286 PROTOPLASMIC ACTION AND NERVOUS ACTION 



were passed through the tissue by non-polarizable 

 electrodes, and the minimal duration required for 

 stimulation was determined by varying the distance 

 between two contacts (one making, the other breaking 

 the current) in a swinging pendulum. 



The essential feature of these results is that below 

 a certain definite intensity of current (0.18 units), 

 increasing the duration has no effect in lessening the 

 intensity required to stimulate; i.e., weaker currents 

 will not stimulate, whatever their duration. This 

 intensity represents the critical or threshold value for 

 any current. But with stronger currents, the duration 

 required for stimulation becomes less as the intensity 

 increases, and a close approximation to Nernst's square 

 root law is found. This signifies that a certain minimal 

 change of polarization is required to initiate the stimula- 

 tion process; with currents above a certain critical in- 

 tensity this polarization is attained with briefer and briefer 

 durations as the intensity is progressively increased. 

 Lapicque and other observers have obtained similar 

 results. The current of threshold value, i.e., of the least 

 intensity that will stimulate with any duration, must 

 traverse the tissue for a certain minimal time in order to 

 stimulate. This time is characteristic for the tissue in 

 question, and apparently is a direct function of the rate of 

 certain specific metabolic processes; probably those con- 

 cerned in the alteration of the surface-film, as indicated by 

 the duration and other features of the refractory period 

 (see below). According to Keith Lucas and Mines, the 

 length of this minimal time varies with the temperature 

 of the tissue in accordance with a somewhat low 

 temperature-coefhcient, similar to that of diffusion 



