INTERFERENCE OF EXCITATIONS 233 



about. On the contrary, my estimate, based upon the investiga- 

 tions of Thorner, refers to the total refractory period of the 

 nerve, that is, to the point of complete recovery of the equilibrium 

 of metabolism and of the specific irritability. Experimental proof 

 of this assumption is already under way. 



I have endeavored to show the elementary principles at the 

 basis of these extremely varied interference effects and to make 

 a few generalizations concerning the complicated conditions here 

 concerned. It has been shown that a great number of interfer- 

 ence effects possess characteristics in common if one takes into 

 consideration the process occurring in the course of a single 

 excitation. The altered state which exists in living substance 

 until the complete disappearance of excitation is the basis upon 

 which to explain the altered effects produced by a second stimulus. 

 This state alters during the whole course of the first stimulus 

 until the original equilibrium of the metabolism of rest is, by self- 

 regulation, again reached. It is, therefore, self-evident that the 

 second stimulus must have different effects depending upon the 

 momentary state of the living system at the time of its applica- 

 tion. The state of the system differs depending on the length 

 of the interval in which the second stimulation follows the first. 

 The most important factor is the phase of the excitation period 

 and the reduction of irritability. The second important factor 

 is the intensity of the second stimulus ; the relation of the two 

 with each other determines the response. But the specific prop- 

 erties of the given systems must also be taken into consideration. 

 It is important to know if the living system possesses isobolic 

 properties, that is, every intensity of stimulation produces a 

 maximal liberation of energy, or if it possesses a heterobolic 

 character, that is, stimuli of different strength bring about the 

 liberation of different amounts of energy. It is further impor- 

 tant to know the rapidity of reaction, whether the system rapidly 

 or slowly fatigues. In all cases it depends whether the second 

 stimulus produces a perceptible excitation or whether it occurs 

 in the refractory period and produces no perceptible effect. Upon 

 these factors depend the results of the interference of two 

 rhythmic series of stimuli, whether a summation or inhibition of 



