168 COMPARATIVE ZOOLOGY. 
brain is heavier in proportion to the body. But quality 
must be considered as well as quantity, else the Donkey 
will outrank the Horse, and the Canary-bird, Man; for 
their brains are relatively heavier. 
The main parts of the brain are the cerebrum, cerebel- 
lum, and medulla oblongata. 
The cerebrum is a mass of white fibrous matter covered 
by a layer of gray cellular matter. In the lower Verte- 
Fia. 136.—Brain of the Horse—upper view, one- 
half natural size: a, medulla oblongata; 3, lat- 
eral and middle lobes of cerebellum; ce, inter- 
lobular fissure; d, cerebral hemispheres; @, ol- 
factory lobules. 
brates, the exterior is 
smooth; but in most 
of the Mammals it is 
convoluted, or folded, 
to increase the amount 
of the gray surface. 
The convolutions mul- 
tiply 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 pro- 
portioned to the intel- 
ligence 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 
matter must be taken 
into account. Save in 
the bony Fishes, the 
cerebram is the largest portion of the brain; in Man, it 
is over eight times heavier than the cerebellum. 
