1888-89.] Mr G. N. Stewart on Electrotonic Variation. 235 
action will pass through the galvanometer from B to A.* This will 
have the opposite direction to the electrotonic current, and will 
therefore look like a diminution or negative variation of that current. 
Similarly, if C he the anode, and the current be descending, the 
excitation will pass over B in greater intensity than over A, and 
again there will he a negative variation of the electrotonic current. 
Hermann, as already stated, seemed at one time to suppose that 
this was a complete explanation of the phenomena. But he was 
afterwards led by rheotome researches to modify his view, and, 
while retaining the law of “ polarisation increment ” as an essential 
factor in the explanation, to postulate besides, as Bernstein had 
previously done in a somewhat different form, an actual diminution 
in the polarisation, a negative variation, so to say, in the capability 
of the nerve to take on polarisation between core and sheath. 
(“ Untersuchungen iiber die Actionsstrome der nerven,” Pfliiger’s 
Arcliiv , Bd. xlii. s. 246, &c.). 
According to Hermann, the electrotonic currents are branches of 
the polarising stream which spread beyond the electrodes, owing 
to the transverse resistance caused by polarisation between this 
hypothetical core and sheath. The greater the polarisation coeffi- 
cient is, the more widely do these branches spread, the stronger are 
the electrotonic currents. If stimulation diminishes this polarisa- 
tion coefficient it will, in general, diminish the electrotonic currents. 
If the excitation be confined to special parts of the nerve, then it 
will depend upon the ratio of the transition resistance between core 
and sheath to the longitudinal resistance of the nerve, and upon the 
magnitude and position of the unexcited or relatively unexcited 
parts, whether the electrotonic variation (as we may for shortness 
call the variation of the electrotonic currents produced by stimula- 
tion) will be negative or positive. All this he deduces from his 
theory, and supports by experiments with the “ Kernleiter Modell,” 
He looked, in vain, however, for a positive phase in his rheotome 
work on nerve, the experimental difficulties being very great. 
It was not from Hermann’s theoretical standpoint that I entered 
* Strictly speaking, if E(a), E(b>, represent the intensity of excitation at A 
and B, fE^dt is >J‘E(B)dt for corresponding limits. Considering time- 
integrals, B may, therefore, he looked on as positive to A during the tetanus. 
The galvanometer deflection produced by stimulation will be a measure of the 
difference of these integrals. 
