318 PHYSIOLOGY 



current at the anode. Thus in the diagram (Fig. 130) 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 tissue is, however, always negative towards 

 adjacent unexcited tissue, and therefore if we connect a to k, there 



Polarising 



. 



Positive polarisation 



FIG. 130. Diagram to show direction of the positive polarisation current, 

 due to a break excitation at the anode. 



must be a current outside the. nerve from k to a, and in the nerve 

 from a to k, viz. in the same direction as the polarising current. We 

 see therefore that negative polarisation is due to polarisation occurring 

 between an electrolytic sheath and a conducting 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 dissected out, and one of the two branches into which it divides be 

 cut, and the central end of this branch stimulated, the muscles supplied 



FIG. 131. Diagram of arrangement for showing paradoxical contraction. 



by the other hah of the nerve contract to each stimulus. Ligature 

 or crushing of the nerve x between the points stimulated and the 

 point which joins the main trunk puts a stop to this effect, showing 

 that it is not due to a mere spread of current. The fibres passing down 

 n are in fact stimulated by the electrotonic current developed in 

 x during the passage of the exciting current. 



