Although this disposition of the cerebral veins is common to all hibernating animals, as envier very justly remarks, it is by no means peculiar to quadrupeds that pass the winter in a state of torpor ; on the contrary, it is met with in many Rodents that do not hibernate ; as, for example, in the rats ; it also occurs in the horse, as well as in many Edentate, Ru minantia, and Carnivora. Cuvier believes this arrangement to be in relation with the situ ation and direction of the head, the differ ence between these quadrupeds and man rather depending upon the position of the latter standing on four legs, than upon any cause connected with the habit of hiber nation.
Nervous system.—The brain in the Rodent order of quadrupeds presents two principal forms ; in the feebler, and more strictly her bivorous species, such as the hare, the rabbit, the agouti, paca, &c., it presents a great re semblance externally in its shape to that of birds, the cerebral hemispheres being broad behind, and gradually tapering towards the anterior lobes. In others, such as the beaver, porcupine, capromys, &c., the contour of the brain is nearly circular (fig. 276.), as in car nivorous quadrupeds. Between these extreme forms there are, however, intermediate grada tions, such as lye met with in the squirrel, the marmot, the water-rat, and others.
The most striking circumstance presented by the brains of these animals is the almost complete deficiency of cerebral convolutions. The hemispheres are almost completely smooth upon their surface, presenting only a few shallow lines instead of the numerous sulci which characterise the brain of the Car nivora.
The cerebellum is of moderate proportions, and is scarcely at all overlapped by the pos terior lobes of the cerebrum.
terior pair (testes) (8); a circumstance which is the converse of what exists in carnivorous quadrupeds. In other respects the structure of the brain in the Rodentia offers no pecu liarity worthy of special notice.
The organs of the senses conform strictly in their anatomical structure to the general type common to mammiferous quadrupeds, and consequently need not occupy our attention in this place.
On separating the hemispheres, the tuber cula quadrigemina (fig. 280. 8, 9) are seen to be of very large size ; and, what is re workable, the anterior pair (notes) are of a roundish form, and much larger than the pos The structure of the kidneys, and the gene ral disposition of the urinary apparatus afford nothing deserving particular description.
Male organs of generation.— The Rodentia are amongst the most prolific of all quadru peds ; a circumstance which may, perhaps, account for the extraordinary development of the appendages to the male generative st.stem, which are met with throughout the order. It is, indeed, difficult to identify the precise ana logies of some of the accessory genital organs, which are much more complex in structure than those of other Vertebrate.
In the greater number of Rodents, as for instance in the rats, the Guinea-pigs, the agoutis, the porcupines, the beaver, the onda tra, and the squirrels, the testicles are not contained in a scrotum, but during the season of impregnation are lodged beneath the skin of the perineum, which is tightly stretched over them. In the hares, however, two dis tinct scrotal pouches exist (jig. 281. k, I), situated in the vicinity of the anus, in which the testes are contained. The testes are moreover, remarkable for tneir great size, these canals are brought into communication by means of this chamber.
In the Alpine marmot, the vesiculx semi nales contain internally a very complicated cavity, the walls of which are glandular.
In the rats, properly so called, the vesiculm seininales consist of large membranous blad ders of a flattened conical form, with their inner margins sacculated and uneven, some thing like a cock's comb. In these animals they are in great part situated out of the pel vis on account of their very la ge size ; in the hamsters, the voles (Arvicolce), the dormice, and the jerboas they present a similar structure, and become remarkably developed during the season for copulation.
which generally exceeds that of the kidneys ; a circumstance which is more remarkably evident during the season of copulation.
From the testicles situated as above, the vasa deferentia ascend into the abdominal cavity, along with the spermatic vessels, through the external abdominal ring. In some tribes, a little above their insertion, the walls of the vasa deferentia become manifestly thicker, and the cavity of their duct considerably dilated ; in some cases they join together, and seem to form but one canal ; but this appearance is merely external, the ducts continuing separate throughout their whole length.