INTRODUCTION. 99 



When, however, unpolarisable electrodes are employed, there is no 

 longer the same difference. 



More recently Biedermann and Hering 1 , the former with 

 parallel-fibred curarised muscles, the latter with exceedingly ex- 

 citable nerve-muscle preparations (both using unpolarisable elec- 

 trodes), made similar experiments, and convinced themselves that 

 excitation of a muscle or a nerve by its own current can, for in- 

 stance, occur if a good external arc of connection is offered to these 

 currents, that is on closure of an arc whose ends are at the cross- 

 section and at the surface, or, what comes to the same thing, on 

 breaking the counter-current in the battery circuit in the case of 

 a nerve- or muscle-current compensated in the ordinary way. 

 Similarly, contractions are observed if the arc of connection is 

 broken, or, what comes to the same, if the compensating counter- 

 current is closed in the battery-circuit. But there is no excitation 

 if, with the nerve- or muscle-current compensated, the key in the 

 nerve-circuit is opened, or if, with the nerve-circuit previously 

 opened, a compensating counter- current is directed through the 

 nerve by closure of the nerve-circuit. 



When the current is stronger than necessary for compensation in 

 the ordinary sense, the conditions become more complicated, but 

 most readily intelligible if what was said at p. 95 on this subject 

 is borne in mind. Hering gives the following explanation of the 

 facts in this case, simultaneously observed by him and by me, namely, 

 that excitation with counter-currents has more effect if the current 

 is made and broken in the main circuit than if in the nerve-circuit, 

 that is, that in the first case weaker currents are able to elicit con- 

 tractions. ' When the counter-current is somewhat stronger than 

 necessary for compensation, after closure of both circuits the current 

 in the nerve is actually ascending, although only traversed by what 

 may be called the remainder of the battery-current. Thus if, with 

 the main circuit already closed, the nerve-circuit is closed, there is 

 no contraction unless the division of the battery-current passing 

 through the nerve-circuit is of very considerable strength. If, on 

 the other hand, the main circuit is closed with the nerve-circuit 

 already closed, the effect of opening the nerve-current is added to the 

 weak and, by itself, insufficient effect of closure of the division of 

 the battery-current, and a contraction occurs. On opening the 

 nerve-circuit with the main circuit previously closed, no contraction 

 occurs, always provided that the division of the battery-circuit 

 1 Wiener Sitzungsberichte, Ixxxv. Part 3. p. 144 ; ibid. p. 237. 

 H Z 



