244 PHYSIOLOGY CHAP. 



Hermann's model consists of a glass tube containing a platinum 

 wire, which makes a good conducting axis. The tube, closed at 

 the ends, is tilled with a saturated solution of zinc sulphate, which 

 forms a moist, less well-conducting sheath for the axis. A pair of 

 zinc electrodes are fastened to the tube, which are in contact with 

 the solution, and serve as the polarising and galvanometer contacts. 

 The electrolytic polarisation which takes place during the passage 

 of the current between the surface of the metallic core and 

 the solution, and drives the kathodic ions towards the anode and 

 the anodic ions towards the kathode, generates a resistance to the 

 passage of the current through the intrapolar portion by which 

 its longitudinal diffusion in the extrapolar parts is promoted. 



Both in the nerve and in Hermann's model, polarisation or 

 post-electrotonic currents are produced on breaking the polarisa- 

 tion circuit. These are opposite in direction to the electrotonic 

 currents, and are due to the accumulation of ions with the opposite 

 charge at either pole of the battery. The reversal of current at 

 the close of electrotonus was demonstrated on nerve by Fick, but 

 according to Hermann it is definite only in the anelectrotonic 

 region. 



Notwithstanding the analogy between the electrotonic pheno- 

 mena in nerve and those which can be reproduced in the core- 

 model, there is no doubt that the former depend not only upon 

 physical conditions, but also upon the anatomical and physiological 

 integrity of the nerve. 



Biedermann pointed out the differences between the electrotonic 

 phenomena in normal and in etherised nerve. In a normal nerve 

 traversed by a polarising current the extrapolar electrotonic effects 

 from two points equidistant from the poles are not equal on the 

 galvanometer. In one case Biedermann found that anelectrotonus, 

 as expressed by the deflection of the galvanometer needle, was 

 equal to 46 and katelectrotonus to a deflection of 25 ; on 

 increasing the strength of the polarising current he obtained 

 anelectrotonus of 96, katelectrotonus of 60. On etherising the 

 nerve these differences disappeared; with the first current the 

 galvanometer deflection was 24 in both the anodic and the 

 kathodic region ; with the second current it was 68 for the former, 

 66 for the latter. Biedermann took these results obtained with 

 etherised nerve to be the expression of the physical electrotonus 

 due to polar electrolytic effects, and those obtained with normal 

 nerve to be the expression of physiological electrotonus due to 

 special vital conditions which make anelectrotonus more pro- 

 nounced than katelectrotonus. He further showed that the effects 

 of anelectrotonus spread over a larger area in normal than in 

 etherised nerve. 



These observations of Biedermann are supported by Waller, 

 who found that anaesthetics, and. all agents in general that 



