ELECTROMOTIVE ACTION IX NERVE 269 



a high resistance (tube of alcohol) into the polarising circuit, 

 or ligaturing the intrapolar tract with a moist thread, the 

 magnitude of electrotonic increment increases with the extension 

 of the electromotive tract traversed by the current, or vice versa 

 diminishes with its restriction. The direction of the polarising 

 current in relation to the lony axis of the nerve has, moreover, a 

 great effect upon the intensity of electrotonic action, and the 

 increment, like the excitation, is found to be greatest when the 

 polarising current flows longitudinally through the nerve nil 

 with transverse passage of current. 



As regards the theory of electrotonic action, its dependence 

 upon the constitution and state of the nerve is of the first im- 

 portance. The conjecture that it is due to ordinary current- 

 escape in the galvanometer circuit is at first sight plausible, in 

 view of the entire reaction, but is at once refuted by the fact 

 that dividing, or crushing of the nerve, between the polarised 

 and leading-off tracts, abolishes all sign of excitation. This proves 

 that the diffusion of electrotonus, as of excitation, is correlated 

 with uninterrupted continuity in medullated nerve. And it is 

 not merely the complete interruption of conductivity, but every 

 modification of it, or of excitability (Leistungsfahigkeit) in the 

 nerve, that affects the magnitude of electrotonus in a greater 

 or less degree. No electrotonic action, or at most only a trace 

 of the ordinary effects, can be detected on dead nerve, or nerve 

 that is fundamentally altered in its physical and chemical 

 properties. The entire manifestation is indisputably bound up 

 with certain structural peculiarities that are present only in living, 

 uninjured medullated nerve. The utmost importance attaches to 

 the fact (to be discussed below) that under uniform conditions 

 the electrotonic incremental current, in the above sense, does 

 not appear either in non- medullated nerve, or in muscle, or 

 other moist conductors (wet threads), so that the presence of a 

 medullated sheath seems to rank first among the necessary structural 

 requirements. 



The fact that division of the nerve between polarised and 

 led -off tracts prevents the development of electrotonus, even 

 when the cut surfaces are replaced as carefully as possible, leads 

 on to the question whether the underlying alterations are, 

 like excitation, transmitted at measurable velocity in the nerve. 

 After du Bois-Eeymond had shown that the development of 



