62 IRRITABILITY 



intensity (in the muscle a continued contraction) and the 

 ''breaking," being a sudden alteration, is followed again by a 

 stronger excitation (in the muscle a contraction). The duration 

 of the change can, however, be so short that its intensity does 

 not remain at two periods of time at the same height, but instead 

 the ascent of the intensity is immediately followed by its descent 

 to zero. Induction shocks of short duration, the duration of 

 which have been observed more in detail especially by Grutsner* 

 offer typical examples. Here a single effect of the stimulus results 

 from the rise and fall of the intensity curve. Hence the induction 

 shocks as momentary stimuli are universally used for experi- 

 mental purposes. 



In contrast to the single stimuli, which find their ideal in 

 induction shocks, another form of stimulation should receive our 

 attention, namely, the series of stimuli which produce a rhyth- 

 mical alteration of vital conditions. These show among their 

 complex combination of simultaneous and successive actions of 

 their single stimuli relatively the simplest and most easily under- 

 stood regularity in their effects. They are of particular interest, 

 because they develop in the normal physiological happenings of 

 the animal body in the form of rhythmical intermittent impulses 

 of the nervous system. 



Here again it is self-evident that with regard to the course of 

 response, we must first consider the character of the single 

 stimulus of the series, and this must be done from all those stand- 

 points already here discussed. However, a new factor is met 

 with here, that is, the frequency of the single stimuli of the series, 

 or that which has the same meaning, the duration of the intervals 

 between them. This is a feature upon which the result of stimu- 

 lation depends in a very high degree. But here, too, however, 

 it is not a case of the absolute frequency of the single stimulus, 

 but simply of the relative frequency in regard to the rapidity of 

 reaction of the particular living system. I should like to remark 

 here that it is of greatest importance whether the interval between 

 the two single stimuli of the series is sufficiently long or not to 



1 Grutzner: "Uber die Reizwirkungen der Stohrer'schen Maschine auf Nerv und 

 Muskel." Pfliigers Arch. Bd. 41, 1887. 



