STIMULATION AND TRANSMISSION 275 



living system by the constant electric current is subject 

 to definite quantitative laws; they indicate also that 

 the current acts primarily through its polarizing action. 

 Upon this polarizing action follows chemical action as a 

 secondary consequence. 



The essential conditions under which the current 

 causes stimulation may be briefly summarized as 

 follows : 



1. The current must exceed a certain minimal 

 intensity; this ''threshold" intensity varies widely for 

 different irritable tissues and for the same tissue under 

 different conditions; e.g., during states of fatigue, 

 narcosis, sensitization, etc. 



2. A current of threshold or greater intensity must 

 traverse the tissue for a certain minimal time; its 

 stimulating action thus depends not only upon its 

 intensity but also upon the duration of its flow. The 

 rule that for equal stimulating action the product of the 

 intensity into the root of the duration is constant 

 {il/t = K) appears to hold for most tissues within a 

 considerable range of intensities.' For the current 

 of threshold intensity this critical duration varies widely 

 in different tissues (from a few thousandths to several 

 seconds) ; it is the expression of a time-factor (chronaxie), 

 which is specific for the tissue in question.'' 



These general statements apply to the case of currents 

 which reach their full intensity rapidly or instantane- 

 ously; e.g., when the stimulating circuit is suddenly 

 closed. Such a condition, however, is apparently not 

 a normal or ''physiological" one, since the bioelectric 

 current — which, according to our present conception, is 



^ Cf. Nernst, loc. cit. ' Cf. Lapicque, loc. cit. (1909). 



