268 ON POSITIVE VARIATION OF THE NERVE CURRENT 



hand, and of the multiplier- or galvanometer-circuit on the other 

 hand, it is not possible to exclude unipolar action, especially with 

 break-induction currents. Unless these are extremely weak, elec- 

 tricity flows appreciably through the intermediate tract into the 

 galvanometer-circuit. Now, seeing that an electrical current excites 

 a nerve especially where it enters or leaves the excitable sub- 

 stance of that nerve, the stimulation is no longer limited to that 

 portion of the nerve to which the stimulating electrodes are applied, 

 but a second direct stimulation occurs in the neighbourhood of 

 the galvanometer electrodes. This unipolar action is greater, the 

 greater the portion of nerve between the stimulating electrodes, 

 and the less it is moistened. I therefore took care in my ex- 

 periments that with stimulations with make and break induction 

 currents, the intermediate tract should not be less than 25 mm., 

 and that the stimulated tract should not be more than 5 mm. ; 

 and although numerous test experiments showed that even with- 

 in these limits, when the coils were more than 13 cm. apart, 

 unipolar action never took place, so long as the primary coil was 

 supplied by I Daniell only, yet I nearly always stimulated with 

 the coils at a distance of 20 cm. apart. If, as an exception, the 

 coils were brought nearer together, a test experiment was always 

 made at the end by crushing the nerve in the intermediate tract. 

 When the stimulated tract was smaller and well-moistened, a much 

 nearer approach of the two coils was possible without noticeable 

 unipolar action. If the end of the nerve is not introduced into the 

 galvanometer-circuit, but is in connection with the well-isolated 

 muscle, and the above-mentioned conditions are fulfilled, there is 

 not the least occasion to fear unipolar action. 



The alterations in the course of the variation of the nerve current, 

 which are brought about in consequence of the application of stronger 

 currents and of the concomitant intermixture of unipolar action, as 

 far as I have yet observed, are in substance the following : 



The positive after- variation is weaker, and when unipolar action 

 becomes to a certain degree considerable, is quite lost. If unipolar 

 action still increases, the magnet falls further short of its original 

 deflection ; or if it has been compensated, does not return to the zero 

 point after cessation of stimulation. Hence the weakening of 

 nerve current, which du Bois-Reymond regarded as the rule. 

 By degrees, however, this weakening passes away, occupying 

 several minutes in so doing if considerable, but only a few seconds 

 if very small. Again, if excitations are repeated at short intervals 



