160 HUMAN BIOLOGY 



and authorities in the practical and scientific spheres, by so many 

 separate acts and hours of work. Let no youth have any anxiety 

 about the upshot of his education, whatever the line of it may be. 

 If he keep faithfully busy each hour of the working day, he may 

 safely leave the final result to itself. He can with perfect certainty 

 count on waking up some fine morning, to find himself one of the 

 competent ones of his generation, in whatever pursuit he may have 

 singled out." 



238. Conditions necessary for a healthy nervous system. — In 

 studying the hygiene of other parts of the body, we found that four 

 conditions were necessary for healthy activity. 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. 



239. Food and air. — In the nervous system of a human being there 

 are milUons of nerve cells. Each of these cells must be suppUed 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 a drowsiness and head- 

 ache, 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. 



240. Varied activity. — To develop a weU-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 finally 

 give way to the relaxation of play. Unfortunate is the boy who fails 

 to find exhilaration in baseball, bicycle riding, or general athletics, 

 for these sports, when properly regulated, besides developing strong 

 lungs and vigorous muscles, are important means of educating the 

 nerve cells and fibers. 



