296 PROTOPLASMIC ACTION AND NERVOUS ACTION 



single stroke will ignite if the interval between the 

 strokes is not too long. 



It will be evident that the general condition common 

 to all cases of this kind is that two active processes or 

 sets of processes, with resultants acting in opposite 

 directions, are concerned; the equilibria are ''dynamic" 

 rather than static. In an irritable tissue through which 

 a uniform electric current is flowing, the effect which the 

 current produces in the direction of stimulation is 

 presumably counterbalanced by contrary processes 

 depending largely on the metabolic or synthetic activity 

 of the tissue. Interruption of an already existing uni- 

 form current, as well as its sudden increase or decrease, 

 disturbs this equilibrium and may result in stimulation. 



Further analysis of the process of stimulation requires, 

 therefore, a consideration of the special nature of the 

 processes occurring in the irritable protoplasmic system. 



According to the foregoing conception of the condi- 

 tions of stimulation in living cells, the primary or 

 initiatory process is a local alteration of the protoplasmic 

 surface-film, or ''plasma membrane," of the irritable 

 element. This alteration has a self-propagating charac- 

 ter, like that shown by other chemically alterable 

 surface-films at the boundary between two electrically 

 conducting phases, and leads secondarily to the character- 

 istic manifestation of cell-activity, or response. If this 

 conception of the stimulation process is a true one, all 

 forms of stimulation should exhibit definite evidence 

 of accompanying surface-processes of the kind indicated. 



Certain effects which apparently accompany all 

 forms of stimulation and activation, whatever the special 

 nature of the response may be, constitute evidence of 



