340 PROTOPLASMIC ACTION AND NERVOUS ACTION 



variation.' It is followed by a second period, the relative 

 refractory period, during which irritability returns 

 progressively to normal; this interval has several times 

 the duration of the absolute refractory period, and in 

 nerves under certain conditions (increased H-ion concen- 

 tration) it may be followed by a brief period of super- 

 normal excitability.^ In cardiac muscle the return of 

 normal excitability is relatively very slow, even when the 

 increased duration of the bioelectric variation is taken 

 into account; in this tissue the period of complete 

 inexcitabihty appears to outlast the entire bioelectric 

 variation, and its limits do not seem to be very clearly 

 defined; at 15° Lucas found its duration to be somewhat 

 more than 0.4 of a second.^ Usually it has been supposed 

 that the regular and somewhat slow rhythm character- 

 istic of this tissue is dependent on its long refractory 

 period, which is almost equal in duration to the period 

 of muscular relaxation. A prolonged refractory period 

 is also characteristic of the nerve cells controlling other 

 physiological rhythms of slow period, such as those of 

 certain motor reflexes in higher vertebrates (e.g., scratch 

 reflex in the dog). 



In any given tissue the duration of the refractory 

 period is influenced by the chemical conditions in the 

 surroundings as well as by the physiological state of the 

 tissue and the temperature. It is lengthened by fatigue, 



^ This is apparently strictly true of frogs' voluntary muscle, but in 

 nerve Adrian finds its duration somewhat longer, equal to that of the 

 whole bioelectric variation. In cardiac muscle it may be still longer. 

 Cf. Adrian, Journal oj Physiology, LV (1921), 193. 



2 Adrian, Journal oj Physiology, L (1920), i. 



3 Lucas, Journal of Physiology, XLI (1910), 368; cf. pp. 383-84. 



