INTERFERENCE OF EXCITATIONS 



211 



secondly, upon the momentary amount of irritability which exists 

 just at the time when the second stimulus produces its effects. 

 It is, therefore, clear that the response produced by interference 

 must also alter with the momentary degree of irritability in a 

 manner analogous with variations of the intensity of the second 

 stimulus. One must, therefore, know the factors which control 

 the momentary degree of excitation. 



The first factor to be considered is the moment of time in which 

 the second stimulus is applied, that is, the interval between the 

 first and the second stimulus. If, for example, a weak second 

 stimulus follows very quickly after the first, the stimulus will 

 bring about no response, as the system at the time of its appli- 

 cation is in a relative refractory period. (Figure 48.) The 

 stimulus is, therefore, under the threshold. If, however, a stimu- 

 lus of the same strength is applied somewhat later, when the irri- 

 tability has already increased to a somewhat greater extent, then 

 at this moment the stimulus is above that of the threshold and 

 a response is obtained which, on account of the state of irritability 



Fig. 52. 



existing, is summated. (Figure 52.) But further, it is not a 

 question of the absolute interval between the stimuli, but rather 

 to the relative interval to the specific rapidity of the reaction of 

 the living substance under consideration. There are living sub- 

 stances, as we have seen, in which the refractory period is un- 

 usually short, as, for instance, the nerve. There are other sub- 



