INTERFERENCE OF EXCITATIONS 231 



with its associated properties, such as the conductivity without 

 decrement and the incapability of summating excitations, have 

 in a system of this character only relative validity. They are 

 realized only in the state of an equilibrium of metabolism. Only 

 when the stimuli follow each other at intervals greater than the 

 duration of the refractory period is there a response of equal 

 extent to stimuli of all intensities which are above the threshold. 

 During the refractory period and consequently in fatigue, 

 asphyxia, cooling and narcosis, etc., in short, in all states in which 

 the refractory period is prolonged this system loses its isobolic 

 properties and becomes heterobolic. In order that there may not 

 be a misunderstanding, we will consider more in detail the capa- 

 bility in this state of summation of excitations. When we refer 

 to a summation of excitation of such a system under the influence 

 of one of these factors, we, of course, at no time mean an increase 

 of response beyond that of the degree of excitation which exists 

 in an isobolic system in a normal state consequent upon the appli- 

 cation of a single stimulus, for this degree of excitation is maxi- 

 mal. We refer rather to a summation which has become reduced 

 as a result of fatigue. 



On the basis of these facts it is readily understood when a level 

 of equilibrium of lower intensity has been reached that excita- 

 tion produced by weak faradic stimulation must have weaker 

 effects than when strong stimuli are applied, for when the system 

 assumes a heterobolic type as the result of relative fatigue weak 

 stimuli bring about weak, and strong, stronger excitation. Conse- 

 quently, during interference induced by a second series of excita- 

 tions, in the first case we have the conditions favorable for inhi- 

 bition, in the second for those of summation. If we also assume 

 that this characteristic alteration of the isobolic character of the 

 elementary nerve fibers which has been shown to occur in fatigue, 

 as seen when continued faradic stimulation is employed, develops 

 immediately after the beginning of stimulation then we can 

 readily understand the various kinds of effects produced by inter- 

 ference observed in the reflex response following weak and strong 

 faradic stimulation to the different nerves in spite of the fact that 

 the nerve in the state of rest is a system isobolic in type. Experi- 



