CHAPTER XVI. 



FATIGUE. 



Expenditure of energy Possible increase of potential energy 

 Results of anatomical disproportion Fatigue affects the 

 material stored at any one time Results of exercise Fatigue 

 a function of the central system Beneficial exercise Fatigue 

 substances Their diffusion Auto-intoxication Drugs 

 Sleep Relation to central system Changes in the circulation 

 Rhythms Dependence of sleep on stimulation Variations 

 with age Depth of sleep Recuperation Length of the 

 period Hodge's experiments Observations of Mann and 

 Vas Fatigue of the nerve fibres Starvation Other tissues 

 Resume. 



IT is conceivable that two individuals of equal age and 

 size may have expended during their lives very different 

 amounts of energy. Further, this expenditure need 

 not in every case correspond with effective performance, 

 for internal resistances may absorb a large share of the 

 energy set free. That individuals differ in their every- 

 day performances is recognised, and these inequalities 

 depend both on a more happy organisation of those 

 most capable, as well as on fundamental differences in 

 the capacity for storing energy. Individual differences 

 in these respects include the minutest details, and are 

 remarkably persistent. 1 For example, Mosso found that 

 in the periodic loss of muscular power following pro- 

 longed activity, the two individuals whom he specially 



1 Mosso. La Fatigue Intellectttelle et Physique, Paris, 1894. 



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