NERVOUS SYSTEM OF VEBTEBRATA. 



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 skull 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 Vertebrata 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 * 

 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 



