ix ELECTRICAL EXCITATION OF XERVK 223 



the interpolar tract, a flowing off in the complex of conductors 

 which forms the galvanometer circuit, and which must (however 

 well insulated) be connected through the nerve with the secondary 

 coil. Hering further showed by special experiments that the 

 sudden charges and discharges in the galvanometer circuit, caused 

 under all conditions by the interpolar flow of electricity, do 

 not as a rule produce any deflection of the magnet. 



The following experiment demonstrates plainly in what 

 degree the connection of the nerve with the galvanometer circuit 

 is responsible for unipolar excitation. " A sciatic nerve, still in con- 

 nection with the leg, was laid across the exciting and galvanometer 

 electrodes just as in the determination of the negative variation 

 (exciting tract = 5 mm., intermediate tract =2 5-30 mm., gal- 

 vanometer tract = 6-8 mm.). The stump of the thigh-bone was 

 fixed in a paraffin clamp with a corresponding bore, so that the 

 leg is as far as possible insulated. After repeated crushing of 

 the intermediate tract, the preparation was stimulated, and the 

 secondary coil gradually pushed up. Unipolar action began at 

 2025 cm. distance of coil, and the muscle went into tetanus 

 even if one galvanometer electrode only was in contact with the 

 nerve. On taking both galvanometer electrodes off the nerve, 

 the muscle remained quiescent " (Hering). The difference 

 between this and ordinary unipolar excitation is that electricity 

 here does not flow over to the muscle, but passes through the 

 galvanometer electrodes into the galvanometer circuit, and thereby 

 excites the nerve, partly below the crushed point, partly at the 

 seat of the galvanometer electrodes (more particularly at the long 

 section). 



This kind of unipolar stimulation is an obvious danger in 

 all experiments on action currents and negative variation in 

 nerve, while it shows what narrow bounds restrict the intensities 

 of current that may be safely used in these experiments. 



BIBLIOGRAPHY 



1. Du BOIS-REYMOXD. Unters. lib. thier. Elektr. I. 



2. E. PFLUGER. Electrotonus. p. 446. 



fE. HERING. W. S.-B. LXXXV. 3. Abth. 1882. p. 1. 

 lv. FREY. Du Bois' Arch. 1883. p. 43. 



4. EXGELMANX. Pfliiger's Arch. III. 1870. p. 403. 



5. C. ECKHARDT. Beitriige. XII. 



6. P. GRUTZXER. P. A. XVII. p. 238. 



