702 



MAMMALIA. 



Fie. 324. 



between the Oviparous and Placenta! Vertebrata, the brain still 

 exhibits a conformation nearly allied to that of the Bird, and the 

 great commissures required in the more perfect encephalon are 

 even yet deficient ; but in the simplest brain of a Placental Mam- 

 mifer the characteristic differences are at once apparent. 



In the Rabbit, for example, (Jig. 322), the cerebral hemispheres 

 (b) are found very materially to have increased in their propor- 

 tionate dimensions ; and although, even as yet, convolutions 

 upon the surface of the cerebrum are scarcely indicated, additional 

 means of intercommunication 

 between the hemispheric 

 masses become indispensable. 

 The corpus callosum there- 

 fore, or great transverse com- 

 missure of the hemispheres, 

 (fig. 322, c,) is now super- 

 added to those previously in 

 existence; while other medul- 

 lary layers, called by various 

 ridiculous names, bring into 

 unison remote portions of 

 the cerebral lobes. 



In proportion as intelli- 

 gence advances, the surface 

 of the cerebral hemispheres 

 becoming more extensive is 

 thrown into numerous con- 

 volutions separated by deep 



sulci ; until at length in the Carnivora, as, for instance, in the 

 Lion, (Jig. 324,) the cerebrum (e, e) attains such enormous di- 

 mensions that the other elements of the encephalon are, as it 

 were, hidden among its folds. 



(823.) But, in addition to this increased complexity of the cere- 

 brum, the cerebellum likewise has assumed a proportionate im- 

 portance. In the Oviparous races this important element of the 

 brain consisted only of the mesian portion, so that no cerebellic 

 commissure was requisite ; but in the Mammal it exhibits in addi- 

 tion two large lateral lobes (Jig. 324, c, c), and co-existent with 

 these the pons Varolii (Jig. 324, d) makes its appearance, em- 

 bracing the medulla oblongata and uniting the opposite sides of the 

 cerebellum. 



