422 THE NEEVOUS SYSTEM 



fibers is removed from the control of the will. Under 

 ordinary conditions they are probably excited reflexly, 

 but the course of the afferent fibers is at present little 

 understood. Most of these reflexes are also unconscious 

 actions, and include such movements as the dilation and 

 contraction of the arteries, the movements of the intestines, 

 and the secretory activities of the digestive glands. The 

 entire field of action and method is still very little imder- 

 stood. 



Hygiene of the Nervous System. 



As has been stated, the manner in which nerve cells 

 generate impulses or give rise to psychical reactions is 

 unknown. We do know, however, that generation of 

 impulses and mental activity is in some way dependent 

 upon the metabolism of the nerve cells. From this, and 

 the fact that nerve cells, like all other cells, are made of 

 protoplasm, it follows that lack of nourishment or of air iS 

 bound to affect their activity, overwork to fatigue them, 

 and rest to restore them. Comparison of the nerve cells 

 of a child with those of an adult show that the latter are 

 much more branched and more intimately connected than 

 those of the child, which is another indication of the rela- 

 tion bet^veen increased nervous action and metabolism. 



Nerve food. — It used to be thought that nerve cells required 

 special kind of foods for their development, notably, phosphorus; 

 and fish, which are rich in phosphorus, were supposed to constitute 

 an ideal brain food. It is now known that their protoplasm is 

 built up of identically the same sort of materials as are used by other 

 cells, and hence a food which is nourishing to the laborer is equally 

 so to the brain worker. The important point to be kept in mind 

 is that there must be a sufficient quantity of food, and that the 



