288 PROTOPLASMIC ACTION AND NERVOUS ACTION 



The characteristic time-factor or chronaxie of a tissue 

 also expresses itself in the rate of variation of intensity 

 required by the stimulating current; this rate is greater 

 the briefer the chronaxie; it is also greater the more 

 rapidly the stimulation-process develops in the tissue, 

 as indicated 'by the rate at which the accompanying 

 bioelectric variation rises to its maximum. The chron- 

 axie also varies directly with the duration of the 

 summation-interval for subminimal stimuli.^ 



STIMULATION BY CURRENTS OF CHANGING 



INTENSITY 



The stimulating effect of a current of continuously 

 changing intensity, or of a change in the intensity of a 

 current already traversing the irritable tissue, varies, 

 in a manner which is characteristic for the tissue, with 

 the rate of change, and is largely independent of the 

 actual intensity. It is significant that this rule, relating 

 stimulating effect to rate of change, applies also to 

 mechanical, chemical, and other forms of stimulation, 

 in aU of which a sudden change is more effective than a 

 gradual one. A general property of living matter is 

 apparently here involved. In the activation of the fore- 

 going metalHc model (passive iron wire in nitric acid) 

 the same rule holds; e.g., in order to activate the metal 

 mechanically by scraping with glass, the movement 

 must be rapid; a slow movement is ineffective. Similarly 

 in electric activation a current which is gradually 

 increased up to a sufficient intensity has no effect, 

 while one of the same intensity, attained suddenly, 

 causes instant activation. 



^ Cf. Lucas, Journal oj Physiology, XXXIX (1910), 463; cf. p. 470. 



