280 ELECTRO-PHYSIOLOGY CHAP. 



logical kathode, or auode, of a parallel-fibred muscle traversed 

 longitudinally, no changes of excitability will be displayed in 

 the intrapolar area, in either a negative or positive sense, 

 when polarising battery currents are applied of not excessive 

 strength. Just as little would this be the case at break of the 

 polarising current. It would thus appear that the electrical 

 current may traverse the muscle without producing any directly 

 demonstrable alteration of the substance with the sole exception 

 of the polar points. Very different, as we have seen, is the re- 

 action at the physiological kathode and anode proper. Here it is 

 easy to demonstrate marked changes of excitability in a positive or 

 negative direction, either resulting from a pre-existing persistent 

 excitation, or as the after-effects of such, or caused by polar 

 inhibitory processes. To this we must refer the observations of 

 v. Bezold on alterations of excitability in the tract of muscle 

 traversed, the method he employed being only adequate to test 

 the excitability of kathodic and anodic points of fibres. Start- 

 ing with the presumption that an induced current does not, 

 like a constant current, act by polar excitation only, but that it 

 excites all points of the area traversed simultaneously, and 

 uniformly, v. Bezold endeavoured to test the so-called " total 

 excitability " of the tract of muscle traversed by the polarising 

 battery current, by making use of a break induction current as 

 test stimulus, led into the muscle by the same electrodes as those 

 which conveyed the polarising current. Yon Bezold's experiments 

 may lie represented by the diagram (Fig. 95). 



The secondary coil of an induction apparatus ($.,) is intro- 

 duced into the battery circuit, the intensity of which can be 

 regulated by a rheochord. If the constant current is opened 

 at (a), an induction current will traverse the muscle in one or 

 the other direction at closure or opening of the primary circuit, 

 giving rise to a twitch in the muscle. If the battery circuit 

 is closed at (.), a part of the current, at any given intensity 

 as determined by the rheochord, will pass continuously through 

 the muscle. If the primary circuit is again made or broken, 

 an induced current of the same strength as before will traverse 

 the now polarised muscle in the corresponding direction, the 

 relations being obviously altered nily in so far as the exciting 

 current no longer starts from, and returns to, a density of 

 zero, but from a density varying with the strength of the 



