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- a 
fii'cHno- o-rnv and Fig. 133. - Brain or the Horse—npper view, one 
SlSllllg O i gidy d,nu fourth natural size: a, medulla oblongata; b, 
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. 
lateral and middle lobes of cerebellum; c , inter¬ 
lobular fissure ; d, cerebral hemispheres; e, ol¬ 
factory lobes. 
