x ELECTROMOTIVE ACTION IN NERVE 305 



regard as core-substance, not merely the axis-cylinder, but the 

 " entire protoplasmic contents of the tube " ; as sheath, not 

 merely the medullary sheath, but also " the neurilemma, and the 

 interstitial connective tissue." 



Hermann brought parallel frogs' nerves, placed together, 

 between two quadratic glass plates, and determined the resistance 

 by Wheatstone's method, passing the current once longitudinally, 

 and once transversely, to the direction of the fibres. " The trans- 

 verse resistance was five times as great as the longitudinal resist- 

 ance ; the former is about twelve and a half million, the latter 

 only two and a half million times as great as that of mercury." 



If the existence of surface polarisation is accepted on 

 the analogy of the core-model for medullated nerve, it must 

 further be asked whether the intensity of such polarisation 

 at the interface of the two electrolytes is sufficient to account 

 for the observed diffusion of current in the nerve. From 

 a purely theoretical standpoint this cannot be disputed. 

 In view, however, of the intensity of the electrotonic effects, 

 we should practically be compelled, with Hermann (40), 

 to recognise in the nerve an " unparalleled " force of surface 

 polarisation, since " the polarisation at the interface of normal 

 fluids (which is very weak as compared with the metal-fluid 

 combinations) could only, in consequence of the resistance, 

 induce a very feeble diffusion, which would be altogether masked 

 by the experimental errors." 



Nevertheless, experimental combinations of moist conductors 

 have been found to give exceptionally strong effects in the sense 

 of a pronounced and regular electrotonus ; this reaction, however, 

 must be referred less to surface polarisation in Hermann's sense 

 than to a peculiar mode of current diffusion, according to a 

 theory advanced by Griinhagen (41) and Hering (24). Hering 

 employs a simple model for "physical electrotonus," which is 

 admirably suited to all demonstrations, i.e. the long stem of the 

 pipe-grass, without internodes, which is first soaked in water, 

 and then filled just before the experiment with concentrated 

 saline. The feelers and bones of the crayfish, preserved in 

 alcohol, and saturated before the experiment with 0'6 per cent 

 saline, are no less convenient. 



The striking similarity of the resulting electrotonic manifesta- 

 tions with those observed under the same conditions in etherised 



VOL. II X 



