ELECTROMOTIVE ACTION IN NERVE 237 



in the part near the transverse section (as was pointed out 

 more particularly by Hermann). If the leading-off electrodes 

 serve at the same time for excitation, i.e. if they lead an artificial 

 current into the nerve, this will either be homodromous or hetero- 

 dromous with the current already present in the entire system, 

 the former when the anode is proximal to the cross-section. 

 Other conditions being uniform, the closure of a current homo- 

 dromous with the intrinsic current increases the excitation, so 

 that it is intelligible that a descending current in the vicinity of 

 the cross-section should prove more effective than an ascending 

 current. In view of the previous argument, it is clear that the 

 interference effects between exciting current and nerve current 

 are due, strictly speaking, not to addition and subtraction of 

 these currents (Griinhagen remarks that an exciting current 

 increased or diminished by the sum of the nerve current exhibits 

 no appreciable alteration of its physiological action), but to 

 polarisation effects discharged at points at which the disposition 

 to response has been altered in one direction or the other by 

 the internal neutralisation of the nerve current. 



If actively electromotive (negative) points occur in the con- 

 tinuity of the undivided nerve, the same considerations of course 

 apply to them. Griitzner (14), indeed, inclines to refer all 

 changes of excitability in the continuity of the otherwise un- 

 injured nerve (in particular the dissimilarity of action with equal 

 but opposite currents in different parts of the same nerve, as 

 described by Hermann and Fleischl) to differences of potential 

 induced by preparation. 



If a frog's sciatic is tested with unpolarisable electrodes 

 5-8 mm. apart, there is regularly, according to Griitzner, a 

 descending current below the point at which the branch to the 

 thigh is given off, an ascending current, on the contrary, above the 

 gastrocnemius. About midway between pelvis and knee there is 

 a point at which no current can be led off to the galvanometer 

 (Fleischl's " equator "). There is no doubt that the P.D. is caused 

 by the side branches given off from the main stem. When these 

 are as far as possible uninjured, the currents are very weak. 

 Each point of bifurcation in the nerve is thus, as it were, pre- 

 destined for the appearance of a P.D., since it presents a suitable 

 point of attack to injuries of all kinds. 



" Wherever the intrinsic currents of the nerve are descending. 



