UN!VE r 



in ELECTRICAL EXCITATION OF MUSCLE 289 



It is less easy to formulate conclusions as to the manifesta- 

 tions of excitability at the " physiological anode " of a muscle, 

 during the passage of current. The attempts at solving this 

 problem by the application of induction currents, opposed in 

 duration to the polarising current, and traversing the whole intra- 

 polar tract, have been too ambiguous to give any decisive results. 

 Von Bezold, who made the same experiments, though from another 

 standpoint, asserts (10) that "both ascending and descending 

 galvanic currents tiowing through the muscle, increase its 

 excitability at first to ascending make induction currents, so long 

 as these do not exceed a certain density, at and after which 

 point the effect is diminutional." Moreover, " the turning-point of 

 the curve of increase of excitability, in ratio with the density of 

 the polarisation current as abscissa, appears earlier when the 

 polarisation current is opposed to the exciting current, than when 

 it is homodromous with it." When the induced current is in 



FIG. 96. 



the same direction as the polarising current, the excitation of 

 the muscle apparently ensues only because the constant current 

 suffers a sudden, evanescent, positive variation at the moment 

 of closure of the primary circuit 1 (Fig. 96, ). 



The converse naturally occurs when the direction of the 

 exciting current is opposed to that of the polarising galvanic 

 current (Fig. 96, b). 



It depends essentially upon the magnitude of variation in 

 intensity of the former, whether a twitch is yielded by the 

 muscle, or not. If the line of intensity of the (presumably) very 

 weak, polarising current is represented by a straight line above 

 the abscissa, and running parallel with it, it is evident that while, 

 with uniform direction of excitation current and polarising 



1 Since the disappearance of induced currents does not usually, on account of their 

 extremely brief duration, give rise to a break excitation, we must not hesitate in the 

 cases cited to regard the action of a homodromous induction current as similar to 

 that of a single, rapidly disappearing positive variation of the galvanic current. 



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