EVOLUTION OF ELECTRICITY. 639 



mediate portion be excited by the transmission of an electric current in the 

 direction z^-*P, the nervous current in the " derived" portion c will be in- 

 creased in intensity, whilst that in the por- 

 tion^) will be diminished. Hence it may Fi S- 170. 

 be inferred that when any portion of the 

 length of a nerve is traversed by an electric 

 current, besides the usual electromotive 

 action of the nerve, a new electromotive 

 action takes place in every point of the 

 nerve, by a polarization of its electromotive 

 elements, which action has the same direc- 

 tion as the exciting current itself; and a 

 current is thus produced in the "derived" 

 portion which is added to the original 

 "nervous current" at that end of the nerve 

 at which the direction of this new current 

 and of the nervous current coincide, and 

 is subtracted at that end at which the 

 directions are different. These variations 



in the intensity of the "nervous current" continue as long as the "exciting 

 current" lasts, and immediately cease when the circuit of that current is 

 broken. It is to the induction of the electrotonic state in the nerve sup- 

 plying it that the contraction of a muscle is due, which ensues on the com- 

 pletion of the circuit; and to the cessation of this state, that the muscular 

 contraction is due which is consequent upon the interruption of the circuit. 

 Hence the electrotonic changes in the condition of nerves may be observed 

 without previously dividing them. When, on the other hand, a nerve is 

 "tetanized" by passing an interrupted and alternating current through a portion 

 of it, the effect is, as in the case of muscle, to produce a diminution in its own 

 proper current; the needles of both galvanometers, in the arrangement last de- 

 scribed, being deflected to the negative side, instead of one going back to zero, 

 and the positive deflection of the other being increased, as happens when the 

 "excited portion" is subjected to a continuous and uniform current. The same 

 negative variation of the nervous current has been demonstrated by M. du Bois- 

 Reymond in nerves tetanized by other means, as by the use of strychnia. And 

 the phenomena both of the " electrotonic state," and of the "negative variation" 

 are precisely the same, whether motor or sensory nerves be subjected to the 

 experiment ; thus making it appear that nerve-force may be transmitted in either 

 direction along each of these orders of nerves. A very remarkable modification 

 of the "nervous current" has been shown by M. du Bois-Reyinond to follow 

 severe injuries of the nerve, by mechanical, chemical, or thermal agencies. If, 

 for instance, a piece of hot metal be brought near to the nerve without touching 

 it, the nervous current will be seen to sink rapidly, and to have its direction 

 reversed, during which the property possessed by the nerve of conveying irrita- 

 tion to the muscle, though somewhat impaired, will not be destroyed ; and if, 

 whilst in this abnormal state, the nerve be divided, every transverse section is 

 found neutral or positive to the longitudinal section, instead of negative. If 

 the nerve-trunk be then placed between muscles, so as to recover its natural 

 moisture, it will at the same time recover its usual electromotive power. 1 



1 The materials^of the five preceding paragraphs have been derived from the sketch of 

 M. du Bois-Reymond's researches, which has been recently published by Dr. Bence Jones 

 ( " On Animal Electricity : being an Abstract of the Discoveries of Emil du Bois-Reymond" ). 

 Having himself had the opportunity of witnessing a considerable number of the experiments 

 above referred to, the Author feels it due to M. du Bois-Reymond to state, that their results 

 correspond so precisely with his predictions in every instance, as to prove that lie had ac- 



