x ELECTROMOTIVE ACTION IN NERVE 309 



ficatiou, the physiological action of one pole may, as in muscle, be 

 excluded, if a greater or less part of the iutrapolar tract is killed, 

 with as little injury as possible to histological structure. This, 

 again, is simply explained by the spatial distribution of the points 

 by which current leaves and enters, as well as the difference 

 already pointed out between abterminal and atterminal induction 

 currents, sent into the transverse end of a medullated nerve. The 

 E.M.F. at the boundary of " altered " and non-altered nerve-sub- 

 stance is presumably very high, since there is a marked action 

 from the deriving currents led off externally ; so that the intensity 

 of the branches of current, which are short-circuited in the 

 proximity of an artificial transverse section of medullated nerve, 

 by the substance of the sheath, must undoubtedly be very great, 

 owing to the low resistance of their microscopic longitudinal path 

 (Hermann). 



VI. SECONDARY ELECTROMOTIVE CHANGES IN NERVE FOLLOWING 

 THE PASSAGE OF A CURRENT 



As in muscle, so in medullated nerve, it was shown by du 

 Bois-lieymond that every part traversed by a current of adequate 

 strength exhibited regular electromotive action in a given direction 

 when the circuit was broken. He referred the phenomena in 

 both cases to " internal polarisation," since it appeared that an 

 opposite (eventually hoinodromous) after -current could also be 

 observed when the two leading-off electrodes to the galvanometer 

 were situated between the exciting electrodes, within the intra- 

 polar region. It has already been shown (I. p. 444) that this view 

 is erroneous, at least as regards muscle. In the case of nerve 

 the investigation is much more difficult, owing to the smaller 

 intensity of effect, and still more from the electrotonic diffusion 

 of the (polarising) exciting current. Nevertheless, it may be 

 affirmed from all the observations made up to the present time 

 that no real difference exists in regard to secondary electromotive 

 phenomena, between nerve and muscle. Du Bois-Eeymond 

 obtained maximal negative effects after prolonged passage of com- 

 paratively weak currents, while maximal " positive polarisation " 

 occurred after a brief closure of a strong battery (2530 Groves !), 

 (43). Hermann, who at first found no fundamental difference in 

 the deflections, on sending current through a tract of nerve 40 



