21-i IRRITABILITY 



recovery of excitation and accordingly a decreased state of irri- 

 tability. In spite of the diminution of the relative response to 

 each stimulus the summation of excitation brings about an 

 absolute increase of the same. At the same time the irritability 

 decreases more and more, for after each stimulation the oxydative 

 disintegration as well as restitution require a progressively greater 

 time and a relative fatigue must, therefore, necessarily develop. 

 The summation, consequently, reaches its limit very soon and 

 then decreases progressively, for, as a result of the increase of 

 fatigue, the oxydative decomposition which occurs at the instant 

 of every stimulation reduces and with this the energy production 

 becomes less and less. The system is relatively refractory for 

 the given intensity of stimulus. Accordingly the response to stim- 

 ulation falls below the threshold of perceptible response (dotted 

 line S) and finally an equilibrium between disintegration and 

 restitution occurs, wherein the small amount of material used at 

 each stimulation by oxydative decomposition is again replaced 

 before the next stimulus. In other words, the irritability is re- 

 duced at each stimulation to an amount equal to that of the 

 recovery in the interval. If this all takes place beneath the thresh- 

 old of perceptible response, the system during the contin- 

 uance of the stimulation seems responseless, that is, inhibited. 

 The inhibition consists then of a reduction of irritability below 

 the perceptible threshold of response of the stimulus concerned. 

 It depends upon a continued lessening of dissimilative excitation 

 to a low level through the delay of the oxydative decomposition 

 processes. The inhibition is according to this a relative fatigue, 

 which is conditioned, as is true of every fatigue, by a lengthening 

 of the refractory period following a relative deficiency of oxygen. 

 The processes of inhibition are simply and solely an expression 

 of a refractory period persisting as a result of dissimilatory 

 excitating stimuli. 



Accordingly the general conditions requisite for summation 

 on the one side and inhibition on the other may be formulated 

 as follows: 



A summation may develop in a heterobolic system and by the 

 use of submaximal stimuli. It always develops when the follow- 



