NATURAL RESISTANCE AND FATIGUE 157 



the exhaustion of available phosphagen, or any other 

 substance essential to the activity of muscle, would 

 likewise lead to fatigue. Similarly, the accumulation 

 within the muscle of any toxic metabolite such as lac- 

 tic or carbonic acids would likewise lead to fatigue. 

 Fatigue due to the latter substances has long been 

 recognized, but, as Bainbridge (1919, page 79) has 

 recently pointed out, neither of these substances "ever 

 attains during exercise such a concentration in the 

 blood as to be directly toxic." 



Vincent and Sheen (1903), and Bayliss (1918), 

 have found that when injected into an animal a neu- 

 tral extract of muscle may cause a loss of tone of the 

 arterioles. This action, as has been shown previously, 

 is a typical cytost reaction, and is of interest in con- 

 nection with the earlier investigations of the author. 



During violent muscular exertion such as rowing 

 or running, the circulatory and respiratory increases 

 necessitated by the increased muscular activity usually 

 cease shortly after the muscular exertion has stopped, 

 although other evidences of fatigue may persist for 

 some time afterwards. This is perhaps due to the 

 accumulation of cytost during the period of exercise. 

 In this connection it is of interest to note that upon 

 reviewing the available literature Bainbridge (1919, 

 page 186) has been unable to find any evidence that 

 metabolites formed as a result of muscular activity 

 have any part in causing a fatigue of the central nerv- 

 ous system. This is of interest in the present discus- 

 sion; for, as has been shown in earlier chapters, the 

 parenteral introduction of even large quantities of 

 cytost does not lead to any involvement of the central 

 nervous system. 



