308 THE GROWTH OF THE BRAIN. 



rises into consciousness and reveals to us some notion of 

 our own plasticity. 



The rhythms which occur mark periods of greater 

 alternating with those of less activity ; the former 

 being those during which energy-producing substances 

 break down, so that the organism is fatigued, and in 

 this process of building up and breaking down is to be 

 found the basis of those alterations in the nervous 

 background which have been here presented. Fatigue 

 is therefore worthy of careful examination, since a know- 

 ledge of the processes causing it can on the one hand 

 suggest why our capabilities periodically diminish, and 

 on the other how this impressive change, the shadow of 

 on-coming death, can be made least harmful. 



