272 ON POSITIVE VARIATION OF THE NERVE CURRENT 



on the actions which occasion unipolar induction contractions. In 

 other words, unless the whole secondary circuit, the apparatus for 

 leading the current to the nerve, the leading- off electrodes and the 

 multiplier are completely insulated (a slate of things not easy to 

 secure as may be well understood) there is a continual current, now 

 negative, now positive, according to the direction of the tetanising 

 current flowing through the tract between the platinum plates 

 and the pads, and through the latter and the remainder of the 

 apparatus to the ground.' * On the needle, indeed, this electricity 

 has no action, but it does not fail slightly to tetanise the nerve, 

 consequently it might show an illusory activity where there should 

 have been none, or an exaggeration of actually existing activity. 

 Hence, for example, it appears that ligature or section of the nerve in 

 the tract between platinum plates and pads does not completely arrest 

 negative variation when induction currents are used for tetanising! 



Du Bois-Reymond discussing, two pages farther on, the in- 

 fluence of the strength of the stimulating current on the amount of 

 negative variation, says expressly, ' There is no drawback to the use 

 of the induction apparatus in this case ' (that is to say, in comparing 

 the actions of two currents of very different strength). 



A few lines farther he remarks, ' In order to show that here like- 

 wise all depends on the density of the current and not directly on 

 its strength, one can go to work in the manner above described 

 for the electrotonic condition. The source of the stimulating current 

 is left unaltered, but a collateral closure is established between the 

 electrodes ; such as a pad of blotting-paper folded several times and 

 steeped in white of egg. It is however advisable in this case to 

 exchange the induction coil for an inversor, because the lessening of 

 density no longer affects unipolar action, as it does when the source of 

 the current itself is weakened.' In like manner du Bois-Reymond 

 rejects the use of the induction coil when dealing with the influence 

 of the length of the stimulated tract, or of the influence of the 

 distance apart of the stimulated and led off tracts. 



The above is sufficient to show that in general it appeared a matter 

 of indifference to du Bois-Reymond, whether the nerve were stimu- 

 lated by induction currents exclusively in the tract between the 

 stimulating electrodes, or whether it were stimulated in the rest of 

 its extent at the same time. Only in particular cases, when it 

 appeared necessary to limit the stimulation to the tract between 

 the electrodes, did he exclude unipolar action by tetanising with 

 battery-currents in alternately opposite directions. 



