THE NERVOUS SYSTEM. 



171 



the gray surface. The convolutions multiply and deepen 

 as we ascend the scale of size and intelligence, being very 

 complex in the Elephant and Whale, Monkey and Man. 

 As a rule, they are proportioned to the intelligence of the 

 animal ; yet the brains 

 of the Dog and Horse 

 are smoother than those 

 of the Sheep and Don- 

 key. Evidently the 

 quality of the gray mat- 

 ter must be taken into 

 account. Save in the 

 bony Fishes, the cere- 

 brum is the largest por- 

 tion of the brain ; in 

 Man it is over eight 

 times heavier than the 

 cerebellum. 



The cerebellum, or 

 " little brain," lies be- 

 hind the cerebrum, and, 

 like it, presents an ex- 

 ternal gray layer, with 

 a white interior. In 

 Mammals, it is likewise 

 finely convoluted, con- 

 sisting of gray and 

 white laminae, and is 

 divided into two lobes, 

 or hemispheres. In the rest of the Vertebrates, the cere- 

 bellum is nearly or quite smooth ; and in the lowest Fish- 

 es it is merely a thin plate of nervous matter. In many 

 Vertebrates, however, it is larger, compared with the cere- 

 brum, than in Man, since in Man the cerebrum is extraor- 

 dinarily developed. 



Fig. 138. — Brain of the Horse— upper view, one 

 half natural size: a, medulla oblongata; b, lat- 

 eral and middle lobes of cerebellum ; c, inter- 

 lobular fissure ; d y cerebral hemispheres ; e, ol- 

 factory lobes. 



