ON NERVE-EXCITATION BY THE NERVE-CURRENT. 155 



This indeed only held good with very weak currents. If I made 

 use of considerably stronger currents, I could then obtain effects 

 by placing the electrodes at various parts of the nerve, and if I 

 placed them very near the origin of the femoral branches, the 

 secondary action was sometimes suddenly and strongly increased 

 owing to the presence of electrotonic currents. From all this it 

 appears that the effects observed by me are above all a function of 

 the excitability of the part of the nerve to which the electrical 

 stimulus is applied, while those which du Bois-R/eymond has 

 recorded are rather a function of current-strength and of the dis- 

 tance of the point of excitation from the secondarily excited nerve. 



In point of fact these experiments of mine and those of du 

 Bois-Reymond, in spite of their superficial analogy, are concerned 

 with two fundamentally distinct classes of phenomena. The true 

 secondary contraction is dependent upon the excitation propagated 

 to the central end of the divided nerve and consequent variation of 

 the nerve-current. If, instead of secondarily excited fibres in con- 

 tiguity with the directly excited fibres in the central end of the 

 nerve, w r e imagine a muscle in natural connection with the directly 

 excited fibres, we may lay down the following rule as regards the 

 result of our experiments : If this muscle contracts strongly, the 

 muscle of a secondarily excited nerve will also contract. This rule 

 signifies, then, that the true secondary contraction must follow the 

 law of contraction, just as would be the case for a muscle in connec- 

 tion with the central end of the primarly excited nerve. Apparent 

 departures from this rule are simply due to the fact that the excita- 

 bility of the secondary nerve does not remain so constant as that of 

 a muscle, but rapidly diminishes after the cross section is made. 



The paradoxical contractions of du Bois-Reymond follow quite 

 another rule. An example will show this best. The sciatic nerve 

 is divided above the knee, isolated up to its femoral branches and 

 placed upon unpolarisable electrodes, so that the interpolar region 

 is not more than a few millimeters long, and the transverse section 

 is but a few millimeters distant from the nearest electrode. The 

 rheochord is in conjunction with 3 or 4 Daniells, and one pole of 

 the battery is joined to the removable binding-screw of the rheo- 

 chord, so that a weak branch current or the total battery current 

 can be alternately sent through the nerve. All the plugs are put 

 in, the slider of the rheochord is placed at zero, and the direction 

 of current is such that the derived current is descending (atter- 

 minal) in the sciatic nerve. The sciatic plexus is now divided, and 



