390 



RODENTIA. 



three inches from its extremity (Jig- 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 

 (/), the walls of which are thick and glandu- 

 lar. At its commencement (e) the colon is 

 quite as capacious as the caecum, 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 caecal 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 

 Leons d' Anatomic 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 

 omentum. 



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. 



In the beaver the internal carotid is larger 

 than the vertebral. 



In the porcupine the internal carotid, after 

 following for some distance the direction of 

 the internal maxillary, without undergoing 

 any sinuous flexures, enters the cranium 

 through the foramen lacerum anterius, where 

 it immediately joins with the basilar, which 

 surpasses it in size, to form the circle of 

 Willis. 



In the Guinea-pig and the agouti there is, 

 properly speaking, only an external carotid, 

 of which the internal carotid is but a small 

 branch. This little cerebral branch is derived 

 from the internal maxillary, of which it seems 

 to be a continuation ; it enters the cranium 

 through the foramen ovale of the sphenoid 

 bone, and joins the circle of Willis, which is 

 here principally formed by the vertebral 

 artery. 



In the squirrel the internal carotid enters 

 an osseous canal in the tympanum, through 

 the jugular foramen, passes between the crura 

 of the stapes, and then penetrates the cra- 

 nium through a hole in the petrous portion of 

 the temporal bone ; it there divides into two 

 branches, the smaller of which enters a deep 

 groove in the os petrosum, issues from the 

 cranium through the foramen lacerum an- 

 terius to enter it again through the oval 

 foramen of the sphenoid bone. It .is only 

 after all these windings that it divides into 

 small branches, and of these only one or two 

 go to form the circle of Willis, the rest being 

 meningeal arteries. The continuation of this 

 branch subsequently becomes the representa- 

 tive of a portion of the ophthalmic artery. 

 The other branches usually given off from the 

 ophthalmic artery are derived from the second 

 branch of the internal carotid above mention- 

 ed, which previously gives off branches to the 

 dura mater. It will thus be seen that the in- 

 ternal carotid supplies very little blood to the 

 brain, and this blood only arrives at its desti- 

 nation by a very circuitous route, -j- 



In the marmot the internal carotid at first 

 follows the same course as in the squirrel ; it 

 enters the canal of the tympanum through the 

 jugular foramen, and then traverses the 

 opening between the crura of the stapes, after 

 which it divides into two branches : of these 

 the internal, which is the smallest, runs 



* Vide Memoire sur les vaisseaux cephaliques 

 de quelques mammiferes qui s'en courdissent pendant 

 1'hiver, par M. Otto, Annales des Sc. Natur. t. xi. 

 p. 200. 



f Vide Barkow, Disquisitiones circa originem et 

 decursum arteriarum animalium. Lipsise, 1829. 



