316 PHYSIOLOGY 



developed instantaneously, and lasts the whole time that the current 

 is flowing through the nerve. Its production is dependent on the 

 occurrence of polarisation between the sheath and the conducting 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 produc- 

 tion is dependent on the vitality of the nerve, and so is not to be con- 

 founded with the simple spread of current. 



M Glass tube 



containing 0-6%NaC|. 



Pt.wire 



FIG. 126. Apparatus for imitating the polarisation phenomena in medul- 

 lated nerve (' Kernleiter ' model). 



The polarisation phenomena resulting from the passage of a 

 constant current through a medullated nerve can be studied on a 

 model made up of a glass tube filled with normal salt solution, con- 

 taining a platinum or zinc wire stretched through it (Fig. 126). 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 



FIG. 127. Diagram to show polarisation at the surface between conducting 

 core and electrolyte sheath in a ' Kernleiter.' 



be made of zinc wire immersed in saturated zinc sulphate solution, 

 so that no polarisation can occur, the spread of current to the extrapolar 

 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 electronegative ions on the core (Fig. 127, d), so we may say that 



