RODENTIA. 



391 



through an ascending canal, which enters the 

 cavity of the skull close to the sella turcica, 

 arriving at the brain much in the same manner 

 as the internal carotid of the human subject. 

 This branch is smaller than the vertebral 

 artery. The other or external branch enters 

 the cranium through a canal that opens upon 

 the anterior surface of the petrous bone, and 

 divides into the middle meningeal and ophthal- 

 mic arteries. 



In the dormouse the distribution of the 

 internal carotid very nearly resembles what 

 is described above, as occurring in the squirrel 

 and in the marmot. In some genera of 

 Rodents the internal condyle of the os humeri 

 is perforated by a canal through which the 

 ulnar artery passes in company with the 

 median nerve : this arrangement exists in the 

 squirrel, the hamster, and the helamys. 



Venous system. In most of the Rodentia, 

 instead of a single anterior vena cava, there 

 are two principal anterior trunks of the venous 

 system, one of which, namely the right, occu- 

 pies the usual position of the vena cava 

 anterior, whilst the left runs along the furrow 

 that separates the base of the ventricle of the 

 heart from the left auricle, to reach the right 

 auricle, into the upper and left side of which 

 it opens. 



In those genera which hibernate the exter- 

 nal jugular vein likewise presents a very 

 remarkable arrangement. This vein receives 

 a considerable proportion of the blood de- 

 rived from the brain through a wide canal, 

 situated between the os petrosum and the 

 temporal bone, into which the anterior division 

 of the transverse sinus opens, so that it is only 

 the smallest moiety of blood derived from the 

 vein which escapes through the jugular for- 

 amen into the internal jugular. The vertebral 

 vein likewise communicates with the external 

 jugular, carrying off its share of the blood 

 from the interior of the cranium. 



Fig. 276. 



Upper surface of the brain of the Porcupine. 

 (After Serres.) 



a, medulla spinalis ; b, hemispheres of cerebellum ; 

 c, median lobe of the cerebellum ; d, e, k, I, cere- 

 bral hemispheres. 



Although this disposition of the cerebral 

 veins is common to all hibernating animals, as 

 Cuvier 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 Edentata, 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 are met with in the squirrel, the 

 marmot, the water-rat, and others. 



Fig. 277. 



Base of the brain of the Porcupine (Histrix cristata). 

 (After Serres.} 



a, anterior pyramid, exhibiting the interlacement 

 of their internal fasciculi ; o, olivary bodies ; t, tra- 

 pezoid bodies ; p, pons Varolii ; h, the lobe of the 

 hippocampus ; g, middle portion of the hemisphere ; 

 r, olfactory tract ; x, external root of olfactory lobe ; 

 y, internal root of ditto. The nerves are indicated 

 by corresponding numbers. 



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. 



c c 4 



