ON NERVE-EXCITATION BY THE NERVE-CURRENT. 157 



contraction, the problem is to excite by means of sufficiently strong 

 electrotonic currents in directly excited fibres, other fibres adjacent 

 to these, and to excite them near the point where they part company 

 from the directly excited fibres. To this end are required strong- 

 currents and a small distance from the excited point to the point of 

 branching. 



If the transverse section of a nerve trunk, of which one branch is 

 to act as the primary and the other as the secondary nerve, is in 

 the vicinity of the point of branching,' the increased excitability of 

 the secondary nerve fibres in the neighbourhood of the transverse 

 section may favour secondary excitation by means of the electrotonic 

 currents. As a matter of fact du Bois-Reymond has already found 

 that when the peroneal or tibial nerve is excited, a fresh transverse 

 section of the sciatic favours the appearance of the paradoxical con- 

 traction in the muscles supplied by the branch which is not directly 

 excited. And in this case the presence or absence of a paradoxical 

 contraction at make or at break is notably affected by the direction 

 of the electrotonising current, since the part of the secondary nerve 

 which is to be excited by the electrotonic currents lies very close to 

 the transverse section of that nerve. 



It is easy to understand further, that when strong currents are 

 applied near the origin of a nerve which is to be excited indirectly, 

 the effects of the electrotonic currents may combine with the effect 

 of the negative variation, if the preparation is excitable enough, and 

 especially if the transverse section is still fresh. It is my intention 

 to consider these complex cases after discussing more completely 

 the paradoxical contraction of du Bois-Reymond. If the above 

 directions are observed, there will be no danger of the admixture 

 of electrotonic action. The current ought to be so weak that 

 when it is ascending (abterminal) it acts only when quite close 

 to the transverse section, or at any rate much more strongly here 

 than at any other part of the nerve. With descending (atterminal) 

 currents even when of excessive strength, the true secondary con- 

 traction may be distinguished from the other by the fact that the 

 former disappears when the current is made so strong that the 

 nerve is blocked in the region of the anode (3 to 4 Daniells), whereas 

 du Bois-Reymond's secondary contraction increases with increasing 

 current-strength. 



