THE REFRACTORY PERIOD AND FATIGUE 159 



of varied duration and then registered the action currents by 

 means of the capillary electrometer. He then found that the 

 second stimulus was ineffective for about .005 seconds after the 

 application of the first stimulus. If the second stimulus follows 

 somewhat later, it produces a contraction which is weaker and 

 has a longer latent period the nearer the second stimulus ap- 

 proaches the first in point of time. (Figure 30.) Massart^ and 

 Jennings^ likewise observed the existence of a refractory period 

 for the myoids of unicellular organisms brought about by mechan- 

 ical stimuli. Massart attributes this cessation of reaction to 

 stimuli following each other at certain intervals, to fatigue, an 

 explanation which has been disputed by Jennings as the result of 

 his investigations made on Stentor and Vorticella. Jennings looks 

 upon the behavior of the infusoria rather as an "adaptation" to 

 the stimulus. P titter was the first to see in this the existence of 

 a refractory period. His experiments on Spirostomun ambiguum 

 in 1900 showed a refractory period in the reaction to rhythmical 

 mechanical stimuli. I wish to state, however, that these observa- 

 tions of Putter have not as yet been published. Thus the exist- 

 ence of a refractory period has even today been proved for a 

 whole series of very different kinds of substances. 



We will now examine the alterations of irritability which are 

 perceptible during the refractory period to complete restitution of 

 the specific irritability of the particular system, and endeavor by 

 the analysis of their special conditions to render them compre- 

 hensible from a physical standpoint of view. 



The first fact to take into consideration is, that, as is shown in 

 the heart, the refractory period begins at the moment of the 

 appearance of the systolic excitation. The irritability of the 

 heart is absent and remains so until the excitation has reached 

 its highest point, that is, shortly before the beginning of the dias- 

 tole. From this point the restitution of irritability begins, which 

 does not reach the maximum until the end of the diastole. In 

 other words : irritability undergoes the greatest reduction by dis- 



1 Massart: Annales de I'lnstitut Pasteur 1901. 



2 Jennings: "Studies on reactions to stimuli in unicellular organisms." IX. 

 American Journal of Physiology, 1902. 



