270 ON POSITIVE VAKIATION OF THE NERVE CURRENT 



False estimates may easily be formed of the influence of the 

 strength of the stimulating- current on the amount of negative 

 variation, unless unipolar action be strictly excluded. Negative 

 variation amounts only to a very small fraction of the current from 

 transverse to longitudinal section so long as it depends solely on 

 propagated excitation, and under these circumstances I have seldom 

 seen it amount to a fourth part of that current. Very weak stimu- 

 lations must be employed at the onset, unless it is desirable to get 

 at once almost maximal action. That increase of the strength of 

 the stimulating current beyond a certain very low limit has scarcely 

 any further influence on the amount of negative variation, can 

 naturally only be observed with methods of stimulation which exclude 

 all unipolar action. For as soon as stimulation is started with the 

 ordinary induction apparatus, negative variation can be made so 

 great with moderate currents, that the whole nerve current tempo- 

 rarily disappears, or even changes its direction. 



In his researches on negative variation, du Bois-Reymond did 

 not exclude the unipolar actions which are occasioned by the escape 

 of electricity into the circuit of the galvanometer. After he had 

 convinced himself that, when tetanising a nerve with induction- 

 currents, the electricity which flowed over and produced unipolar 

 action did not deflect the needle, he took measures to exclude that 

 action in special cases only. His general purpose was to show that 

 the electromotive force of the nerve decreased during excitation. 

 Whether this excitation was produced exclusively by stimulation 

 of the tract between the stimulating electrodes, or whether it was 

 produced at the same time by stimulation of the tract through which 

 electricity was discharging itself unipolarly, appeared to him in 

 general of no consequence, so long as the electricity led into the 

 nerve did not influence the deflection of the needle. 



In order to prove that in du Bois-Reymond' s researches he did 

 not as a rule exclude unipolar action when tetanising with induction- 

 currents, I will quote a few passages from his * Researches,' remark- 

 ing that the words in italics are not so in the original text. 



In vol. i. p. 436 *, du Bois-Reymond says : 



* The question is whether the electricity which with this method 

 of experiment continually spreads through the nervej considering that 

 the apparatus is not completely isolated, may not have an action 

 on the needle of the multiplier. That in this case, as in that pro- 

 duced by unipolar induction-contractions, electricity continually Jlows 

 1 Untersuchungen uber thierische Elektricitat. 



