392 



RODENTIA. 



The cerebellum is of moderate proportions, 

 and is scarcely at all overlapped by the pos- 

 terior lobes of the cerebrum. 



278. 



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. 



Fig. 280. 



Upper surface of the brain of the male Agouti. 

 (After Stares.) 



a, the medulla spinalis ; b, posterior pyramid ; c, 

 median lobe of the cerebellum; d, hemisphere of 

 cerebellum ; e, cerebral _hemispheres ; /, olfactory 

 lobe of the brain. 



On separating the hemispheres, the tuber- 

 cula quadrigemina (fig. 280. 8, 9) are seen 

 to be of very large size ; and, what is re- 



Fig. 279.1 



Base of the brain of the male Agouti. (After Serres.) 



p, pons Varolii ; h, lobe of the hippocampus ; g,f, 

 lateral portion of cerebral hemisphere; k, anterior 

 part of the lobe of the hippocampus ; e, olfactory 

 lobe ; u, infundibulum. The nerves are indicated by 

 corresponding numbers. 



markable, the anterior pair (nates) are of a 

 roundish form, and much larger than the pos- 



Interlor of the brain of the same animal. 

 (After Serres.) 



a, medulla spinalis ; b, restiform body ; c, arbor 

 vitse cerebelli ; 1, 2, 3, 4, 5, 6, ramifications of ditto ; 

 d, superior peduncle of the cerebellum ; 7, nervus 

 patheticus ; 8, posterior quadrigeminal tubercle ; 

 9, anterior quadrigeminal tubercle ; 11, optic tract; 

 12, posterior pillar of the fornix ; 13, corpus stria- 

 turn ; n, corpus callosum ; i,f,g, horizontal section 

 of the hemisphere on a level with ditto ; /, m, lateral 

 portion of the cerebral hemisphere. 



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 system, 

 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 Vertebrata. 



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 (fig. 281. k, I), 

 situated in the vicinity of the anus, in which 



