THE CHARACTERISTICS OF STIMULI 53 



the rapidity of the alterations in the factors which act as stimuli, 

 the behavior varies greatly. Many stimuli because of their nature 

 never have a steep ascent or descent of intensity, as, for instance, 

 alterations in the concentrations of soluble substances, that is, 

 chemical or osmotic stimuli ; likewise temperature variations may 

 be mentioned. They always act relatively slowly. On the con- 

 trary there are forms of stimuli which have now a rapid, now a 

 slow, ascent or descent of their intensity, such as the photic and 

 mechanical stimuli. Finally, there are other stimuli that nearly 

 always show a very abrupt change of intensity, such as the 

 electrical form. 



The most important factor to be considered in producing the 

 response to variations of intensity, is not the absolute rapidity, but 

 rather the relative rapidity; that is, the rapidity in relation to the 

 characteristic rapidity of reaction of the particular living sub- 

 stance concerned. The rapidity of the reaction to stimuli is very 

 different in various forms of living substance. On the one 

 hand, we have forms reacting very quickly, as the nerve and the 

 striated muscle; on the other, those which respond very slowly, 

 such as a great number of unicellular organisms. Between these 

 are a great number of living substances which, as far as the 

 rapidity of the reaction is concerned, occupy intermediate posi- 

 tions of every varying degree. It is clear that the adequate 

 stimuli for slowly reacting substances must be those having also 

 a slow change of intensity; for quickly reacting, those having a 

 rapid change of intensity.^ If a nerve muscle preparation is 

 simulated with the single induction shock, the "break" as well as 

 the "make" shock has effect. But even here a difference is 

 noticeable. The "make" shock has a weaker effect than the 

 "break" shock. This difference is due to the difference of 

 abruptness in its course, which when the current is made is less 

 than that of opening, for, when the current is made, the ascent 

 of the primary current is retarded by the extra current flowing 

 in the opposite direction^ whereas, when broken, with the fall 

 of the intensity of the primary current, the extra current in the 



1 Vergl. Julius Schott: "Ein Beitrag zur electrischen Reigung des quergestreiften 

 Muskels von seinen Nerven aus." Pfliigers Archiv Bd. 48, 1891. 



