242 ELECTRO-PHYSIOLOGY CHAP. 



such a manner that one rests upon the transverse section, or point 

 of ligature, even minimal currents will produce a very marked effect, 

 when the opening current in the nerve is abterminal (i.e. parallel 

 with the nerve current). With atterminal direction of current, 

 on the contrary, the effect albeit with uniform position of elec- 

 trodes and strength of current is very much weaker, or fails 

 altogether. If with an abterminal opening current the electrodes 

 are removed farther and farther from the cross-section, the effect 

 of the current declines quickly, and finally vanishes altogether. 

 If, on the other hand, the opening current is atterminal, its 

 action on moving the electrodes away from the cross-section 

 increases rapidly, attains its maximum, and then subsides with 

 further shifting, until it disappears entirely." 



II. ELECTROMOTIVE CHANGES PRODUCED BY ELECTRICAL 

 EXCITATION (CURRENT OF ACTION) 



The state of activity in nerve-fibres is not characterised by 

 any directly visible alterations in the nerve, so that in order to 

 recognise it the nerve must be left connected with its muscle or 

 other peripheral organ. This acts as indicator for the nerve, 

 since neither optic, nor chemical, nor any other demonstrable 

 alterations can be recognised in the nerve itself. In its electro- \ 

 motive reaction, however, du Bois-Eeymond perceived a means of 

 detecting the state of activity by means of the nerve itself. 

 Immediately after discovering the nerve current, du Bois- 

 Eeymond found in the year 1843 that it diminished duruuj 

 tetanus i.e. underwent a " negative variation" the signs of which 

 agree essentially with those of the negative variation of the 

 muscle current. Du Bois-Eeymond showed that, as in the latter, 

 the effect is the expression of an altered state of the nerve, and 

 not of any experimental errors whatsoever. Apart from other 

 facts to be mentioned below, this is more particularly seen in the 

 circumstance that the negative variation can be observed with 

 even very weak alternating induction currents, quite independent 

 of the length of tract between the leading-off and exciting 

 electrodes in the nerve; so that it essentially depends upon a 

 diminution of E.M.F., concomitant with the state of tetanic excita- 

 tion in the divided nerve. The extent of the negative variation, 



