322 THE GROWTH OF THE BRAIN. 



other systems a relation the utility of which is plain, 

 but the mechanism of which is quite unexplained. 



In general the fatigue which controls us is, in so large 

 a measure, dependent on the nervous system that there 

 is reason to make the changes occurring there the most 

 prominent, though it can also be shown that the glands 

 and muscles undergo changes as the result of activity, 

 and a complete discussion of the subject would involve 

 all the active tissues. 



There is, then, an anatomy as well as a physiology of 

 fatigue, and the facts grouped together permit the 

 following general statement. After recuperative sleep 

 the cells in the central system are full-sized and 

 granular, the blood flows with a medium pressure 

 through the nerve centres ; slight stimuli elicit a ready 

 response, and there are general sensations of vigour 

 and well-being. From the beginning of the day the 

 process of running down goes on, all the constant 

 stimuli hasten it, meals retard it, drugs modify it, 

 according to their nature. In general there is a 

 tendency to run down towards the middle of the after- 

 noon, with a return of vigour later in the day. On this 

 long rhythm is superposed one by which in the evening 

 the blood supply to the brain diminishes at the accus- 

 tomed hour of retiring. This change in the blood 

 supply appears to depend on the waste substances pro- 

 duced by the active cells. These accumulate faster 

 than they are removed, and render activity more 

 difficult. At the beginning of sleep these substances 

 are abundant, the stored material in the cells is small, 

 and the cells themselves are shrunken in various ways. 

 Slowly the toxic products of metabolism are removed, 

 and at the end of two or three hours the sleeper is in 

 a state to be readily awakened, though physiological 

 recuperation has but just begun. The circulation has 



