NATURAL RESISTANCE AND FATIGUE 159 



demanded by the nature of the exercise, such as row- 

 ing a boat, or throwing a ball. Studies directed toward 

 this end have demonstrated that with cessation of the 

 physical activities, the excess metabolic activity is 

 utilized in connection with the repair and synthesis 

 of tissue; and the latter may and usually does take 

 place to a greater extent than is warranted by the 

 tissue substance loss during the period of exercise. 

 This is attested to by the fact that an individual's mus- 

 cular development is conditioned by the extent to 

 which his muscles have been used in past perform- 

 ances. 



Conversely, the disuse of muscles leads to atrophy. 



The manner in which functional activity causes 

 an increase in growth and incidentally in natural re- 

 sistance has up to the present been shrouded in mys- 

 tery, although the writer feels that at least a partial 

 explanation of the observed facts may be efifected as 

 follows. Let us recall the facts demonstrated previ- 

 ously: that (a) cytost in low concentration stimulates 

 the proliferation of cells in tissue culture; (b) cytost 

 in too high a concentration brings about the death of 

 cells exposed to its action ; (c) severe muscular fa- 

 tigue is accompanied by an accumulation of cytost. 



From the last it follows that cytost is liberated by 

 the muscle cells as a consequence of their activities 

 incident to contraction. If it be imagined that the 

 amount of cytost liberated during muscular activity 

 is determined by the extent of such activity, it be- 

 comes apparent that if formed in the proper amount 

 the cytost so liberated should by analogy to the tissue 

 culture experiments stimulate the growth of the cells 

 of the muscle. In this manner we may account for 



