GENERAL SURVEY AND CONCLUSION 185 



Fatigue. There is, again, another type where fatigue 

 is exhibited. 



The explanation hitherto given of fatigue in animal 

 tissues that it is due to dissimilation or breakdown of 

 tissue, complicated by the presence of fatigue-products, 

 while recovery is due to assimilation, for which 

 material is brought by the blood-supply has long- 

 been seen to be inadequate, since the restorative effect 

 succeeds a short period of rest even in excised bloodless 

 muscle. But that the phenomena of fatigue and recovery 



FIG. 113. FATIGUE (A) IN MUSCLE, (P) IN PLANT, (M) IN METAL 



were not primarily dependent on dissimilation or assimila- 

 tion becomes self-evident when we find exactly similar 

 effects produced not only in plants, but also in metals 

 (fig. 113). It has been shown, on the other hand, that 

 these effects are primarily due to cumulative residual 

 strains, and that a brief period of rest, by removing the 

 overstrain, removes also the sign of fatigue. 



Staircase effect. The theory of dissimilation due to 

 stimulus reducing the functional activity below par, and 

 thus causing fatigue, is directly negatived by what 

 is known as the ' staircase ' effect, where successive 

 equal stimuli produce increasing response. We saw an 



