THE NERVOUS SYSTEM OF MAN 1 67 



(interoceptors) ; thirdly, the stimuli that pass 

 from the brain to the voluntary muscles (somatic 

 motor) ; fourthly, the stimuli that pass to the in- 

 voluntary muscles (visceral motor). Naturally 

 there are several subdivisions of these, especially 

 the first one, but the more important fact to be 

 kept in mind is that a stimulus must act on some 

 form of receptor, be conducted to the central 

 nervous system and a motor stimulus be sent to 

 certain muscles. This means that a center located 

 in the brain for hearing, seeing, or speaking has 

 reference to but one cluster of ceils, associational 

 in character, which belong to the chain of nerve 

 cells necessary for this response. They do not 

 have any significance apart from their relation to 

 the group with which they are accustomed to act. 

 They are simply one of the links in a chain, — a 

 link that happens to be located on the surface of 

 the cerebrum at a given spot. 



During the passage of a stimulus along a nerve 

 fiber, energy is required and physical work Is done. 

 This Is proved by the formation of a definite chem- 

 ical substance, carbon dioxide, which results from 

 the breaking down of a very small amount of liv- 

 ing protoplasm. Such a change is a vital one. 

 This is really one of the great discoveries of the 

 past twenty years in connection with the nervous 

 system as it verifies what has been thought to be 



