ELECTROTONIC CURRENTS 



831 



should be polarization (separation of ions) at the boundary between 

 the core and the sheath i.e., between the wire and the liquid, where 

 the current passes from the one to the other. 



A current led into the sheath tries, so to speak, to pass mostly by 

 the good conducting wire. If this is not polarizable if it is, e.g., a 



Fig, 302. Diagram to show Dis- 

 tribution of ' Positive Polar- 

 zation ' after opening Polar- 

 izing Current. B, battery; 

 G, galvanometer. The dark 

 shading signifies that the ex- 

 citation to which the current 

 causing the positive deflection 

 after the opening of the polar- 

 izing current is due is greatest 

 in the immediate neighbour- 

 hood of the anode, and fades 

 away in the intrapolar region. 

 + indicates the anode, and 

 the kathode of the polariz- 

 ing current. 



Fig. 303. Ritter's Tetanus. 

 A strong voltaic current 

 was passed for some time 

 through the nerve of a 

 muscle - nerve preparation. 

 On opening the circuit, the 

 muscle gave one strong con- 

 traction, and then entered 

 into irregular tetanus, which 

 continued for four minutes. 

 (Only the first part of the 

 tracing is reproduced.) 



zinc wire surrounded by saturated zinc sulphate solution there is 

 little or no spreading of the current outside the electrodes : it passes at 

 once into the core, and so on to the other electrode. If, however, there 

 is polarization when the current passes from the liquid into the wire, 

 as is the case in the platinum-zinc sulphate or the zinc -sodium chloride 



combinations, the stream 

 spreads longitudinally in 

 the sheath, since the 

 polarization introduces a 

 virtual resistance at the 

 surface of the wire, in 

 comparison with which 

 the difference in resis- 

 tance of an oblique and a 

 direct transverse path 

 through the liquid becomes small. It has been supposed that in 

 medullated nerve a similar polarization takes place at the boundary 

 between some part of the nerve-fibre which may be called a core, and 

 another part which may be called a sheath for instance, between the 

 axis-cylinder and the medullary sheath, or between the latter and the 

 neurilemma. It is known that the electrical resistance of nerve, in the 



Fig. 304. Diagram showing Direction of the Extra- 

 polar Elcctrotonic Currents. + is the anode 

 and - the kathode of the polarizing current. 



