Space distribution of excitability in the frog's sciatic nerve stimulated by slot electrodes

The foregoing paper reached the conclusion that when a nerve is excited by slot electrodes, the impulse arises, not at the cathode, but 2 or 3 mm. away on the cathodal side. Fig. 2 (a) of that paper showed a curve representing the distribution of excitability about the electrodes which may be called the 'slot excitability curve ',. but the form was largely conjectural. The only point clearly established was that the maximum of the curve was some 3 mm. distant from the manifest cathode. The first object of this paper is to determine experimentally the whole slot excitability curve, and the second, to apply; the curve so obtained to predict verifiable excitability results. In Parts I and II two different methods are described by which the slot excitability curve may be found, each strengthening some weak points in the other. In Part III the results are used to predict both the threshold and the latency of each point on the family of curves partly shown in Fig. 8 of the previous paper. It is of course impossible to derive any of these conclusions from excitability measurements' alone without assuming some guiding principle in their' interpretation. The fact that Pig. 6 of the previous Ipaper shows a linear relation at all seems to justify the'ass'umption that excitability (=reciprocal threshold) at a point is an additive quantity. This assumption, which has been accepted by most' workers in the past, forms the basis of the present analysis. For the sake of concreteness the principal assumptions will now be set' out in a form rather more definite and restricted than is necessary to justify the conclusions to be drawn from them. Their suibstantiation or modification will be postponed till the Discussion section at the end of the paper so that the bearing upon the results and conclusions may be appreciated.