434 Mr. C. Brooke on the Dynamical 



more appropriately be termed) have long since been recognized 

 as the extra-currents of Faraday; but their relative direction and 

 intensity have been assumed to coincide with those of the induced 

 secondary currents, and therefore to be the reverse of what dyna- 

 mical considerations have suggested; it has, however, been demon- 

 strated by an experiment of M. Chauveau, that the dynamical view 

 is the correct one. It has been proved by numerous experiments 

 that physiological effects are produced, and produced only, by a 

 sudden efflux of electricity from a nerve or muscle into & nega- 

 tive electrode, whether the active agent be a discharge of Frank - 

 linic electricity, or either the induced or the extra currents, 

 already mentioned. Starting from this as an admitted fact, the 

 experiments of M, Chauveau appeal to the most sensitive of all 

 tests of the direction of an electric impulse (using that term as 

 synonymous with " shock " or " momentary current") — a living 

 nerve. He places the electrodes of an electromotor over the op- 

 posite facial nerves of a horse, and, having duly adjusted the 

 strength of the current, he finds that, on closing the circuit that 

 side of the face only is convulsed (by the initial extra-current) 

 the nerve of which lies under the negative electrode, and on 

 opening the circuit the contrary side is less strongly convulsed 

 (by the terminal extra-current), the nerve of which lies under the 

 positive electrode. M. Chauveau also found that with a still fur- 

 ther reduced current, convulsion occurred in relation with the 

 negative electrode only, the contrary or terminal extra-current 

 being then too feeble to affect the nerve. And if several horses 

 were similarly included in one circuit, the same results were ob- 

 served in each of them. 



The same fact has been observed by M. Claude Bernard * in 

 a prepared frog's limb in which the vitality of the nerve is 

 unimpaired : with a sufficiently reduced current convulsion oc- 

 curs on closing the circuit, and only then, whether the current 

 be direct or inverse, because the terminal extra-current is then 

 inoperative. 



It may here be remarked that the well-known relative direc- 

 tion and intensity of the initial and terminal secondary or in- 

 duced currents in a secondary coil are the necessary dynamical 

 consequences of the above assumed conditions of the extra- 

 currents. The initial extra-current will excite a similar im- 

 pulsive motion in the secondary coil, just as one stretched chord 

 will excite another capable of vibrating in unison with it (for 

 electro-dynamic induction is probably quite analogous to the re- 

 ciprocation of sound) ; and the recoil of this impulse (the initial 

 induced current) will be weakened in opposing the continuous 



* Legons sur la Physiologie et la Pathologie du Systeme Nerveuw, 

 Paris, 1858, vol. i. p. 163. 



