MEMBRANE CHANGES DURING STIMULATION 339 



in unfatigued tissues, while the change in irritability- 

 follows a markedly asymmetric course; i.e., there is a 

 rapid and complete loss of irritability on stimulation, 

 followed by a relatively gradual recovery. Evidently 

 in the recovery process additional factors enter which are 

 independent of the bioelectric variation. It is, however, 

 highly significant that in all cases irritability seems to 

 disappear completely during the rising phase of the 

 variation. This partial correlation between the time- 

 relations of the two phenomena indicates that the 

 conditions determining the electromotor variation are 

 closely connected with those determining the temporary 

 loss of irritability. 



The division of the whole refractory period into two 

 distinct subperiods, known respectively as ''absolute" 

 and ''relative," is its most interesting feature. The 

 absolute period is the brief interval of complete insensi- 

 tivity immediately following a single effective stimulus. 

 During this interval a second stimulus, no matter how 

 strong, has no appreciable effect. It is as if the tissue 

 for a brief time lost its irritability completely; hence, 

 two stimuli succeeding each other within this interval 

 produce the same effect as a single stimulus; while if 

 the second is sent in after the completion of this brief 

 interval, its effect is seen in an increased response or 

 summation-eft'ect. The completely inexcitable period 

 in a frog's motor nerve at 20° lasts about o.ooi to 0.0015 

 of a second;^ in the sartorius muscle it is from two to 

 three times longer; and in both of these cases its duration 

 is closely similar to that of the upstroke of the bioelectric 



' Cf. Adrian, Journal of Physiology^ XL VIII (191 4), 453; L(i9i6), 

 345. 



