THE REFRACTORY PERIOD AND FATIGUE 165 



period must be present in every metabolic self-regulation after 

 an excitation, during which stronger stimuli produce response, 

 while weaker are still without result. This is a fact which, as we 

 shall see later, is of fundamental importance for the compre- 

 hension of the various kinds of interference responses to stimuli. 



From the information here gained on the nature and origin 

 of the refractory period the conclusion must inevitably be drawn 

 that in all living substance there must exist, directly following an 

 excitation, a period of time in which its irritability is reduced, 

 that is, under proper conditions a refractory period can be 

 demonstrated for every living organism. Every living system 

 possessing irritability undergoes a period of reduced irritability 

 at the time of and subsequent to every excitation, for every 

 excitation momentarily decreases the amount of products capable 

 of disintegration and increases the disintegration products in the 

 unit of space. As restitution involves time, a stimulus occurring 

 in the phase preceding complete restitution cannot break down 

 the same quantity of molecules as would be the case after the 

 establishment of complete restitution, that is, the response is 

 weaker, the irritability is decreased. The refractory period during 

 and subsequent to excitation is as much a general property of the 

 living substance as irritability and metabolic self-regulation. 



This conclusion appears so self-evident that it would seem 

 hardly to call for emphasis were it not that even at the present 

 time the view is still widely held that the refractory period is a 

 special characteristic of certain forms of living substance. This 

 assumption is explained on the one hand by the fact that our 

 information concerning the refractory period is still of compara- 

 tively recent date and that few physiologists are in the habit of 

 connecting special observations with general physiological con- 

 ceptions, but also for the reason that some investigators have 

 vainly tried to find a refractory period in certain forms of living 

 substance. Langendorff and Winterstein? for instance, have not 

 succeeded in proving a refractory period for the spinal cord of 

 the frog. Langendorff stimulated the central sciatic stump 



1 Langendorff u. Winterstein: "Beitrage zur Reflexlehre." Pfliiger's Arch. Bd. 127, 

 1909. 



