THE NERVOUS SYSTEM 285 



the same conditions ; that refraining from doing something 

 toward which we are inclined similarly renders more easy 

 the inhibitory processes concerned when the same conditions 

 impel us toward it again. We are largely what we make 

 ourselves by the training which our actions give to the 

 nervous system. 



And what activity thus does for the development of power 

 it does also for the maintenance of power. An efficient nerv- 

 ous mechanism of any kind once acquired does not remain 

 efficient without use. The man who has developed a rugged 

 constitution in colder climates and then lives for years in 

 the tropics, constantly exposed to a warm climate, finds on 

 return to the home of his youth that the mechanism of heat 

 regulation does not readily adjust itself to cold damp winds 

 and blizzards; the athlete who has learned to execute the 

 greatest variety of " tricks " in the gymnasium and then 

 settles down to a sedentary life finds after some years that 

 he is almost as helpless as the man who gave no attention 

 to such training. It is unnecessary to multiply examples. 

 Efficiency in any direction is the result of continued use of 

 organs and especially of continued training of the nervous 

 system. As we fit ourselves to do some few things, and to 

 do them well, we have not time to conserve by use the effi- 

 ciency of all the nervous mechanisms we have acquired ; we 

 must to some extent sacrifice the more general actions for 

 those which are more special and useful. But it must not 

 be forgotten that this can be carried too far ; that a certain 

 amount of general activity is a condition of healthy living and 

 that one of the problems of life to solve, and to solve aright, 

 is how to distribute our activity between the two. To the 

 consideration of these questions we shall return in our study 

 of personal hygiene. 



