POLARISATION PHENOMENA IN NERVE 317 



the core is positively polarised at the anode and negatively polarised 

 at the cathode. This polarisation, while opposing the primary current, 

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

 the arrows in Fig. 128, the current passing from a to 6 and from 6 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 



FIG. 128. Diagram to show polarisation currents in a ' Kernleiter,' or in a 



medullatcd nerve. 



will cause an extrapolar current in the cathodic area, the catelectro- 

 tonic current. 



This polarisation will not disappear at once on breaking the 

 polarising current. The nerve or nerve-model will still be positively 

 polarised at the anode and negatively polarised at the cathode. On 

 connecting therefore these two points with the galvanometer, we shall 

 get a current in the opposite direction to the previous polarising 

 current, viz. from anode to cathode (Fig. 129). This is the so-called 



Polarising 



/* Negative polarisation. 

 FIG. 129. Diagram to show direction of the negative polarisation current. 



negative polarisation of nerve. Similarly in the extrapolar regions of 

 the nerve we shall have currents in the same direction as the previous 

 polarising current, as shown by the arrows. So far then the nerve 

 behaves exactly like the mechanical model. If, however, we pass a very 

 strong current through a nerve, and then quickly switch the nerve on to 

 a galvanometer, we find a momentary current through the galvano- 

 meter in the same direction as the previous polarising current. This 

 is known as positive polarisation of nerve. It is absolutely dependent 

 on the living condition of the nerve, and is in fact an excitatory pheno- 

 menon due to the strong excitation which occurs at break of the 



