282 PHYSIOLOGY 



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 direction 

 opposite to the previous polarising current, viz. from anode to cathode 

 (Fig. 128). This is the so-called negative polarisation of nerve. Similarly 

 in the extrapolar regions of the nerve we shall have currents in the same 



Polarising 

 k 



,'Ve Positive polarisation 



Fia. 127. Diagram to show polarisation ~ 



currents in a JSTeritfciter, or m a medul- the megatiye p g olarisation current . 



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 

 galvanometer 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 phenomenon 

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

 anode. Thus in the diagram (Fig. 129) a strong current is passing through 

 the nerve from a to k. When this current is broken, excitation occurs, 

 as we have already learnt, at the anode, and this excitatory state may, 

 if the previous currents were strong, last two or three seconds. An excited 



Polarising 



S*f Neqative polarisation. 



I i'.. 129. Diagram to show direction of the FIG. 130. Diagram of arrange - 

 positive polarisation current, due to a break ment for showing'poradoxical 



excitation at the anode. contraction. 



tissue is however always negative towards adjacent unexcited tissue, 

 and therefore if we connect a to k, there must be a current outside the 

 nerve from k to a, and in the nerve from //. to A*, viz. in the same direction 

 as tin- polarising current. We see therefore that negative polarisation is 

 to polarisation occurring between an electrolytic sheath and a con- 

 core, whereas positive polarisation is hardly a polarisation effect at 

 all, but is a current of action. 



PARADOXICAL CONTRACTION. If the sciatic nerve of a frog be 



