286 ELECTRO-PHYSIOLOGY CHAP. 



dition of excitability at the kathode, varying as it does with 

 strength and duration of current, must be regarded solely as the 

 consequence of localised persistent excitation, and the question is 

 only how there comes to be sometimes an increase, and some- 

 times a depression, of excitability. 



It must be admitted that the electrical current traversing a 

 muscle acts as an exciting agent, not merely at, but during the 

 whole period of closure. Further, it has been established ex- 

 perimentally, that the alterations in the contractile substance 

 which underlie the excitatory process are confined to those points 

 of fibres by which the current leaves the muscle. Each experi- 

 ment shows, moreover, that the appearance of a make twitch, i.e. 

 discharge of a wave of excitation, or contraction, at the point of 

 stimulation, as a rule implies the condition that the oscillations 

 of current from zero, or any finite value, should occur with a 

 certain rapidity ; and it should be added that the absolute 

 intensity of the excitation current also must surpass a given limit, 

 if visible manifestations of excitation are to be elicited. Sup- 

 posing that a muscle is persistently traversed by a battery 

 current of such low intensity that its presence is not betrayed 

 by any trace of visible excitation, we are none the less justified 

 in assuming that a "' latent condition of excitation " is, as it were, 

 present at all those points of fibres which collectively repre- 

 sent the " physiological kathode," during the passage of current : 

 for that altered state of the contractile muscle-substance, which, 

 by its rapid appearance at the point where current escapes, sets 

 up a wave of contraction as soon as the intensity of the current 

 exceeds a given minimal limit, and the continuance of which 

 during the closure of stronger currents is expressed in the con- 

 tinuous closure contraction, must obviously exist on closure of 

 the weakest currents also, albeit in a lesser degree. 



Only a small sudden increase, greater or less according to 

 circumstances, will then be required, in addition to the constant 

 but inadequate stimulus, in order to produce a wave of 

 excitation at the kathode. In other words, a rapid, positive 

 variation of a very weak current flowing through the muscle 

 may effect an excitation, even w T hen the same variation, starting 

 from zero at the abscissa, produces no effect, or only minimal 

 excitation of the muscle. The increased excitation occasion- 

 ally to be observed with an induction current, homodromous 



