558 NERVE. 



is one that produces electrolysis ? The only absolutely reliable evi- 

 dence of excitation is the production of an undoubted response in an 

 attached muscle or other recipient organ ; the only reliable evidence of 

 electrolysis is the presence of the electromotive state which we term 

 polarisation. That either of these may exist apart from such evidence is 

 by no means improbable ; but the present inquiry may be advantageously 

 limited to the indications above mentioned. With this limitation, it will 

 be seen that there is little difficulty in the assumption that, when a 

 current is able to excite, it must have produced polarisation ; on the 

 other hand, there are serious objections to the second assumption, 

 namely, that if a current produces polarisation of sufficient intensity, 

 such production must, in an excitable structure, lead to excitation. 



It is remarkable what a large number of conditions influence both 

 exciting efficiency and the production of polarisation in a similar 

 manner. The intensity of the current, its duration, the length of the 

 intrapolar region, are all factors the variation of which influence both 

 effects in the same way ; and most conditions which increase the rapidity 

 of polarisation development are, cceteris paribus, those which increase 

 exciting value. Finally, the nerves most susceptible to polarisation 

 effects (e.g. medullated nerves of the frog) are those most readily 

 stimulated by electrical currents. 



Against the second assumption is the well-known fact, that currents 

 directed across the fibres of either nerves or muscle are unable to excite, 

 yet, owing to the multiple anodes and cathodes between the poles (there 

 being a pair for each fibre), the polarisation is extremely pronounced. 

 The seat of electrolytic change in nerve must be at the surface junction 

 of each medullated axis cylinder with its envelope, and the counter after 

 effect is a clear indication, both of its existence and of the separation 

 of the two groups of products. Polarisation may thus be produced in 

 large amount, and yet all evidence of excitation may completely fail. 

 A still greater difficulty is the behaviour of nerve when subjected to 

 local change of temperature ; local cold increases nerve susceptibility to 

 excitation by galvanic currents and condenser discharges ; it decreases 

 such susceptibility for make or break induced currents. Local cooling 

 undoubtedly modifies polarisation, but it is difficult to frame any 

 hypothetical link between excitation and polarisation which may 

 account, on the one hand, for the increased exciting efficiency of pro- 

 longed condenser discharges as the temperature falls, and, on the other, 

 for the increased exciting efficiency of rapid condenser discharges as 

 the temperature rises. Finally, the fact that excitation can itself 

 produce electromotive changes of sufficient magnitude to overpower all 

 evidence of polarisation, as in the anodic opening after effect, appears 

 to indicate that the two conditions cannot be connected so intimately 

 that the one is an essential concomitant of the other. 



Some of these difficulties have been recognised by Bernstein, who, 

 anxious to bring excitation and polarisation into complete accord, framed 

 an electro-chemical hypothesis to explain the most conspicuous of the 

 difficulties. 1 He supposes that the internal conductor is in reality a 

 discontinuous one, and that electrolytic change takes place botli on the 

 surface and in the intervals between the portions of such an interrupted 

 core. Surface polarisation alone does not constitute excitation ; it is 



1 Bernstein, Untersuch. a. d. physiol. Inst. d. Univ. Halle, 1888 (1); " Lehrbuch d. 

 Physiol.," 1894, pp. 359, 450. 



