x ELECTROMOTIVE ACTION" IN NERVE 253 



thermal stimuli affect centripetal more than centrifugal nerves 

 must therefore, as concerns the magnitude of the negative varia- 

 tion, remain undecided. Nor did Griitzner elicit negative 

 variation from mechanical stimuli (e.g. scissors-cut) at any distance 

 from the seat of excitation. Only when the incision came 

 within 10 mm. of the longitudinal electrode was there a slight, 

 and that a permanent diminutioji of the current. On the 

 other hand, in dividing the non-medullated olfactory nerve of 

 the pike, Hering (24) found not merely a strong negative varia- 

 tion, but a distinct positive after-effect also, and analogous results 

 are exhibited by non-medullated molluscan nerve (Biedermann). 

 Steinach (21) has recently succeeded in proving that in suitable 

 frogs' nerves (e.g. from cooled animals) each single cut will, under 

 certain conditions, produce a marked negative variation, the time- v/ 

 distribution of which corresponds as a rule with that found in 

 electrical stimulation. The mirror swings back rapidly, and reaches 

 the point of rest again more slowly. This is obviously in line 

 with the slow subsidence of the persistent excitation, as expressed 

 in the inclination of a muscle to tetanus, when its nerve is excited 

 by the constant current, or by short-circuiting of its own current. 

 Boruttau (p. 31) also noted positive results in frog nerve 

 with mechanical excitation, both with simple division and in 

 mechanical tetanus. 



With chemical excitation by Nad, Griitzner observed a 

 gradual diminution of current, while Kiihne and Steiner (2) 

 obtained a negative variation of the demarcation current on the 

 pike's non-medullated olfactory nerve, under the same conditions. 

 Whether the poverty of effect is due solely to asynchronous 

 excitation of the separate fibres of the nerve-trunk (Griitzner), 

 or to other factors also, is uncertain. 



After cutting off' the part of the nerve that had been excited, 

 or bathing it in physiological saline, Steinach found that the 

 diminution of the demarcation current due to chemical stimula- 

 tion was completely neutralised. Alcohol proved the most 

 effective excitant, and here again there was a marked difference 

 between cooled and warmed frogs. With the former, immersion 

 of the central end of the nerve produced tetanus of the flexors, 

 succeeded by a vigorous extensor-tetanus, while, under similar 

 conditions, a nerve-muscle preparation from a warmed frog gave 

 only a few twitches, and was then quiescent. 



