312 THE GROWTH OF THE BRAIN. 



Under natural conditions the blood is always streaming 

 through the active tissue, and thus not only removing 

 the waste products, but supplying fresh nutritive 

 materials. In some measure the constructive and de- 

 structive processes occur together, but the curve of 

 normal fatigue indicates that the loss of power is first 

 rapid and then slow, exhaustion being long deferred. 



The neuro-muscular reactions are under the control 

 of the central system, and it is of interest to recall that, 

 as shown in the last chapter, it is the variations in the 

 nervous system which are most important for them. 

 So, too, in the last stages of extreme fatigue, it is 

 the nerve cells, not the muscles, which are exhausted. 



The exercise of cells finally causes both their better 

 irrigation by the blood, and the greater and more rapid 

 storage of fresh material. In this respect the structural 

 element stands related to the nutrient lymph as does 

 the entire organism to its food supply ; a good appetite 

 and vigorous digestion indicating health. Closely con- 

 nected with these processes is the determination of the 

 point to which exercise may be carried, in order that it 

 shall be followed by the maximum increase in power. 

 If the cell activities are viewed as dependent on the 

 partial decomposition of complex molecules, then it is 

 conceived that this decomposition, when carried to a 

 certain point, leaves the cell best prepared to recuperate. 

 This optimum can, however, be overstepped, and in- 

 stances where persons, after one form and another of 

 excessive fatigue, have never completely recovered, are 

 too familiar to require specification. Just what this 

 limit is, must be determined by personal experiment, 

 always keeping in mind that the individual differences 

 are very wide. 



Both the sensation of fatigue, and in part the loss 

 of power, are regarded as due to substances resulting 



