POLAR EXCITATION OF NERVE. 505 



Apart from such variations in the general excitability of the nerve, 

 the effectiveness of these polar excitatory phenomena is dependent upon 

 the intensity of the polar change itself. This is mainly determined by 

 four factors : intensity of current, direction of current, length of nerve 

 subjected to the influence of the current, duration of current. 



These have all been briefly referred to in the section dealing with 

 modes of excitation. It has been already stated that a current loses 

 its exciting value if the direction of flow is across the nerve fibres ; this 

 is not due to the increased resistance, nor to the failure of polarisation, 

 since polarisation, owing to the multiple anodes and cathodes (each nerve 

 fibre traversed having its own set), is more prominent than when an 

 equal length of nerve is subjected to a longitudinally directed current 

 flow. It must therefore be due to the interference of the two nerve 

 disturbances, the cathodic lessened stability and the anodic augmented 

 stability of the nerve equilibrium so balancing each other on the 

 current closure that excitation (i.e. sudden lessened stability) becomes 

 impracticable. 



The local separation of these two states is thus an important factor 

 in determining efficient polar excitation. It is for this reason that a 

 nerve is excited more effectually the greater the length which is sub- 

 jected to the longitudinal flow. In order to ascertain the fact, it is 

 essential to eliminate the effect of increased electrical resistance, and 

 the consequent diminution in current intensity produced by the greater 

 length of nerve through which the current is led. The elimination may 

 be approximately effected by using a very large external resistance, in 

 comparison with which the alteration in nerve resistance becomes a 

 negligible quantity. In this way du Bois-Eeymond showed that the 

 extrapolar electromotive changes associated with electrotonus are 

 augmented by increase in the length of nerve between the poles. 1 



Marcuse placed the nerve in a trough of physiological NaCl solution, 

 arranged so that a current could be led through it; alteration in 

 the length of nerve dipping in the trough now makes no appreciable 

 difference in resistance.' 2 He found that the minimal exciting current 

 was of less intensity in proportion as the length of nerve traversed was 

 increased ; beyond a certain limit, the favourable influence of polar 

 separation became gradually diminished, and disappeared. The result 

 was the same whether the direction of current flow was ascending or 

 descending. The simple method described in connection with excitability 

 (see Fig. 260) also affords a ready means of ascertaining the same fact. 

 A more remarkable influence is that afforded by multipolar contacts, 

 first suggested by Setschenow, 3 and employed in a modified form by 

 Danilewsky. 4 



The arrangement for such contacts consists in placing upon the nerve three 

 electrodes (A, B, C), at equal distances as in Fig. 264 ; these are so connected 

 with the electromotive source that the current flow has its point of exit at 

 the middle electrode, and its points of entry at the peripheral ones; by re- 

 versing the current direction, the points of exit are peripheral and central to 

 the point of entry. A special side attachment with another nerve permits of 



1 E. du Bois-Reymond, " Untersucliuiigen," (1), Bd. ii. S. 337. 



2 Verhandl. d. phys.-mcd. Geselhch. in Witrzburg, 1877, Bd. x. ; also Tschiriew, Arch, 

 f. Physiol., Leipzig, 1877. 



3 Ccntralbl.f. d. med. JVissensch., Berlin, 1895. 



4 CentralU.f. Physiol., Leipzig u. Wien, 1895, Bd. ix. 



