DIFFERENT TYPES OF RESPONSE II9 



allowed, give uniform responses to uniform stimuli. And 

 when the period of rest is shortened, all alike exhibit 

 fatigue. 



From the conditions of experiment it is clear that 

 these effects are physical or molecular. The molecular 

 derangement caused by stimulus is thus gauged by the 

 amplitude of response. Recovery is brought about by the 

 restoration of molecular equilibrium, and for this purpose it 

 has now become evident that the process of assimilation is 

 not essential. When sufficient time, however, is not allowed 

 for recovery, we have a residual molecular strain, and a 

 substance in this strained condition is less responsive, as 

 seen in the diminished height of its response. Fatigue is 

 thus due to molecular strain, and its 

 cumulative effects. But when the 

 fatigued substance is allowed sufficient 

 time for the strain to disappear, its 



1 . 1 -1 -^ ii Fig. 61. Fatigue-Reversal 



subsequent responses exhibit the nor- ;„ Arsenic, under Con- 



mal amplitude. tinuous Stimulation of 



Hertzian Radiation. 



It was explained in the last chapter 



. r -IT- -1 ^^^ horizontal line repre- 



that in the case of Mimosa, as in that sents the duration ot 

 of muscle, a complete reversal of stimulation. Curve ob- 



r- tamed by conductivity 



response is brought about by extreme variation method. 

 fatigue, under continuous stimulation. 



The following record shows a similar reversal in Arsenic, 

 under the continuous stimulation of electric radiation (fig. 61). 

 It was only after a sufficient interval of rest that this sub- 

 stance gave renewed normal response. It maybe added that 

 these fatigue-reversals, as in the longitudinal response of the 

 Uriclis Lily, are sometimes found to be recurrent. 



This curve of fatigue-reversal in Arsenic under continuous 

 stimulation was obtained by recording the changes of electric 

 conductivity in the substance. A still more striking analogy 

 with the mechanical records of fatigue in plants and animals 

 is afforded, however, by the automatic record given in fig. 62 

 of contractile responses in india-rubber. When this substance 

 is excited by rapidly succeeding thermal shocks, we obtain 



