284 STUDIES IN PHYSIOLOGY 



Throughout life, too, continual use of the nerve cells in- 

 volves a destructive metabolism of their protoplasm. This 

 wasting process must, therefore, be succeeded by repair. 



Necessary Conditions for a Healthy Nervous System. In 

 studying the hygiene of the muscles we found that four 

 conditions were necessary for healthy muscular activity (see 

 p. 202). That the nervous system, too, may develop as it 

 should and that it may do its work properly, the same four 

 conditions are essential; namely, food, fresh air, various 

 kinds of activity, and periods of rest. 



Food and Air. It is estimated that in the nervous system 

 of an adult human being there are at least three thousand 

 millions (3,000,000,000) of nerve cells. Each of these cells 

 must be supplied with nutritious food and pure air, or it 

 becomes stunted in its growth and unable to do its proper 

 work. These busy cells are constantly giving off carbon 

 dioxid, water, and other wastes, and if these are not re- 

 moved and fresh oxygen supplied, one feels (as we have 

 learned on p. 226) a drowsiness and headache, and is unable 

 to think clearly. Well-ventilated rooms, both by day and 

 by night, are of prime importance in the hygiene of the 

 nervous system. 



Varied Activity. Fortunately for the well-being of the 

 race, genius is rare, for genius is usually a kind of one- 

 sided mental life. To develop a well-balanced brain one 

 must be active along many lines. Experience tells us, too, 

 that we cannot work successfully at the same task hour 

 after hour without some change. Hence, varied activity is 

 an important principle in sound education. The young 

 child must, of necessity, turn, after a short time, from one 

 lesson to another, and all lessons must at length give way 

 to the relaxation of play. In planning the school curricu- 

 lum we seek to develop the sensory areas of the child's 

 brain by nature study and science; the motor cells are 

 trained by manual training and physical exercise. Unfor- 

 tunate is the boy who fails to find exhilaration in baseball, 



