THE REFRACTORY PERIOD AND FATIGUE 155 



system in the process of excitation, brings about an alteration of 

 its specific irritability. Likewise as the original state of the system 

 is restored by the metabolic self-regulation after the course of an 

 excitation, the specific irritability of the system must be re- 

 established. The specific irritability is, therefore, a property of 

 the living system, which, like the metabolic equilibrium, under- 

 goes restitution by the process of self-regulation after variation 

 produced by a stimulus of any kind. It is scarcely necessary to 

 repeat each time that this is only applicable within the physiologi- 

 cal variations and for a limited period, during which the altera- 

 tions in development need not be considered. 



These alterations of the specific irritability following an excita- 

 tion and their compensation through the metabolic self-regulation 

 will now claim our attention. 



That the specific irritability of a living system undergoes a 

 diminution as the result of a stimulus of long duration has been 

 long known through the study of fatigue. This is especially so 

 with frequently recurring excitating stimuli. It is only within the 

 last decade, however, that the observation has been made in a few 

 instances that a single momentary excitation is likewise followed 

 by such a reduction of the specific irritability. But that this is a 

 fact of general physiological fundamental importance for the 

 whole field of response to stimulation in the living substance has 

 only been recognized within the last few years. 



In 1876 Marey'^ found that the irritability of the heart in re- 

 sponse to artificial stimulation was greatly reduced during the 

 systole, and that recovery took place during the following dias- 

 tole. (Figure 39.) This fact was already apparent from the 

 observations made by Bowditch' and Kronecker,^ that by stimula- 

 tion of the isolated frog's heart with single induction shocks, an 



1 Marey: "Des excitations artificielles du cceur." Travaux du lab. de M. Marey 

 II, 1875. The same: "Des mouvements qui produit le coeur lorsqu'il est soumis a des 

 excitations artificielles." Comptes rendues de I'academie des sciences T. L. XXXII, 

 1876. 



2 Bowditch: "Xithcr die Eigenthiimlichkeiten der Reizbarkeit welche die Muskel- 

 fasern des Herzens Zeigen." Arbeiten aus der physiologischen Anstalt zu Leipzig, 1872. 



3 Kronecker: "Das charakteristische Merkmal der Herzmuskelbewegung." Beitrage 

 zur Anatomie und Physiologic als Festgabe f. Carl Ludwig zum 15, Oct. 1874, gewid- 

 met von seinen Schiilern. Leipzig 1874. 



