CH. XVI.] 



REACTION OF DEGENERATION. 



187 



as for motor nerves; in a case of neuralgia, relief will often be 

 obtained by passing a constant current through the nerve ; but 

 the pole applied to the nerve must be the anode which produces 

 diminution of excitability, not the kathode which produces the 

 reverse. 



Waller has pointed out that Pfliiger's law of contraction, as formulated 

 for frogs' muscles and nerves, is true for human muscles and nerves in the 

 main, but there are certain discrepancies. These arise from the method 

 necessarily employed in man being different from those used with a muscle- 

 nerve preparation. In a muscle-nerve preparation the nerve is dissected out, 

 the two electrodes placed on it. and the current has of necessity to traverse 

 the piece of nerve between the two electrodes. In man, the current is 

 applied by means of electrodes or rheophores which consist of metal discs 

 covered with wash leather, and soaked in brine. One of these is placed on 

 the moistened skin over the nerve, and the other on some indifferent point 

 such as the back. The current finds its way from one electrode to the other, 

 not necessarily through the nerves to any great extent (though it will be 

 concentrated at the nerve as it leaves the anode or reaches the kathode), 



Shin 



fig. 194. Electrodes applied to the skin over a nerve-trunk. In A the polar area is 

 anelectrotonic. and the peripolar katelectrotonic. The former condition, therefore, 

 preponderates, since the current is more concentrated. In B the conditions are 

 reversed, the polar zone corresponding here to the kathode. (After Waller.) 



but diffuses widely through the body, seeking the paths of least resistance. 

 Thus it is impossible to get pure anodic or kathodic effects. If the anode is 

 applied over the nerve, the current enters by a series of points (polar zone). 

 and leaves by a second series of points (peripolar zone). The second series 

 of points is very close to the first, as the current leaves the nerve as soon 

 as possible, seeking less resistant paths. The polar zone will be in the con- 

 dition of anelectrotonus, the peripolar in that of katelectrotonus, so that 

 although the former effect will predominate, the points being more con- 

 centrated, the latter effect may prevent a pure anelectrotonic effect being 

 observed (fig. 194). 



Excitability andCondiictirity. Wehave already seen that these two pro- 

 perties of nerve do not necessarily go together. We learn the same lesson 

 from the following experiments. The nerve of a frog's leg is led through 

 a glass tube, the ends of which are sealed with clay, but the nerve must not 

 be compressed. The tube is supplied with an inlet and outlet, so that gases 

 may be passed through it. Two pairs of electrodes are arranged, so that the 

 nerve can be stimulated either within or outside the tube. If carbonic acid 

 is passed through the tube, and the nerve stimulated by an induction shock 

 within the tube, the muscle does not respond ; but on stimulating outside the 

 tube, the muscle contracts. The nerve is, therefore, not excitable, though 

 it will conduct impulses. If alcohol vapour is used, conductivity vanishes 

 before excitability. Cold acts like carbonic acid ; localised cold applied to 

 nerve, however, increases its excitability to the constant current, and also 

 to mechanical and thermal stimuli (Gotch). 



