CH. XVI.] THE LAW OF CONTRACTION IN MAN 189 



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), but diffuses 

 widely through the body, seeking the paths of least resistance. Thus it is impos- 

 sible to get pure anodic or kathodic effects. If the anode is applied over the nerve, 

 the current enters by a series of poifits (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 condition of anelectrotonus, the peripolar in that of 

 katelectrotonus, so that although the former effect will predominate, the points being 

 more concentrated, the latter effect may prevent a pure anelectrotonic effect 

 being observed (fig. 202). 



Pfliiger's law of contraction according to which excitation occurs at the kathode 

 on the make of a constant current, and at the anode on the break, holds good for 

 all excitable tissues. The excitation at the break is probably really due to the 

 make of a polarisation current having its kathode at the former anode, and is 

 therefore fundamentally of the same nature as the make contraction ; or, in general 



FIG. 202. 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.) 



terms, excitation occurs only at the place where a current leaves the excitable 

 tissue. No doubt the effect is determined by the electrolytic changes occurring at 

 the point of entry and exit of the current ; the development of kat-ions must there- 

 fore be the chemical change that results in excitation. It is difficult to imagine that 

 in a degenerated muscle there should be a reversal of such a fundamental law, and 

 that excitation should be associated with the development of an-ions. Yet this is 

 supposed to occur in the qualitative change known as the " reaction of degenera- 

 tion." Page May has investigated this question afresh, and finds that the reversal 

 of the law is only apparent, not real, and is due to the imperfect method which 

 clinical observers must necessarily employ when testing the electrical reaction of 

 muscles through the skin. By the use of appropriate electrodes on the degenerated 

 muscles of animals, it is possible to detect the source of error. Let us substitute a 

 muscle for a nerve in the diagrams of fig. 202 ; the current enters a few fibres at 

 the anode, then spreads in all directions, and leaves the muscle by a number of 

 diffused kathodic points. If the muscle is degenerated, its excitability is high, 

 and the ready response at the anode when the current is made does not really occur 

 at the actual anode, but in the neighbouring and more widespread peripolar 

 kathodes. In other words, degenerated muscle obeys the general law of excitable 

 tissues, and excitation occurs only at the situation where the current leaves the 

 muscle. At the actual anode there is relaxation or absence of effect ; this is 

 obviously not observable through the human skin because the change is very 

 limited in extent ; it can be actually seen in the exposed muscles of an animal. 



