56 IRRITABILITY 



also, the intensity of which I found quite unbearable, remained 

 utterly without effect.^ These two extreme cases, the nerve and 

 the foraminifera, show plainly that the effect of a stimulus is not 

 produced by the absolute rapidity of the increase of intensity, but 

 is solely influenced by the relative rapidity of the same. 



A further point for consideration in the duration of an altera- 

 tion in a vital condition in producing a stimulant action is the 

 length of time the stimulus remains after reaching its highest 

 point. In the forms of stimuli occurring in nature the duration 

 of the alteration after reaching its highest level can vary consider-^ 

 ably. The stimulus may remain indefinitely at a certain level, 

 v.hen this is once reached. (Figure 5, A.) The alteration like- 

 wise persists. This would be the case, for instance, with the 

 changes of concentration in the transfer of an organism from 

 fresh into sea water. The alteration can also, however, imme- 

 diately after attaining its highest level, return, so that the original 

 state is at once reestablished. (Figure 5, B and C.) Here it is a 

 case of a quick deviation in the external vital conditions. A 

 sudden jar would be a case in point. Between these two extremes 

 we have all variations in the duration of all natural and experi- 

 mental forms of single stimuli. 



Now we arrive at the question : Has a prolonged stimulation 

 really a prolonged effect ? This question might seem superfluous, 

 as from a conditional standpoint it is self-evident that every 

 alteration in any one of the conditions of a system is followed 

 by an alteration in the system. But this very question played an 

 important role in older physiology and led to prolonged discus- 

 sions for the reason that a special case was taken into considera- 

 tion in this connection, which at that time was not clearly under- 

 stood. Du Bois-Reymond,- as a result of his investigations on 

 the nerve muscle preparation of the frog, formulated a law of 

 nerve excitation, according to which it is not the absolute value 

 of the intensity of the constant current which produces an exci- 

 tation of the nerve and contraction of its muscle, but an alteration 



1 Max Verworn: "Untersuchungen iiber die polare Erregung der lebendigen 

 Substanz durch den constanten Strcm." Ill Mitteilimg, Pfliigers Arch. Bd. 62, 1896. 



2 Du Bois-Reymond: "Untersuchungen iiber tierische electricitat." Bd. I. Berlin 

 1848, p. 258. 



