ix ELECTRICAL EXCITATION OF NERVE 219 



IX. UNIPOLAR EXCITATION 



Under certain conditions, excitation with induced currents 

 brings to light phenomena which are not merely of theoretical 

 interest, but have great practical value in all experimental observa- 

 tions. Among these is the so-called unipolar excitation by in- 

 duction, first noted and worked out by du Bois-Eeymond (I.e. p. 

 423). The facts underlying the entire subject are as follows : " If 

 the nerve of a rheoscopic leg is connected with one end of an open 

 induction circuit, either the leg or the other terminal of the circuit 

 being led off to earth, a twitch occurs each time that an excitatory 

 process is set up near the circuit, sufficient to have produced a 

 secondar}^ current in the circuit if it had been closed" (I.e. p. 

 429). This occurs even with complete insulation of the prepara- 

 tion, and also, at a given (short) distance of the coils, when there is 

 no lead-off to earth (by touching the preparation, or connecting the 

 other free pole with the ground). The excitation fails when 

 the metallic end of the induction circuit is led off by contact above 

 or below the point on which the nerve rests, or when, with the 

 nerve hanging freely, contact is made with the preparation, and 

 the muscles are led off by touching them. In the first case, 

 ligaturing or crushing the nerve does not inhibit the excitation, 

 seeing that the nerve is traversed by electricity in its entire length, 

 inclusive of the crushed part. Pfliiger (2, pp. 57, 121, 410) found 

 that break shocks were markedly the most effective explained by 

 du Bois-Eeymond as due to delay in charging the secondary coil, 

 by the development of the extra current. 



In order to produce unipolar excitation, it is not necessary to 

 lead off from one pole to an infinite conductor (such as the earth). 

 The effect on the contrary appears, as first pointed out by Pfliiger 

 (I.e. p. 128 f.), even when the led-off pole is in contact with a 

 comparatively small surface, the more so in proportion as the 

 P.D. arising from the E.M.F. due to induction is higher. "The 

 degree of unipolar action increases rapidly with the magnitude 

 of the lead-off, unipolar stimulation being greater at any given 

 point, with a restricted leading-off surface, in proportion as this 

 point lies nearer the seat of unipolar action at the metal pole." 

 Pfliiger placed a row of frogs' legs (4-6) upon a glass plate, after 

 insulating all the apparatus as carefully as possible, so that only 

 the nerve of the first touched the (single) metal pole, that of the 



