154 ELECTRO-PHYSIOLOGY CHAP. 



with him on the ground of his own experiments, finding invari- 

 ably a diminution of excitability in the region of the anode, an 

 augmentation in that of the kathode. Erb, on the contrary, ob- 

 served on his own ulnar nerve an initial decrease of excitability 

 near the kathode, and increase near the anode, which, as was sub- 

 sequently pointed out by Helmholtz and admitted by Erb, is 

 essentially due to the formation of secondary electrode points pro- 

 duced by relations of conductivity within the arm. Samt again 

 obtained contradictory results in different cases, and referred this 

 apparent inconstancy of reaction to an inconstancy of the nerve itself. 



No one who is unprejudiced can doubt (in spite of these 

 contradictions) that if it were possible to test human motor 

 nerves in the same unexceptionable manner as the nerve of a 

 frog's leg, there would be essentially the same reaction of excit- 

 ability under the influence of a battery current this effect being 

 only masked in man and other intact animals by the masses of 

 tissue which surround the nerve, and the complications that arise 

 in applying the electrodes. De Watteville (27) in fact finds, 

 with due precaution against every possible source of error, that 

 there is complete conformity between the electrotonic alterations 

 of excitability in the nerves of man and in frogs' nerves, both 

 as regards the effects during closure of the modifying constant 

 current and the after-effects of opening the circuit. 



These considerations, which apply more especially to the 

 difficulties and fallacies attending the investigation of electro- 

 tonus in man, are no less apparent in all experiments brought 

 forward in proof of the law of contraction on other intact living 

 animals. There is again great uncertainty among the different 

 authors, both as to results and in method. It appears to be 

 quite impossible to speak of any definite direction of current 

 in the nerve, as long as it is still in situ, since, as Hermann 

 points out, "the current necessarily divides so that it flows 

 through both apparently extrapolar tracts, and that in a direction 

 contrary to the intrapolar current" (H. Handb. ii. 1, p. 62, 

 Fig. 187). Eor this reason it has been usual to fall back 

 upon the so-called " unipolar " (more correctly inonopolar) method 

 of stimulation, the application of which is (supra) sometimes of 

 value in direct excitation of the muscle, in testing the local 

 visible action of current at the point of greatest density. For 

 nerve, however, it must be regarded as illusory to test, at any 



