3. The arguments against the antiquated doctrine of transudation, through parts of a living body, are already so numerous and satisfactory, that it may be thought unnecessary to notice in this place, a decisive one so far as relates to the skin of frogs, at least, which may be deduced from the fact already mentioned; that so long as the skin was suffered to remain upon the limbs of frogs, placing them in water, very evidently preserved the contractility of their muscles, whereas when the skin was taken off, the muscles became hard, and incapable of contracting, in a very few hours. Had there been a possibility of water soaking through the skin, this difference could not possibly have had place.
4. In speaking of some of the relations, which subsist between the influence discovered by Galvani and the nerves, I omitted mentioning the following facts.
A very different effect is produced by applying the metals to the brain or spinal marrow of frogs, from what is produced by applying them to their nerves. In the latter case, I have observed, that every muscle, to which a nerve below the part touched is distributed, is brought into instant contraction. But no muscles are brought into contraction, when the metals are applied to the brain or spinal marrow, except such as derive their nerves from the part immediately in contact with the metals. The influence does not stimulate or pass along the spinal marrow, as it would along the trunk of a nerve, to affect all other nerves branching off from it.
I first became acquainted with this fact, while making the following experiment. Having laid bare the brain of a living frog, and put a stop to its spontaneous motions, by gently pressing upon the brain, I introduced a long slip of tin-foil doubled underneath a part of the skull, which had not been removed, and placed a silver probe upon its tongue. The only muscles which contracted, when the tin-foil was bent over the nose of the frog, so as to come in contact with the probe, were those which move the eyes, and the transparent membrane which defends them, those of the tongue and of the throat. When the tin-foil was twisted into a thin roll, and passed a little way down the spine, the muscles of the upper extremities and of the thorax contracted; when a little further, those of the back and of the abdomen contracted; and when, introduced still further, to where the sciatic nerves leave the spine, the posterior extremities were, for the first time, strongly convulsed. I have repeated this experiment very frequently; and have always found, that, as soon as the spontaneous motions of frogs had ceased, the contractions, excited by the metals, were uniformly progressive from the head downwards, corresponding exactly to the progress of the metals down the spine. The experiment sometimes succeeds when neither the brain nor the spinal marrow have been laid bare, and when even the skin has not been divided, but, when the frog is placed upon a plate of zinc, and one of the ends of a bent silver wire is placed upon any part of its spine, while the other is made to touch the plate.
5. As it has not been till very lately, that I have been able to procure an electropherus, I have as yet made but few experiments with it; their result, however, is such as tends still more to confirm me in the opinion, that the influence, discovered by Galvani, has no relation whatever to electricity.
Having, first, so far freed the instrument, from the small quantity of electricity collected, by wiping it, that none was indicated by a very sensible electrometer of linen-yarn, suspended from the wooden part of its handle; I placed it within a few inches of a glass stand, upon which I had laid a plate of zinc, supporting a frog recently killed, and with its sciatic nerves within the abdomen laid bare. A bar of zinc formed the communication between the frog and the metal plate of the electropherus. Contractions were then excited in the frog, by placing one end of a bent silver wire, insulated in sealing wax, upon the nerves of the frog, and the other end upon the bar of zinc. After strong contractions had, in this way, been kept up for about half a minute, I carefully removed the bar of zinc, by means of a stick of wax, that there might be no possibility of the electricity escaping, if any should have been collected. The metal plate was then raised from the varnished surface. The electrometer attached to its handle was very slightly affected; but a fine thread, presented to the plate, was perceptibly attracted by it.
I had a strong suspicion, that the electricity, thus collected, had been excited solely by the friction of the frog’s legs during contraction, against the insulated plate of zinc upon which it lay; and I soon found that my conjecture was just; for an equal quantity of electricity was obtained from another frog similarly disposed, when contractions were excited in it, by merely mechanical irritation.
The result was the same when these frogs were laid successively upon the metal plate of the electropherus itself, and excited, the one in M. Galvani’s method, the other by mechanical irritation only.
These experiments were very frequently repeated, but the quantity of electricity collected was always greater where the contractions, or, in other words, the friction had been most considerable, and did not, in any instance, appear to depend on the means employed to excite the contractions.