POLARISATION PHENOMENA IN NERVE 281 



on the occurrence of polarisation between the sheath and the conductin 

 part of the nerve fibre and may be exactly reproduced on a model consisting 

 of a core of zinc or platinum wire in a casing of cotton soaked with ordinary 

 salt solution. Although thus physical in origin, its production is dependent 

 on the vitality of the nerve, and so is not to be confounded with the simple 

 spread of current. 



The polarisation phenomena resulting from the passage of a constant 

 current through a medullated nerve can be studied on a model made up of 



Glass tube 



containing 0-6%NaCI. 



Pt.wire 



FIG. 125. Apparatus for imitating the polarisation phenomena in medullated 

 nerve (' Kernleiter ' model). 



a glass tube filled with normal salt solution, containing a platinum or zinc 

 wire stretched through it (Fig. 125). On leading a current through a and b, 

 and connecting c and d with a galvanometer, a current will be observed in 

 the extrapolar portion of the model in the same direction as in the intrapolar. 

 That this spread of current is due to polarisation is shown by the fact that, 

 if the model be made of zinc wire immersed in saturated zinc sulphate solu- 

 tion, so that no polarisation can occur, the spread of current to the extra- 

 polar area is also wanting. If we examine the phenomena taking place at 

 the anode, we see that a current passes here through an electrolyte to the 

 conducting core. Every passage of a current through an electrolyte must 

 be accompanied by dissociation, the current being carried by the ions. 

 We get therefore a movement of negative ions up into the electrode, and 

 a deposition of electropositive ions on the core (Fig. 127, a). In the same 

 way at the cathode there 

 will be a deposit of electro 

 negative ions on the core 

 (Fig. 126, d), so we may 

 say that the core is posi- ^ v% + -*;-+-*-; 

 tively polarised 'at the = :-:-~:-~s~M-Mz. ^P^T^ 

 anode and negatively polar- " 



^A 4- 4-1* 4-1, A TT,' FIG. 126. Diagram to show fpolarisation at the surface 



ised at the cathode. This between conducting core and electrolyte sheath in a 

 polarisation while opposing ' Kernleiter.' 

 the primary current, will 



set up currents in the surrounding electrolytic sheath, as shown by the 

 arrows in Fig. 127, the current passing from a to b and from b to c in the 

 electrolyte, returning towards a in the core. Hence if we lead off from 

 the sheath in the neighbourhood of the anode from a and c, a current 

 will pass in the galvanometer from a to c, that is along the core in the 

 same direction as the intrapolar current. The same factors will cause an 

 extrapolar current in the cathodic area, the catelectrotonic current. 



