RALPH S. LILLIE 109 



prolonged "compensatory pause" following an "extra-contraction" 

 in heart muscle. 



It is generally assumed that the refractory phase corresponds essen- 

 tially to the period required for recovery or recuperation. The act 

 of stimulation appears to involve the more or less complete breakdown 

 or removal of some material or structure which is essential to stimula- 

 tion; the refractory interval represents a period of reconstruction or 

 restoration. When this process of repair is rapid the refractory period 

 is brief, and vice versa. In any single tissue the rate of this recon- 

 struction exhibits a general parallelism with the rate of the structural 

 or chemical breakdown associated with stimulation ; hence rapidly re- 

 sponding tissues have as a rule brief refractory periods. This paral- 

 lelism can, however, be artificially disturbed, as in Tait's experiments, 

 by drugs like protoveratrine and yohimbine which abnormally pro- 

 long the refractory period. In such cases an abnormal prolongation of 

 the descending or return phase of the bioelectric variation is also ob- 

 served.^ There appears thus to be a correlation between the length 

 of the refractory period and the duration of the bioelectric variation, 

 especially of the return phase of the latter. The relation, however, is 

 not simple, for it is certain that in some cases, e.g. heart muscle, the 

 whole period of diminished excitability (including both the "abso- 

 lute" and the "relative" refractory phases) may greatly outlast the 

 electrical variation.^ This is also true of the metallic model under 

 consideration in the present paper. It is thus not sufficient to regard 

 the entire refractory phase as corresponding to the time required to 

 restore the normal or "resting" semipermeability and polarizabiHty 

 of the altered plasma membranes of the irritable elements. Recovery 

 of semipermeability is no doubt necessary to a recovery of irritability, 

 since electrical stimulation requires polarizability in the membranes;^ 

 but some further change appears also to be essential. This question 

 will be discussed later, after the processes in the metallic model 



5 Tait, J., Quart. J. Exp. Physiol, 1910, iii, 221. 



^ Trendelenburg, W., Arch. ges. Physiol., 1912, cxliv, 39. The case of nerve, 

 with a brief refractory period, is similar (c/. Adrian, E. D., J. Physiol., 1914, 

 xlviii, 453). 



^ The inference from the work of Nernst, Lapicque, Lucas, Hill, and others on 

 electrical stimulation. 



