338 PROTOPLASMIC ACTION AND NERVOUS ACTION 



The duration of the refractory period varies greatly 

 in different irritable tissues, and under normal conditions 

 is specific for each tissue. The most significant general 

 correlation is that it is brief in tissues with brief chronaxie, 

 and vice versa. As already pointed out, in any irritable 

 tissue the length of the chronaxie is closely related to 

 the duration of the single bioelectric variation; and the 

 duration of the refractory period shows a similar correla- 

 tion. This parallelism has frequently attracted attention, 

 and its significance has recently been discussed in con- 

 siderable detail by Tait,^ who has reached the conclusion 

 that the first part of the period, the interval of complete 

 inexcitability or '^absolute refractory period," which is 

 of brief duration, corresponds with the period of upstroke 

 of the curve of electromotor variation, while the ''rela- 

 tive" part of the period corresponds with the downs troke 

 or return phase. Tait has shown that the relative 

 refractory period is greatly prolonged by drugs 

 (yohimbine, protoveratrine) which retard the return 

 phase of the bioelectric variation. In general, he finds 

 the return of irritability to run parallel with the return 

 of the normal resting potential of the muscle. 



While it seems clear that the delayed recovery in 

 such cases has a connection with the delay in the recovery 

 of the normal electromotor properties of the tissue, the 

 correlation is apparently not a simple one. More recent 

 evidence shows that under normal conditions the relative 

 refractory period in muscle and nerve may last several 

 times longer than the return phase of the electric varia- 

 tion.^ The curve of the latter is very nearly symmetrical 



' Tait, Quarterly Journal of Experimental Physiology, III (191 o), 211. 



' Cf., for heart muscle, Trendelenburg, Arch. ges. Physiol, CXLIV 



(1912), 39; for nerve, Adrian, Journal of Physiology, XL VIII (1914), 453. 



