312 ELECTRO-PHYSIOLOGY CHAP. 



than when it is absent. Hermann has communicated similar 

 experiments which he carried out in 1875-76, with the same 

 result, but did not publish. 



From these facts it appears that the polarising heterodromous 

 current is, under the given conditions, implicated in the discharge 

 of the opening twitch, although we must by no means conclude 

 that it is invariably the sole factor. This conclusion seems, how- 

 ever, to Griitzner and Tigerstedt to be justified, mainly by the fact 

 that all those circumstances which are favourable to the appear- 

 ance, or increase, of a negative polarisation current are also con- 

 ducive to the appearance of the break twitch. 



The normal, vigorous, and uninjured nerve is characterised (as 

 was remarked above) by a certain resistance to excitation from the 

 break of an electrical current, so that tolerably strong battery 

 currents are required to discharge opening twitches after a brief 

 closure. When, however, a break twitch has once been dis- 

 charged by an adequate current, then even weak currents (that 

 previously took effect at make only) will excite directly after- 

 wards, provided that in both cases the same tract of nerve 

 is traversed by the current. After a brief period of rest this 

 excitatory effect disappears again completely. Griitzner and 

 Tigerstedt interpret this reaction to mean that the negative polar- 

 isation current, set up by the stronger current in the tract 

 traversed, and gradually subsiding at break of the exciting 

 current, disposes this tract during its passage to the discharge 

 of " false " opening twitches, in which case the short-circuiting 

 of the polarisation current can only (with the normal method of 

 opening the exciting current) be internal, occurring within the 

 nerve itself. 



Tigerstedt arrived at the following results from his investiga- 

 tion of the time-relations of negative polarisation in frog's nerve, 

 as well as its dependence upon intensity and duration of the 

 exciting current : 



(i.) Within certain limits of current intensity the (negative) 

 polarisation of the nerve is directly proportional to the strength 

 of the exciting current. 



(ii.) If the polarising current acts upon the nerve during an 

 indefinite period, polarisation increases ; it rises quickly at the 

 beginning, then more slowly, reaching its maximum with extreme 

 sluggishness. 



