290 PROTOPLASMIC ACTION AND NERVOUS ACTION 



The more slowly the current changes its intensity the 

 less effective it is as a stimulus. The sensitivity to 

 rate of change varies with temperature and the composi- 

 tion of the medium; the necessary rate of change is 

 greater at higher temperatures; it is also greater when 

 calcium is present (B) than in the pure NaCl solution 

 (A). According to Lucas, "an increase in the concentra- 

 tion of the calcium would appear to necessitate a more 

 rapid concentration of the ions concerned in excitation."^ 

 Observations by Mines, on the minimal duration of the 

 threshold current of constant intensity,^ have shown 

 that in this case also the duration is briefer when Ca 

 is present. Such facts indicate that the chronaxie of a 

 tissue is determined not only by its specific constitution 

 but also by the external conditions to which it is exposed. 

 This is well shown in certain studies, by Adrian, on the 

 effects of peripheral nerve injury.^ 



The chronaxie of a tissue appears to be closely 

 related both to the rate of response and to the rate of 

 recovery of the irritable elements. Thus it shows a 

 close correlation with the characteristic duration of both 

 the bioelectric variation of the tissue and the refractory 

 period. The more slowly a tissue responds to a constant 

 current, i.e., the longer the minimal duration of the 

 current of threshold intensity, the more gradual is the 

 rate of change required for excitation by a current of 

 changing intensity. The length of the summation- 



^ Op. cit. (1908), p. 480. 



2 Cited in Lucas' paper, op. cit. (1908), p. 472. 



3 The chronaxie of a muscle with nerve supply interrupted increases 

 progressively until innervation is re-established {Archives oj Radiology 

 and Electrotherapy^ May, 19 17). 



