522 



COMPARATIVE ELECTRO-PHYSIOLOGY 



the first series are by the abnormal positive variation, that is 

 to say, by expansion. The tissue here being sub-tonic, the 

 impinging stimulus could not induce the true excitatory effect. 

 The tissue was now subjected to short-lived tetanisation. 

 But the absorbed stimulus was not yet sufficient to induce 

 the normal responsiveness. The next series of. records, 



therefore, still exhibited the 

 abnormal positive response. 

 Tetanic shocks of longer 

 duration were next applied. 

 This gave rise to a short- 

 lived positive twitch down- 

 wards, succeeded by large 

 contractile response upwards. 

 After the cessation of the 

 second tetanisation, the 

 absorbed energy is seen to 

 have brought the tissue to 

 a condition of more or less 

 normal responsiveness. This 

 is seen in the third series, 

 where the first responses are 

 diphasic, but the positive 

 component (the downward 

 twitch) becomes perceptibly 

 ,-, . i T> i t, smaller and the negative 



FIG. 321. Photographic Records show- 

 ing Gradual Disappearance of Positive larger, in each of the succeed- 



Re^L "of D ^T N^vftd ing responses. It should 

 Plant-nerve also be noticed that the 



Note also the staircase increase. recovery from positive is much 



quicker than from negative response. This fact is important, 

 in connection with certain psycho-physiological phenomena 

 to be described in a later chapter. 



The effect of successive stimuli, in enhancing normal 

 response, when the nerve is not yet in maximum tonic 

 condition is illustrated in a still more striking manner in the 

 record given in fig. 321, obtained with a different specimen 



