THE NERVOUS SYSTEM. 
171 
the gray surface. The convolutions multiply and deepen 
as e 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 
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. 
1 Fig. 138. —Brain of the Horse—npper view, one 
d 1 half natural size: a, medulla oblongata; 6, lat¬ 
eral and middle lobes of cerebellum; c, inter¬ 
lobular fissure; d, cerebral hemispheres; e, ol¬ 
factory lobes. 
