Correspondence. 357 
current, inferior in intensity, and opposite indirection to the 
qnitial currents. 
These initial and terminal currents (or impulses, as they 
might more appropriately be termed) have long since been 
recognized as the extra currents of Faraday; but their rela- 
tive direction and intensity have been assumed to coincide 
with those of the induced secondary currents; and, there- 
fore, to be the reverse of what dynamical considerations 
have suggested ; it has, however, bcen demonstrated 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. I produced only, 
by a sudden efflux of electricity /vom a nerve or muscle 
into a negative electrode, whether the active agent be a dis- 
charge of Franklinic electricity, or either the induced or the 
extra currents already mentioned. Starting from this as 
an admitted fact, the experiment 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 elec- 
trodes of an electromoter over the opposite facial nerves of 
a horse, and, having duly adjusted the strength of the cur- 
rent, he finds that, on closing the current, that side of the 
face only is convulsed, (by the zuztzal extra current,) the 
nerve of which lies under the xegative electrode, and on 
opening the circuit the contrary side is less strongly con- 
vulsed (by the terminal extra current,) the nerve of which 
lies under the fosztive electrode. M.Chauveau also found 
that witha still further 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 observed 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 unimpared: witha sucffiently reduced current, con- 
vulsion occurs 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 
direction and intensity of the initial and terminal secondary 
or induced currents in a secondary coil, are the necessary 
* “TVecons sur la Physiologie et la Pathologie du Systeme Ner- 
veaux,’ Paris, 1858, vol., p. 163. 
