146 ELECTRO-PHYSIOLOGY CHAP. 



resistance of the circuit. If the effect of length of tract upon 

 excitation, or upon electromotive alterations of excitability, is to 

 be studied, care must be taken, in view of the great resistance 

 of nerve, that the total resistance is not unduly altered with the 

 enlargement of area traversed. Du Bois-Eeymond accomplished 

 this by introducing an alcohol rheostat as resistance, against 

 which the resistance of the nerve-tract is practically minimal. 

 He then found that the extrapolar electrotonus (i.e. its galvanic 

 manifestation), and the negative variation as the expression of 

 excitation, were more strongly developed when the intrapolar 

 tract was lengthened. Pfliiger subsequently reached the same 

 conclusions. 



Willy (22) then tested the different magnitudes of twitch 

 under uniform .conditions. He employed two nerves, traversed 

 one in longer, the other in shorter portions by current. 

 Stronger excitation of the longer portion was found to be 

 effective with the closure of descending currents only, while 

 closure of ascending currents gave the contrary effect. Willy 

 formulated his observations as follows : " Excitability is cceteris 

 paribus the stronger, in proportion as the muscle is nearer the 

 kathode, and farther from the anode." 



Marcuse (22) investigated the same problem under Pick's 

 direction, the nerve being placed in a small parallel-epipedic 

 glass trough filled with physiological saline. Its opposite walls 

 were made of amalgamated zinc, through which an induced 

 current was led in. As a longer or shorter portion of the nerve 

 was bathed, it was traversed by the current at constant density ; 

 with increasing length the minimal, just effective strength of 

 current diminished, at first rapidly and then more slowly, 

 " appearing to approach a limit asymptotically, or after passing 

 a minimum to increase again." With the constant current also, 

 Marcuse found a beneficial effect from a longer intrapolar tract, 

 with both ascending and descending direction of current, since 

 the first perceptible twitch made its appearance earlier than in 

 a shorter tract. Tschirjew (16) and Clara Halperson (23) 

 arrived at much the same results. 



The time-development of all the modifications in the nervous 

 substance which are characteristic of electrotonus, including the 

 above alterations of excitability, will be discussed at a later period. 

 Pfliiger places the katelectrotonic increase of excitability im- 



