192 ELECTRO-PHYSIOLOGY CHAP. 



met by the significant fact that rapid variations of density in 

 a current are an effective stimulus to highly mobile kinds of 

 protoplasm (striated muscles), while they are ineffective towards 

 more sluggish portions. This is clearly shown by the fact that 

 normal striated muscle, when excited with the constant current, 

 twitches conspicuously at the moment of appearance and dis- 

 appearance (closure and opening) of the current. TJie visible 

 manifestations of persistent excitation fall into the background, 

 while the excitatory effects of current variation come prominently 

 forward, in proportion as the excitable protoplasm is more highly 

 mobile. This dictum is sufficiently borne out by the total results 

 of experiments on contractile substance. It finds character- 

 istic illustration when the action of a gradually increasing current 

 on different irritable tissues is examined. If the circuit is closed 

 as usual by hand, e.g. with a wire dipping into mercury, the intensity 

 naturally rises with excessive rapidity from zero to its maximum, 

 so that the form of the curve of variation is unrecognisable in 

 detail. But by using a contrivance, by means of which the 

 intensity of the current can be gradually increased from zero 

 as in the slow and uniform gradation of the slider in du Bois' 

 rheochord it may easily be demonstrated that (although the 

 sudden closure of the same current produces a maximal twitch 

 with subsequent persistent contraction) it now gives no indication 

 of shortening, or, in the most favourable case, a weak persistent 

 contraction only, in striated muscle. If the same experiment is 

 repeated on a preparation of smooth muscle, e.g. the adductor 

 of the shell in anodonta, the effect is quite different. Tick (4) 

 indeed asserts that he has succeeded " in passing currents of con- 

 siderable strength " through this muscle also, " without contracting 

 it," but a phenomenally slow increment of current intensity was 

 required, extending over several minutes. Under these conditions 

 it can hardly be a matter of surprise that no visible manifestations 

 of excitation make their appearance, considering that the effect of 

 the constantly increasing fatigue changes in the muscle-substance 

 must be accentuated in proportion with the slowness of increase 

 of intensity, at every point at which (as will be shown below) an 

 excitatory process is discharged by, and during, the passage of the 

 current. At each successive moment, i.e., the current acts upon 

 points of the fibre, which have already been modified by the whole 

 preceding passage of current, in proportion with its duration. 



