100 INTRODUCTION. 



ascending in the nerve is not so strong as to produce a break- 

 excitation by itself, in spite of its partial compensation by tlie 

 nerve-current. On opening the main circuit with the nerve-circuit 

 closed, a new path is afforded to the nerve-current, and a con- 

 traction occurs, which is here further reinforced by the influence of 

 the Voltaic alternatives. For, if the influence of the latter depends 

 on the fact that a point on a nerve which had previously been the 

 point of entrance of a current has become more sensitive to excita- 

 tion by a current leaving the nerve at the same point, the effect of 

 closing the nerve-current will be greater. I shall here entirely 

 neglect any effects which polarisation-currents might have, since 

 these currents would find a path for discharge by the nerve-circuit 

 at break of the main circuit/ 



While refraining from all further criticism of Hering's explana- 

 tions, I yet think that they are at least insufficient. It appears to 

 me that the circumstance which Hering puts aside altogether, the 

 formation namely of polarisation-currents, is of far greater import- 

 ance than the other facts. I believe that all these phenomena may 

 be explained in the most simple manner as follows. If a battery 

 of inconstant strength is closed through an arc of connection of 

 small resistance, the electromotive force of the battery, as is well 

 known, quickly falls off from the formation of a polarisation-cur- 

 rent, and any one who has once used such a battery for galvano- 

 caustic experiments knows how quickly the wire ceases to glow 

 when the current has been passed for some time, and, on the 

 other hand, how quickly and certainly it again becomes red-hot on 

 closure after the battery has been left open for a short time ; 

 further, how little the electromotive force diminishes if there is a 

 circuit of great resistance. 



Now a nerve, or at any rate certain parts of it, when an 

 arc of connection of small resistance is applied to it, is analogous 

 to a battery whose current diminishes very rapidly; while a nerve 

 without any such arc of connection is analogous to a battery closed 

 by a circuit of very great resistance. 



Hence if the current is broken in the battery circuit, thus leaving 

 for the nerve an external arc of connection always closed, the 

 counter-current has at closure only to contend with a weak current 

 in the portion of nerve concerned, and will therefore produce an 

 effect sooner ; hence the earlier appearance of excitation on closing 

 the counter-current in the main circuit. The opposite of course 

 occurs on closure in the nerve-circuit. On opening in the battery 



