which to convey their harvest into their store-houses.