306 APPLIED PHYSIOLOGY 



impressions. Any one will find study interesting if he will 

 concentrate his mind upon a subject so that he gains 

 knowledge. Then when playtime comes he will enter 

 into the sport with zest and satisfaction. No one who has 

 not been working can truly enjoy play. 



552. Persistence of mental impressions. Brain work re- 

 quires heat and energy like muscular work. The cells of the cerebrum 

 retain an impression of each thought, which is deep and permanent in 

 proportion to the power expended upon it. A lesson learned in a 

 minute makes some impression upon the cells, but it is gone in another 

 moment. A dull boy hammers away at a lesson by the hour, but at the 

 end of a year he will have retained far more than the brilliant boy who 

 loses his impressions as fast as he gains them. 



It is extremely difficult to efface impressions once really made upon 

 the cerebral cells. Apparently, knowledge may be forgotten, but some 

 day something will cause the cells to recall the impressions. Thus it 

 is very important to avoid all thoughts which we should be ashamed to 

 recall. 



553. Habit. Memories of thoughts often repeated may 

 arise in spite of the will to restrain them, and may compel 

 the motor region to do acts which the will utterly abhors. 

 At first a man's will has to direct the thoughts to speak 

 profane words. Soon the words become so imprinted in 

 thought that they arise even without his knowledge. 

 Habits grow faster and stronger than the will to overcome 

 them. On the other hand, one can form a habit of study 

 and of mind cultivation so that mental work is a pleasure. 

 The more one works with his mind, the more he enjoys 

 his work. The mind is constantly forming habits of 

 thought. Even if it thinks nothing bad, yet it may soon 

 acquire a lazy habit of not thinking at all. 



554. Heredity. Impressions of any kind may become 

 so permanent that one's children have a tendency to 



