DIFFERENT TYPES OF RESPONSE I 19 



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 



t_ 1 -l*^ -1 Fig. 61. Fatigue-Reversal 



subsequent responses exhibit the nor- in Arsenic under Con- 



mal amplitude. tinuous Stimulation of 



. Hertzian Radiation. 



It was explained in the last chapter 



1 _, , * The horizontal line repre- 



that in the case of Mimosa, as in that sen ts the duration of 

 of muscle, a complete reversal of £-{*-■ O™* 

 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 may be 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 



