STIMULATION AND TRANSMISSION 283 



difference of the local bioelectric circuit in a conducting 

 tissue like a nerve must exceed the critical value re- 

 quired for the electrochemical process which initiates the 

 chemical reaction of stimulation. The general nature of 

 the conditions will be considered more fully later when 

 the phenomena of transmission are discussed in detail. 

 For the present we may conclude that the significance 

 of the polarization change involved in electrical stimula- 

 tion is simply to furnish the condition required for some 

 critical chemical decomposition at the cell surface. 

 Presumably this chemical change alters locally the 

 physical properties of the surface-film in such a way as 

 to involve local breakdown or increase of permeability; 

 and then, just as in the passive iron model, an auto- 

 matically self-propagating wave of chemical decomposi- 

 tion is initiated. In this process the altered and the 

 unaltered portions of the cell surface act as two electrode 

 areas, in a manner analogous to that observed in the 

 passive wire and similar systems during transmission. 



In living tissues the conditions are more complex 

 than in the simple model considered by Nernst, which 

 takes account of only one of the conditions of electrical 

 stimulation. Two chief conditions which this simple 

 theory disregards are: (i) the existence of a critical 

 threshold current-intensity, independent of duration; 

 and (2) the character of the response to currents of 

 changing intensity. Nernst's theory, however, explains 

 the essential fact of polar stimulation, in addition to 

 assigning a definite condition, viz., change of polarization, 

 for the initiation of the stimulation-process. On the basis 

 of the law of polar stimulation we may now say further 

 that a change of polarization in a definite direction^ such as 



