NERVOUS SYSTEM OF VERTEBRATA. 
87 
resembling those of the back, but simpler in their form, and 
not possessing a cavity for the spinal cord. We commonly 
find that in those animals in which the sknll is very large, 
the tail is short; and that where the tail is very long or 
powerful, the head is small. Thus in man and in the apes, 
the head is large, and there is no 
external appearance of a tail; but there 
are some very imperfect vertebrae at the 
lower end of the spinal column, which 
constitute the rudiment of it. In the 
long-tailed monkeys and in the kan¬ 
garoo (whose tail is like a third hind¬ 
leg), the head is comparatively small. 
But this rule does not hold good uni¬ 
versally. 
72. The Nervous system of Verte- 
brated animals consists of a Brain and 
Spinal Cord (fig. 28), which are lodged 
within the skull and vertebral column ; 
and of nervous trunks proceeding from 
these, which are distributed to all parts 
of the body. The Brain is not (as 
commonly reputed) a single organ, but 
is composed of a number of ganglionic 
masses, differing considerably in their 
functions. Thus each of the nerves 
of special sense (smell, sight, hearing, 
and taste) has its own proper centre; 
and there is another of considerable 
size, which seems to perform the same 
office in regard to common sensation. 
These are found in Yertebrata generally; 
and their proportionate size corresponds 
with the relative development and ac¬ 
tivity of the several organs of sense 
with which they are connected. The 
bulk of the brain of Man, however, is Fis ' 28 ^d A 0 T / man S pINA1 
made up by two large masses of nervous 
matter, which are known as the Cerebral Hemispheres ; these, 
as will be shown hereafter (chap, x.), are so small in the brains 
of Fishes as to be scarcely distinguishable; and their relative size 
