100 COMPARATIVE ELECTRO-PHYSIOLOGY 



individual response to individual stimulus, into that of 

 multiple response, either to continuous, or to a single strong 

 stimulation. The excess of energy derived from impinging 

 stimulus is in the latter case held latent in the tissue to find 

 subsequent expression in phasic alternations of negative and 

 positive variations in series (cf. Chapter XVII). 



There can be no doubt that these effects of periodic 

 alternation of phase are due to two antagonistic reactions, 

 becoming effectively predominant by turns. Thus the con- 

 tinuous impact of stimulus on a tissue may first give rise to 

 the negative phase of response. But by the continuous 

 absorption of incident stimulus, the internal energy is in- 

 creased, with its opposite 

 reaction of positivity. 

 Hence, the negativity will 

 be gradually diminished, 

 and the positive phase 

 become predominant. The 

 existence of these two 

 antagonistic factors will be 

 understood, from an inspec- 



FIG. 71. Photographic Record of tion f the to P f a tetatl!c 



Periodic Fatigue under Continuous curve. Here, the more Or 

 Stimulation in Contractile Response . . .11- 



(Filament of Uridis Lily) less horizontal line repre- 



sents a state of balance 



between the two opposite forces of excitatory response by 

 contraction, with galvanometric negativity and recovery or 

 expansion, with galvanometric positivity. When this state 

 of balance is disturbed, by a sudden cessation of the 

 hitherto continuously acting stimulus, a brief overshooting 

 of the response in the negative direction is sometimes seen, 

 followed by recovery. We shall meet with examples of 

 this in, among others, the responses of retina and certain 

 vegetable tissues under light. Such facts it has been 

 suggested afford a demonstration of the two antagonistic 

 processes of assimilation and dissimilation, characteristic of 

 living tissues. But that they are really to be accounted for 



