THE CHARACTERISTICS OF STIMULI 63 



allow the living system time to completely recover from the effect 

 of the preceding stimulus. In the cases, for instance, where we 

 have recovery, we have the same rhythm of stimulation as that 

 of response. When recovery does not occur, interferences of the 

 response are developed, which are of great physiological impor- 

 tance, with the analysis of which we shall later on find occasion 

 to occupy ourselves in detail. The physiological example for these 

 stimuli is the rhythmical discharge of impulses of the nerve 

 centers; the physical method, which is most widely used for 

 experiments, is the faradic current. 



It is apparent that the question of frequency must again be 

 combined with all those factors previously discussed in connec- 

 tion with the single stimulus. In consequence another compli- 

 cation arises and with this another point must be taken into 

 consideration, namely, the fact that the duration of the single 

 stimulus in a series undergoes alteration by increasing frequency 

 beyond a certain limit. Beyond this limit the duration of the 

 single stimulus must become less and less. As the result of the 

 fact that stimulation is, as we have seen, dependent on the dura- 

 tion of stimulus, it is evident that, depending upon the rapidity 

 of response of the living system, sooner or later the rhythmical 

 stimulation must become ineffectual. Nevertheless, this effect 

 of shortening the duration of the single stimulus can be compen- 

 sated by a corresponding increase of its intensity. In this con- 

 nection Nernst^ showed a very simple relation for induction cur- 

 rents of higher frequency of interruption, which furnishes a law 

 according to which such a compensation takes place. In conjunc- 

 tion with Barratt he found, namely, that the intensity must in- 

 crease proportionately to the square root of the number of single 

 stimuli if the threshold value of the stimulus is to be maintained, 

 that is, I : Vm = const., in which / is the intensity of the current 

 and m the frequency of interruptions. The limits of the validity 

 of this law cannot at present be conclusively established. 



This exhausts the small number of elementary factors con- 

 cerned in the course of the stimulation, and which are of impor- 



1 Nernst und Barratt: "Ueber electrische Nervenreizung durch Wechselstrome." 

 Zeitschrift fur Electrochemie 1904. 



