VARIOUS TYPES OF RESPONSE 95 



responses are approximately equal, where the residual strains 

 are similar. The first response in fig. 63, A, shows this, because 

 there had been long previous rest. The first of B shows it, 

 because we are there passing for the first time to an increased 

 intensity of stimulus. The first of c does not show it, because 

 of the strong residual strain from the preceding excessive 

 stimulation. And the first of D, again, does show it, because 

 the strain has now been removed, by the interval of fifteen 

 minutes' rest. 



Of the antagonistic elements of positivity and negativity 

 which are present in response, we have seen that the positive 

 becomes predominant when the excitability of the tissue is 

 in any way depressed. And since a tissue under fatigue has 

 its excitability lowered, it follows that in this condition it 

 may be expected to exhibit a tendency towards positive 

 response : that is to say, expansion in the case of mechanical 

 and galvanometric positivity in the case of electrical, response. 

 Thus, when a tissue is subjected to continuous stimulation, 

 the first effect will be the maximum negative response, 

 contraction and galvanometric negativity. But on the setting- 

 in of fatigue, the positive effect will predominate, inducing 

 a fatigue-reversal of the response. In cases where such 

 fatigue is very great, as, for instance, in certain muscles, 

 the top of the tetanic curve undergoes rapid decline (fig. 64, a). 

 The normal contraction now exhibits a reversal, or relaxation. 

 In the sensitive plant, Mimosa, similarly, continuous stimula- 

 tion by electrical shocks gives rise to results which are essen- 

 tially the* same. It will be noticed that after the responsive 

 fall of the leaf it returns to its former erect position, in spite 

 of the fact that stimulus is still being continued. Here also, 

 as in the corresponding case of muscle, we have the usual 

 sequence, of (i) normal contraction and (2) fatigue relaxation 



(fig. 6 5 ). 



In electrical response, also, under continuous stimulation, 

 the normal galvanometric negativity, owing to the increasing 

 positive effect, undergoes decline or abolition. x This is seen 

 in fig. 64, b, which exhibits the decline of electrical response 



