240 HUMAN PHYSIOLOGY CHAP. 



The amount and kind of air breathed is next in im- 

 portance to the food eaten. Since the brain needs so 

 much food, it must have an equally abundant supply of 

 pure fresh air to oxidize the food. The " school head- 

 aches " are the result of hot, close, poorly ventilated 

 schoolrooms, in which the same air must be breathed 

 over and over again, until the pupils become so drowsy 

 and listless that they cannot get their lessons. Such a 

 condition is sure to cause severe headaches and other 

 disorders. 



164. Exercise. The brain, like the muscles, must be 

 given a moderate amount of exercise daily to keep it 

 well and strong. It becomes delicate, weak, and sickly 

 if not used, just like the muscles of an arm carried in a 

 sling. Studying may at first be hard work, and very 

 fatiguing, but if kept up day after day, with proper 

 intervals of rest, it will soon become easy, so that diffi- 

 cult tasks are a real pleasure. 



In studying exercise, we learned that, in order to use 

 all the muscles of the body, a great variety of move- 

 ments is essential. In the same way a single power of 

 the mind can be exercised and all the others neglected. 

 Variety in mental work is just as important as in physi- 

 cal work to call into activity all the capacities and powers 

 of the human mind. The cerebrum alone contains, on 

 its outer surface, over 9,200,000,000 cells which can be 

 used. In order to bring all these cells into action, so 

 that they may grow and develop as they should, a very 

 great variety of mental exercise and a wide range of 

 interests are indispensable. 



