STIMULATION AND TRANSMISSION 289 



The rate of change which a current requires in order 

 to stimulate a tissue varies with the nature of the tissue, 

 and is a function of the characteristic chronaxie. When 

 the chronaxie is brief, the rate of change must be rapid. 

 When the rate of change of an increasing current is 

 gradual, a greater final intensity of current is needed 

 for stimulation than when this rate is rapid. The 

 following observations of Lucas, on the stimulation of the 

 frog's sartorius, illustrate the conditions for a single 

 typical tissue. The rate of change of the exciting current 

 was controlled by varying the rate of movement of a 

 shutter which opened and closed a slot in a partition set 

 across a zinc sulphate solution, forming part of the 

 stimulating circuit.^ Comparison was made between the 

 current-strength required: (i) when the circuit was 

 closed instantaneously; and (2) when the intensity 

 was increased from subminimal to a stimulating value 

 at varying rates. The muscle was also stimulated by 

 currents of the same linear gradient or rate of change 

 under two conditions, (A) while immersed in pure 

 0.7 per cent NaCl solution and (B) in a mixture of 0.65 

 per cent NaCl plus 0.05 per cent CaCla. The following 

 results are typical:^ 



Strength of Current Required 

 Time Required to Reach Full for Stimulation 



Intensity (Seconds) ^ (NaCl^ B (NaCl+CaCh) 



o (instantaneous) ... i i 



0.1 sec I I 



0.27 1.05 1.07 



0.50 I.I 1.27 



0.97 1. 15 1-5 



^ Lucas, Journal of Physiology , XXXVI (1907), 253. 

 2 Lucas, iUd., XXXVII (1908), 459; cf. p. 473. 



