POLARISATION CURRENTS 189 



(Fig. 36). Nerve consists of a conducting core of electrolytes 

 and colloids surrounded by a sheath enclosing electrolytes and 

 colloids. The passage of a constant current through this mass 

 gives rise to certain physico-chemical changes in core and in 



Polarising 



"X5> x '' 



Negative polarisation. 

 FIG 36. Diagram to show direction of the negative polarisation current. 



sheath. These electrolytic and polarisation changes set up a 

 current which tends to restore equilibrium (back E.M.F.). This 

 is negative polarisation. 



The changes that take place may be studied conveniently by 

 means of a model (Fig. 37). A platinum wire, forming a con- 



Glass tube 



containing 0-6/oNaCI. 



^^^ Pt.wire 



c d a b e f 



FIG. 37. Apparatus for imitating the polarisation phenomena in medullated nerve. 



ducting core, runs inside a glass tube filled with cotton wool 

 soaked in 0-6 per cent, sodium chloride solution. 



The points a and b are connected to a battery and the extra- 

 polar points c and d, e and / are led off to galvanometers. On 

 the passage of a current from, a to b it is found that an extra-polar 



FIG. 38. Diagram to show polarisation at the surface between conducting core 

 and electrolyte sheath. 



current is flowing from c to d and from e to f (cf. Fig. 40). The 

 cause of these extra-polar or, as Du Bois-Reymond called them, 

 electrotonic currents, is to be sought in the movements of the 

 dissociated salts in the sheath. Opposite the anode (Fig. 38), 

 the positive ions are concentrated on the surface of the core. 

 This causes opposition to the passage of the primary current and 



