In the water-rat (Arvieola amphiNus) the small intestines are of equable diameter throughout their whole extent, but their calibre is small, as indeed is that of the large intestine. The cmcurn is, however, of enor mous proportions (fig. 274. n, o, p, q), and is divided at intervals into pouches by deep constrictions. The commencement of the colon (r) is extremely voluminous, but it soon diminishes in its diameter, and is twisted in a remarkable manner, so as to form several close spiral turns ; the walls of the small in testine (1,m) are very thin and transparent ; at the commencement of the colon its lining membrane is thrown into regular folds, which, as they appear through the transparent coats of the intestine, resemble a series of spiral muscular fibres.
In other species belonging to the genus arvicola, the same disposition is observable.
In the Cape moles (Bathiergus) the struc ture of the caecum varies. In the orycterus of the Downs (Bathiergus mar:dims) the crecum is short, and has its walls sacculated and puckered up, as it were, by tendinous bands. The colon begins by a wide pouch, and preserves through nearly its wholelength a considerable diameter and sacculated ap pearance, but on approaching the anus it be comes contracted and of equable diameter.
In the white-spotted orycterus (Bathiergus eapensis) the cmcum is much longer in pro portion and of more equal calibre, although still very wide, in proportion to the size of the small intestine, and much sacculated; the com mencement of the colon is at first of the same diameter as the ca-cum, but it soon be comes narrower and spirally convoluted, much in the same way as in the water-rat.
In the hare and in the rabbit the small intestine is nearly of the same diameter throughout its whole length ; the cmcum is of a very remarkable size, and forrns an enormous elongated conical sac, divided, at intervals, by deep constrictions into numerous compart ments, as far as about the distance of two or three inches from its extremity (fig.275.). The constrictions, apparent externally, correspond to the windings of a spiral valve, which runs nearly along the whole length of its cavity. The small intestine, at the point where it is about to enter the colon, dilates into a cavity (f), the walls of which are thick and glandu lar. At its commencement (e) the colon is quite as capacious as the crecum, but it soon begins to contract in its diameter. (g) At its commencement there are three rows of sacculi, divided by as many tendinous bands, but further on these sacculi disappear. The rectum is much dilated, and contains, at in tervals, small pellets of excrement moulded in the sacculi of the colon. In all the species belonging to this genus, as well as in the rats and hares, including the Lagomys, the ex tremity of the cwcal bag, opposite to that which receives the termination of the small intestine, is terminated by a long, smooth, cylindrical appendage (fig. 275. d,b)the walls of
which are glandular, and somewhat resemble those of the glandular stomach of a bird.
The above examples will suffice to put the reader in possession of the general structure of the alimentary canal in the rodent order of quadrupeds ; and for farther details we must refer him to the last edition of Cuvier's Lecons d'Anatomie comparee, where the principal varieties met with in the different genera are recorded.
Liver. —In the Rodentia the liver is very largely developed, and presents the usual division into five principal lobes. The gall bladder, though generally present, is some times wanting, a circumstance more particu larly observable in the family of rats. Another circumstance which may be noticed is that the bile is frequently poured into the intestine at a point remote from that where the pancreatic fluid enters it ; when such is the case, the biliary secretion enters the duodenum very near to the pylorus, above the entrance of the pancreatic duct.
In the porcupine the ductus communis choledochus is formed by the union of two hepatic canals with the cystic duct; it enters the intestine close to the pyloric ring, opening into a furrow excavated in the latter, in such a manner that the bile would seem to flow as easily into the stomach as into the duo denum. The opening of the pancreatic canal is at a considerable distance from the pylorus.
The pancreas is very large, and generally divided into two portions.
The spleen occupies its usual position sus pended from the stomach by the gastro-splenic o men tum The lymphatic system of the Rodentia con forms in all respects to the usual arrangement of these vessels met with in other quadrupeds, and exhibits nothing worthy of particular remark.
The arterial system, as far as the general distribution of the blood-vessels is concerned, offers a few peculiarities worthy of notice. In all those genera of rodent quadrupeds which become dormant during the winter months, the vertebral artery considerably surpasses in size the internal carotid ; to such an extent, indeed, that some authors have described the latter vessel as being entirely wanting. In this case the basilar artery forms by itself a very considerable part, and some times the whole of the circle of Willis, giving off the anterior cerebral arteries as well as the posterior arteries of the brain.* The arrangement of the carotids, moreover, varies remarkably in different genera.