x ELECTROMOTIVE ACTION IN NERVE 331 



" In non-medullated nerve, e.g. olfactorius, and in muscle, 

 where the excitable substance, unlike medullated nerve, has no 

 imperfectly -conducting sheath, the characteristic diffusion of 

 entrance and exit points is wanting. The electrical phenomena 

 which depend upon this diffusion (due, in the first place, to rela- 

 tions of conductivity), together with the physiological local con- 

 sequences of the same, are accordingly absent. On the other 

 hand, the phenomena caused by transmission of the ascending or 

 descending alteration induced at the anode or kathode of the 

 foreign current are more or less plainly exhibited both in non- 

 niedullated nerve and in muscle-fibre. 



" If a tract of nerve has been traversed for some time by a 

 foreign current, and the current is then reversed, the excitable 

 matter at the point of exit (i.e. former point of entrance) will be 

 absolutely, or relatively, above par, and thus has a greater dis- 

 position to ' down ' change ; the current accordingly produces a 

 more rapid descending alteration than would otherwise be the 

 case (Volta's alternative). 



" Muscle-fibre, as compared with nerve-fibre, has the great 

 advantage of expressing the excitation due to descending altera- 

 tion, by change of form of the part affected ; while a foreign current 

 can, moreover, enter and leave at the natural ends of the fibres. 

 In the latter case, the allonomous change which occurs on closure 

 at the point of exit is, in the first instance, transmitted along the 

 fibre, but when the closure twitch has expired, it persists only 

 near the point of exit during closure (persistent kathodic con- 

 traction), and steadily decreases. Meantime, the autonomous up 

 change continues at the point of entrance, and may raise the 

 living matter considerably above par, given adequate strength and 

 duration of current. At break there will accordingly be an 

 autonomous down change, which, if sufficiently rapid, may pro- 

 duce an opening twitch, or persistent opening contraction, near the 

 point of entrance. Even when this autonomous down change is 

 so weak that no visible alteration of form can be detected in the 

 muscle, it may express itself in the physiological homodromous 

 current (supra), which appears on connecting the anodic end with, 

 e.g., the centre of the muscle. 



" Autonomous ascending alteration cannot always be demon- 

 strated at the point of exit, on breaking the internal current, 

 because the autonomous assimilation of the living matter in excised 



