52 



IRRITABILITY 



(Figure 3, A.) A regularly rising pressure would, for instance, 

 represent a stimulus in its simplest form. But such forms of 

 stimuli are only very rare in nature and are also experimentally 

 very difficult to produce. It is, for example, not easy to give the 

 electrical stimulus, so much used for experimental purposes, this 

 form. Fleichl and v. Kries have only accomplished this by means 

 of complicated apparatus. The usual form of the individual 

 stimulus is not a straight line, but a logarithmic curve. (Figure 

 3, B.) The alteration hardly ever progresses with equal rapidity 

 from its beginning until it reaches its highest point, but as a rule, 

 with decreasing rapidity. This is the usual course of alterations 

 of concentration, also of chemical and osmotic stimuli, of changes 

 of temperature and of electric stimulation. 



Fig. 3. 



The rapidity of alterations in vital conditions has quite an 

 important influence on the development of the response to stimu- 

 lation. It is well known that if a constant current, which reaches 

 its highest intensity rapidly, is permitted to act upon a muscle, 

 the effect differs from that following the application of a current 

 of the same intensity but in which this is reached very slowly. In 

 the first case there is a sudden strong twitch, in the second none 

 at all. In spite of this there can be no doubt whatever of the 

 current in the last case being effective. That the muscle is also 

 excited when the current is slowly increased is shown by the 

 contracture, which grows more and more plainly perceptible with 

 the increasing intensity of the current and in higher intensities 

 by the so-called Porrefs phenomenon, which consists in a curious 

 wave-like movement of the muscle-substance. In reference to 



