236 Proceedings of Royal Society of Edinburgh. [sess. 
upon the work of which this paper is an account. But from certain 
experiments on the effect of stimulation on the intrapolar current 
during the flow, and on both extra and intrapolar currents after 
the opening of the polarising stream, I suspected that, if one of the 
galvanometer electrodes were placed very near the polarising 
circuit, and the strength of the current increased sufficiently, a 
positive electrotonic variation ought to appear on the side of the 
anode, but not on that of the cathode. For the explanation of those 
experiments it was assumed, and the assumption was supported by 
direct experiments on muscular contraction, that during the flow 
of the polarising current the conductivity of the nerve for the 
excitatory change is less around the cathode than around the anode, 
and that, with increasing strength of current, complete block occurs 
sooner at the former than at the latter, although eventually it 
prevails at both. Whether, when this last stage is reached, the 
whole intrapolar area has lost its conductivity, was left an open 
question, and need not be considered here. 
Going back now to fig. 1, let us inquire what the effect would be 
on the side of the anode, i.e ., with descending current, at a time 
when complete block was established there, and at the same time 
let us suppose that the galvanometer circuit is brought quite close 
to the anode, so that the lower galvanometer electrode is within the 
non-conducting region. If stimulation be now made at I, the 
excitation will pass A with a certain intensity, but will altogether 
fail before reaching B. B will, therefore, be strongly positive to 
A. We leave out of account for the moment any possible effect of 
the excitation on the electrotonic currents as such. There will be 
a current of action developed in the descending direction through 
the nerve — that is, in the same direction as the anodic electrotonic 
current. If this true action current be not masked by an over- 
whelming negative electrotonic variation, it will appear as a positive 
variation of the electrotonic current. 
Now let us take the case of the ascending current in fig. 1, 
Here the lower galvanometer electrode is in the cathodic region, 
and we know that even with comparatively weak currents the 
cathodic block appears. B will therefore, above a low limit of current 
density, be positive to A when the nerve is excited, and the true 
action stream will be descending. The cathodic electrotonic current, 
