ELECTROMOTIVE ACTION IN NERVE 265 



the intrinsic arid suddenly short-circuited current of this nerve " 

 (Hering). 



If under these conditions excitability is heightened as much 

 as possible by making a new transverse section simultaneously 

 at the peripheral end of the primary and central end of the 

 juxtaposed secondary nerve with a scissors-cut, and then weakly 

 tetanising the central end of the primary nerve, Hering invariably 

 noted a weak tetanic disturbance of the secondary preparation. 

 Current escape and unipolar stimulation were excluded, since the 

 weak exciting currents only took effect when the electrodes were 

 placed near the transverse section, all secondary action failing 

 when they were applied to other points of the primary nerve 

 nearer the second preparation. Electrotonic action is excluded 

 by the great distance between the point of stimulation and the 

 position, of the secondary nerve, so that the possibility of true 

 secondary excitation from nerve to nerve was no longer doubtful. 

 Obviously, the result would be even less ambiguous if the tedious 

 process of applying the two nerves together could be replaced by 

 a preparation in which the bundles of nerve-fibres serving as 

 primary and secondary nerves should lie naturally in a common 

 sheath. Hering accordingly, in a cooled frog, exposed the sciatic 

 nerve above the knee, ligatured its two branches together, divided 

 them below the ligature, dissected out the nerve to the place 

 where the branch is given off to the thigh, and then divided the 

 sciatic plexus, and (when the muscles were quiet again) stimulated 

 the knee-end of the nerve with weak currents. The muscles of 

 which the nerves were still in connection with the plexus fell at 

 once into strong secondary tetanus. The experiment never fails, 

 provided the preparation be so excitable that the division of the 

 sciatic plexus produces a slight muscular disturbance in the leg, 

 in addition to the twitch, and that there is a fresh transverse 

 section. The proof that this is not due to current-escape, 

 electrotonus, or unipolar excitation, again lies in the fact that there 

 is regularly no effect so soon as the electrodes are moved slightly 

 away from the cross-section, and approximated to the muscle. 

 In three preparations, moreover (and in two cases, twice or three 

 times consecutively), a weak partial twitch of one of the muscles 

 of the thigh was observed by Hering on crushing the primary 

 nerve. This, together with the inevitable failure of other kinds 

 of stimuli, is hardly surprising, in view of the difficulties (supra] 



