1877.] 



On the Excitability of Motor Nerves. 



211 



I. ct Further Observations on the Modification of the Excitability 

 of Motor Nerves produced by Injury. v By George J. 

 Romanes, M.A., F.L.S. Communicated by Prof. Burdon 

 Sanderson, M.D., LL.D., F.R.S. Received April 11, 1877. 



In my former paper on this subject* I showed, among other things, 

 that injury of a motor nerve is attended with a very marked and peculiar 

 alteration in its relative excitability towards the stimuli which are 

 respectively supplied by closing and opening a voltaic circuit. In the 

 present paper I propose to detail the results which have been obtained 

 by continuing this hue of research ; and, in order to render them more 

 easily intelligible, I shall begin by briefly restating such of the previous 

 results as form the basis of the present ones. 



It will be remembered, then, that my method of experimenting was as 

 follows. Having pithed a frog and laid it on a frog-board in such a 

 position that one of the hind legs should hang over the edge of the board, 

 I divided the tendo Achillis, dissected out the gastrocnemius as far as its 

 point of origin, and removed the tibia just below the knee. The exposed 

 though uninjured gastrocnemius was then laid with its flat surface on 

 non-polarizable electrodes, in such a way that while one electrode supported 

 the extreme tarsal end of the muscle, the other supported its extreme 

 femoral end. By means of a rheochord it was then ascertained what 

 strength of voltaic stimulus the muscle required to give its earliest 

 response, («) to the anodic t make, (6) to the kathodic make, (c) to the 

 anodic break, and (d) to the kathodic break. It will be remembered that, 

 under these circumstances, " the muscle is usually more sensitive to 

 minimal stimulation supplied by closure of the constant current when the 

 femoral end rests on the kathode [case (6)], than when this end rests on 

 the anode [case (a)]. Conversely, under similar circumstances, the 

 gastrocnemius is more sensitive to minimal stimulation supplied by 

 opening of the constant current when the femoral end rests on the anode 

 [case (c)], than when this end rests on the kathode [case (c?)]. In view of 

 the other facts of electrotonus the present ones are of interest, because, 

 as the sciatic nerve enters the gastrocnemius near the femoral end of the 

 latter and then spreads out its peripheral ramifications as it advances, 

 in the experiments just mentioned one electrode is in almost immediate 

 contact with the nerve-trunk where it enters the muscle, while the other 

 electrode supports the part of the muscle that contains only peripheral 



* Proc. Boy. Soc, May 4, 1876. 



t Throughout this paper I shall designate the direction of the voltaic current 

 through the gastrocnemius by employing the terms "anodic" and "kathodic" with 

 reference to the femoral end of the muscle, i. e. where the nerve-trunk first enters the 

 latter. Thus, for instance, " anodic make " means closure of the current in a direction 

 descending from the femoral' to the tarsal end of the gastrocnemius. 



Q2 



