84 THE EVOLUTION OF MAN 



THE NERVOUS SYSTEM OF MAN AND OTHER HIGHER 



ANIMALS 



The nervous system of man is without doubt one of the 

 most intricate structures in the animal kingdom. It is com- 

 posed of a most complicated mechanism of end-stations and 

 of intercommunicating lines which far outruns in intricacy the 

 telephone system of such a place as Chicago or New York. 

 It is made of nerve cells, or neurones, whose processes, the 

 nerve fibers, stretch like telephone wires for relatively pro- 

 digious distances through the body. When these neurones are 

 studied individually they are seen to fall into three classes. 

 One of these classes is composed of elements that reach from 

 the skin and other outlying parts, such as the organs of smell, 

 taste, hearing, and the like, to the central organs. These 

 neurones are called sensory or afferent neurones. Other 

 neurones, commonly called motor or, better, efferent, stretch 

 from the central apparatus to the muscles and other organs 

 activated by the nerves. These form the second class. The 

 third class, the internuncial neurones, connect one part of the 

 central nervous system with another; they consequently lie 

 entirely within the limits of the central organ and may be in 

 accordance with their connections either functionally afferent 

 or efferent. These three classes of neurones make up the whole 

 of the real nervous substance of the human body and col- 

 lectively they constitute a system by means of which many of 

 the bodily activities are controlled and in which take place such 

 remarkable operations as sensation, memory, volition, and 

 the host of other performances which together constitute per- 

 sonality. 



When we survey the organization of the nervous system 

 and its appended parts we find that, though it is made up of 

 neurones, the boundaries of its organs do not conform to the 

 limits of the neurones, but have a topography of their own. 



