ON NERVE-EXCITATION BY THE NERVE-CURRENT. 147 



It was during- experiments on excitation of the central end of 

 the vagus that I was first struck by the fact that it is necessary, 

 in order to obtain an effect, to bring- the secondary much closer to 

 the primary coil when the break-induction shocks are atterminal 

 than when they are abterminal. Since then I have noted and 

 found this confirmed in the excitation of various nerves of warm- 

 blooded animals. Nerves of frogs behave similarly. But in these, 

 unless it is specially desired to investigate excitability in the 

 neighbourhood of the transverse section, it is seldom necessary to 

 place an electrode at or near a transverse section or ligatured point, 

 whereas in warm-blooded animals, owing to shortness of the end of 

 nerve, this frequently has to be done. In such cases, it is easy 

 to convince oneself of the great difference in the effect when the 

 electrodes are shifted, or when the current is reversed. Nor does 

 the Helmholtz modification entirely abolish the difference. 



Although the peculiar conditions of excitability of nerve in the 

 neighbourhood of its transverse section have been frequently inves- 

 tigated since Heidenhain called attention to them, I think that 

 the facts which are here mentioned, and which are, in many 

 respects, important, have not been sufficiently regarded. To ex- 

 perimenters familiar with the department of nerve physiology, this 

 is perhaps no novel statement. 



On the excitation of nerve consequent upon the negative variation 

 of the current of another nerve lying upon it. 



If no one has yet succeeded in obtaining true secondary contraction 

 from nerve, this want of success must be attributed to the fact that 

 all the attainable conditions which favour it have not been utilised. 

 When I took advantage of these, I succeeded in eliciting from 

 nerve true secondary contraction and secondary tetanus which 

 were as strong and as lasting as if the nerve of the secondary 

 preparation were itself directly excited electrically. 



In designating these as ' true ' secondary effects in contradistinc- 

 tion from the secondary effects discovered by du .Bois-Reymond, and 

 caused by the electrotonic currents of the primary nerve, I am 

 justified on the one hand, by the convenience of the term, on the 

 other, by the consideration that the secondary effects to be presently 

 described are truly analogous with the secondary contraction from 

 muscle. This, as the longest known kind of secondary contrac- 

 tion, deserves to be regarded as the prototype of such actions. 



