Experiments and Observations Relative to the Influence Lately Discovered by M. Galvani and Commonly Called Animal Electricity

by Richard Fowler

After this full detail of these curious phenomena, I hardly need remark, that they demonstrate the free communication, which subsists between the several branches of the fifth pair of nerves, and consequently give strong support, if not absolute confirmation, to the well known doctrine of nervous sympathy, or of the reciprocal influence, which different parts exert upon each other, through the medium of nerves.

If I might be allowed to hazard a conjecture, where we cannot have recourse to demonstration, I should say that the flash, observed in the above experiments, was the effect of contractions excited in involuntary muscles by the application of a stimulus to their nerves; or, in other words, that the effects of the application of the metals to the nasal branch of the first division of the fifth pair of nerves, had been propagated through the ciliary ganglion, along the ciliary nerves, and to the choroid coat, whose vessels it had excited into instantaneous action; and that their action again (as in the case of action excited by pressure, or a blow upon the eye,) had by stimulating the retina occasioned the sense of light.

This supposition is, I think, rendered probable by several considerations. I have already shewn that this influence can excite contractions in involuntary muscles, through the medium of their nerves. And certainly no reason can be assigned, a priori, why it should not act equally upon every description of involuntary muscles; upon those which make a part of the minutest vessels in the body, as well as upon the heart, or upon the iris.

That it excites to increased action the arteries of the tongue in the experiment, in which a sense of warmth is produced along its surface by the application of the metals to the lips, seems to be almost demonstrated; for it would be difficult to point out the presence of another cause competent to occasion the evolution of the heat, in this case, besides the increased action of the arteries: and that this cause is competent to the effect we know from numberless experiments, too familiar to need being particularized here.

Whether the metals, however, do or do not affect the action of the blood vessels, is a question which admits of solution by experiment. The following, I confess, was not quite satisfactory, and I have not yet found leisure and opportunity to repeat it with all the attention it requires.

I inspected the foot of a living frog with a microscope of very high powers. In fixing the foot so as to keep the web expanded, a considerable degree of inflammation was excited, notwithstanding every precaution to avoid it. The current of blood was seen distinctly in several vessels, now flowing rapidly, now slowly, and now in a direction contrary to that in which it was first observed, but with equal rapidity. A thin plate of zinc was introduced between the fleshy part of the foot and its supporter, and a silver probe was used as an excitor. To me, the circulation appeared very decidedly to be quickened several times when the metals were made to touch each other: but the gentlemen who assisted me could observe no change. To prevent the contractions in the muscles of the leg from producing any fallacy, the crural artery should be laid bare, and insulated from surrounding parts, by passing a thin plate of glass, or sealing wax, between it and them.

That the flash is the effect of such an increased action of the vessels, composing the choroid coat, might be somewhat more difficult to prove. It is however known to every one, that a blow, and that pressure upon the eye, are capable, as I have before observed, of producing a similar effect. And the following case, which Bonetus quotes from Hermannus Cummius, if it may be credited, affords an almost positive proof, that vision depends upon the stimulus given to the retina by the activity of blood vessels in some part of the eye. ‘Quando theologus, plaga dolorifica, a rupta instrumenti musici chorda accepta, nocte subsequenti jam adulta, e somno evigilans, cuncta clare, ac si de die esset, vidit, adeo, ut minimos picturarum et tapetum tractus observare, characteresque ex libro legere posset. Oculo vero læso clauso, tenebras densissimas adesse ille percepit, eodemque iterum aperto, conclave illustratum visum est, lucem tamen candelæ allatae solisque splendorem de die, ægre tulit oculus affectus, quod per aliquot dies duravit, tandemque sensim remisit.’

Haller speaks of such cases as by no means uncommon, and quotes the names of several authors, who have related similar ones.

The direction of this influence, when suffered to pursue its natural course, appears to be the same with that of most other stimuli, i. e. from the place at which it first affects a nerve, onwards to the part, in which that nerve terminates. I have repeatedly caused electrical sparks to be passed into my own ulnar nerve at its passage over the inner condyle of the humerus, but both the sensations and the contractions produced by them have been entirely confined to the hand and fore arm.

It appears too, both from the experiments of Dr Monro, and of Dr William Alexander of Halifax in Yorkshire, that when no communication is left between the trunk and posterior extremities of a frog, except by its sciatic nerves, a strong solution of opium, injected under the skin of its posterior extremities, deprives them both of their sensibility and of their contractile power; but that it does not in the least affect the trunk of the body. If, on the contrary, it be applied to the trunk, it exhausts both the trunk and the extremities.