x ELECTROMOTIVE ACTION IN NERVE 287 



appearance of electrotonic action in the ordinary sense are most 

 unfavourable, and any negative variation of the nerve current 

 observed under such conditions may presumably be referred to 

 persistent excitation from the kathode. Other data are also of 

 great importance, theoretically, to this point. 



In the first place, the magnitude of the initial, deflection is 

 independent both of intensity of exciting current, and also, within 

 certain limits, of length of intermediate tract. The maximum 

 effect usually appears with very weak currents, and it is 

 immaterial whether a fraction of the current from a single cell or 

 the full current of several cells is employed as excitant the 

 effect may indeed be less in the last case than with weaker 

 currents. Nor, with uniform intensity of current, can the effect 

 be increased by bringing the exciting electrodes nearer to the 

 galvanometer tract (up to a certain limit). If, on the other 

 hand, the intermediate tract is shortened by gradually shifting 

 the galvanometer electrodes away from the transverse end of the 

 nerve, with unaltered position of the exciting electrodes, a 

 diminution of the negative variation is regularly observed at first 

 with descending direction of current, amounting under some con- 

 ditions to its complete disappearance (cf. Table I. infra). 



If the galvanometer electrodes are approximated to the kathode 

 beyond a certain point, a new series of homodromous (negative) 

 deflections will appear, which, both as regards character during 

 passage of current, and intensity, are quite distinct from the first 

 series, and in all respects exhibit the same characteristics as those 

 generally accepted as the signs of the electrotonic incremental 

 current. These are, in first degree, dependence on strength of 

 exciting current, and strikingly rapid increase of effect with 

 approximation to the exciting tract. During the closure of the 

 excitatory circuit these deflections either remain constant, or 

 exhibit a gradual diminution, never amounting to disappearance. 



The following tables illustrate this reaction. Both sciatic 

 nerves of a very sensitive (cold) frog (R. esculenta) were simultane- 

 ously excited at their central end. NS= magnitude of deflection 

 produced by the demarcation current ; E W, the rheochord 

 resistance ; ZS, the length of the intermediate tract ; SR, the 

 direction of current. The sign > indicates the diminution of the 

 deflection during passage of the current. 



Much stronger negative deflections have frequently been 



