THE NERVOUS SYSTEM. 
171 
the gray surface. The convolutions multiply and deepen 
as we ascend the scale of size arid intelligence, being veiy 
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 , 55 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 
fourth natural size: a, medulla oblongata; &, 
lateral and middle lobes of cerebellum ; c, inter¬ 
lobular fissure ; d, cerebral hemispheres; e , ol¬ 
factory lobes. 
