162 IRRITABILITY 



integration produced by the stimulus and is restored by the meta- 

 bohc self-regulation following the decomposition. 



This point of view enables us to interpret this state from a 

 physical standpoint. In this discussion on the relations between 

 irritability and the extension of excitation, I have taken the 

 amount of energy which is produced during the time unit and 

 space unit in a living system as the general standard for the 

 degree of irritability, at the same time duly regarding the indi- 

 vidual components involved. This amount of energy is deter- 

 mined in a given system by the quantity of substance broken down 

 by a stimulus of a given intensity. It is, therefore, clear that 

 during the time in which an increased disintegration produced 

 by a stimulus takes place, the irritability in response to a second 

 stimulus must be reduced, as during this period the second stimu- 

 lus has less of necessary decomposable substances at its disposal, 

 and at the same time there are more products of disintegration 

 in a given space. If a living organism is the subject of consider- 

 ation, to which the "all or none law" is applicable, as, for instance, 

 the heart at the moment of the beginning of excitation, irrita- 

 bility is completely obliterated, as shown by the fact that the 

 second stimulus of any strength remains without response, for 

 during the excitation there is a complete breaking down of all 

 the substances capable of decomposition. If, on the contrary, a 

 system is the subject of observation, for which the "all or none 

 law" is not valid, then irritability is merely reduced but not wholly 

 obliterated during an excitation, and whether or not a response 

 is obtained to the stimulus depends upon its strength. To impress 

 the relations between the degree of irritability and the intensity 

 of the stimulus, I have, therefore, employed the term "relative 

 refractory period" in contrast to the "absolute refractory period," 

 in which irritability is obliterated even for the strongest stimuli. 

 It is self-evident that irritability must again increase in the same 

 degree as the restitution of the living system by metabolic self- 

 regulation takes place, for the more molecules capable of disin- 

 tegrating are restored and the more products of disintegration 

 removed, the more molecules necessary for decomposition in the 

 unit of space are attacked and broken down by the stimulus. All 



