220 IRRITABILITY 



response does not occur, whereas such is the case immediately 

 upon the discontinuation of the stimulus. In inhibition, then, the 

 dissimilatory excitation produced by a single stimulus falls to a 

 low level as a result of the reduction of irritability and remains 

 at this level continuously. Inhibition as well as tonus is based 

 upon the development of a state of equilibrium between excita- 

 tion and recovery, or disintegration and restitution of the living 

 substance under the continuous effect of a rhythmic series of 

 stimuli. They differentiate themselves essentially by the height 

 of this equilibrium, which is dependent upon the intensity of the 

 stimulus. 



We have to the present considered only the siynplest conditions 

 existing as a result of the effect of a single series of stimuli and 

 also of the interference of its individual members. These ele- 

 mentary conditions are at the basis of an understanding of com- 

 plicated interference effects which arise when two series of stim- 

 uli interact. In that these processes can be readily explained by 

 the elementary processes previously described, I will, therefore, 

 dwell but briefly on this subject. From the standpoint already 

 taken it may be readily presumed what will happen when two 

 series of stimuli act upon the same system. 



When there is interference of two series of stimuli, there are 

 two resultant possibilities. In one type the stimuli of the one are 

 active simultaneously with that of the other. In this instance 

 both stimuli would act as a single stimulus of greater intensity, 

 and we have essentially the same condition as exists when a single 

 series is operative. Nevertheless, such cases are practically hardly 

 realized in the physiological happenings of the organism. More 

 often a state exists wherein the single stimuli of one series occur 

 in the intervals of the stimuli of the other. In these cases there 

 is an increase in the frequency of the stimuli applied in a given 

 length of time. We have here, then, in principle the same condi- 

 tions as when a series of greater frequency is operative. (Figure 

 55.) The effect of such alteration in the frequency consists in an 

 increase of the velocity of the development of summation or inhi- 

 bition, as the general scheme (Figure 55) has shown us. Depend- 

 ing upon the special combination of the factors involved in inter- 



