146 ON NERVE-EXCITATION BY THE NERVE-CURRENT. 



2. On true secondary contraction and secondary tetanus 

 derived from nerve. 



On the behaviour of nerve when it is excited by induction currents in 

 the vicinity of a transverse section. 



Before passing to the subject-matter proper of this section, I will 

 refer to the peculiar behaviour which a nerve manifests when it is 

 excited by induction currents in the neighbourhood of its transverse 

 section, for this is of importance to what has to follow. 



If to a freshly cut or tied nerve the electrodes of the secondary 

 coil are applied, separated from each other by a distance of only 

 two to three mm., so that one electrode is at the point of section 

 or of ligature, the very weakest currents give strong contractions 

 if the direction of the break-currents is abterminal in the nerve. 

 With an atterminal direction of these currents the contraction is 

 much weaker, or absent, in spite of unaltered position of the elec- 

 trodes and unaltered current strength. If, the direction of the 

 break-currents being abterminal, the electrodes are pushed further 

 and further from the transverse section, the effects diminish rapidly 

 to zero. If, on the other hand, the direction of the break-currents 

 is atterminal, the effect rapidly increases as the electrodes are moved 

 away from the transverse section, soon reaching a maximum, and 

 finally diminishing and disappearing as the electrodes are moved still 

 further away. All this naturally presupposes that one is working 

 with minimal current strengths and freshly-made transverse sections. 



The rule here given is however liable to exceptions. If there are 

 points of section of branches in the vicinity of the transverse section, 

 if the latter is not quite fresh, if the nerve has been injured in its 

 course, or sometimes from other undemonstrable causes, departures 

 from the rule may present themselves. I am here concerned only 

 with the rule itself which anyone will easily find to hold good. 

 From it one may generally determine, with complete certainty, the 

 direction of the break-current in given cases. 



This behaviour of the nerves is especially valuable in all those 

 cases in which it is desired without further trouble to exclude 

 unipolar effects. For on the one hand the low intensity of the 

 currents used makes these effects very improbable, and on the other 

 they ought to persist or indeed become stronger as the electrodes are 

 moved away from the transverse section, and consequently towards 

 where the nerve produces its action ; whereas, indeed, under the 

 circumstances in question, the opposite is the case. 



