THE INJURY CURRENT OF NERVE 
247 
upon electrotonic currents (when Hermann' identified it as being due to this 
cause, confirming his statements by experiments upon the core-model), nevertheless, 
it has not been directly examined. 
The experiments given below may serve to supply such a deficiency, although 
not made with this intention, since they were undertaken in ignorance of the general 
principle enunciated by Helmholtz, and with a view to experimentally investigate 
the disturbance produced by the placing of an 'external arc' The examples, which 
are quoted, all exhibit special instances of the manner in which the pre-existing 
difference of potential is affected, e.g. : — 
(«) The creation of differences of potential where none existed previously. 
(b) The diminution, elimination, or reversal of pre-existing differences of 
potential. 
The agreement of the results actually found with those anticipated by a law 
unknown to the investigator is evidence of the general exactness of the measurements 
taken in this research, and also, and this is of importance, to the correctness of certain 
assumptions made in dealing with the measurements of resistance. 
Experiments 
In each experiment the nerve, having been removed from the animal, was 
placed upon four nonpolarizable electrodes — A, B, C, D ; the cross section being 
always placed upon electrode A, and the other end of the nerve, extending beyond 
electrode D, was suspended from the wall of the moist chamber by a silk thread. 
The wires connected to the four electrodes — A, B, C, D — were fitted into brass 
plugs, which could readily be inserted and removed from positions in circuits arranged 
for the measurement of potential differences or resistances, or could be placed into 
adjoining holes in an insulated piece of brass. In this way it was possible to measure 
the resistance, take the potential difference, or complete the 'arc' joining the elec- 
trodes simply, or through an inserted resistance. 
The length of the nerve and of its various sections, as divided by the position 
of the electrodes, was carefully measured. 
The resistances of the nerve and of its various sections were also measured. 
Each value given for a resistance being the mean of two measurements taken with 
the nerve placed in the two positions possible in the limb of the Wheatstone bridge. 
This precaution was taken to avoid the error due to the presence of differences of 
potential. 
The resistances between the electrodes themselves were measured, for each pair, 
before and after the experiments. The values obtained for these have in each case 
been subtracted from the values obtained of the resistances of pieces of nerve in- 
clusive of electrodes. 
1. Hermann, Pfl'ugers Archiv., XX, 1879 ; also H. Weber, Borchardts Journ. of Math., LXXVI, p. 13. 
