Experiments and Observations

I relate the following fact, in this place, because at the same time that it gives further confirmation to the above opinion, it affords an instance in which insulation diminished the effect of the metals. I had one day laid the nearly exhausted leg of a frog upon my hand, with a piece of zinc in contact with its nerve only; and, when I touched these with a silver probe, tolerably strong contractions were excited, even when the nerve appeared dry: but when both the leg and the metals, thus disposed, were insulated by means of glass and sealing wax, the contractions were scarcely perceptible. My hand, it would appear, had, in these instances, supplied the place of the moisture in the other; and been the conducting medium between the muscles and the metals.

This communication of the muscles with the nerve, through the medium of the metals, had appeared to Dr Valli a circumstance so essential to the production of Galvani’s phenomena, that (taking it for granted they were occasioned by the action of the electrical fluid), it seems to have suggested the hypothesis, which he has offered in order to account for them.

Aware that no electrical phenomenon can possibly have place, except between the opposite states of positive and negative electricity, or, in other words, where there is a breach of equilibrium in the distribution of the electrical fluid; he supposes it to be one office of the nerves, to produce this breach of equilibrium, by continually pumping (to use his own expression) the electrical fluid from the internal parts of muscles, and in this way rendering them negative, with respect to the external surface. The brain, he makes the common receptacle for this fluid. The metals, he seems to consider in the light of a conductor, interposed between the outside of muscles and their nerves. And the rapid transmission of the fluid to restore the equilibrium, as the cause of the contractions.

He presumes his hypothesis proved from the following considerations:

I. The interval which commonly takes place between the contractions; which interval, according to him, is necessary for the restoration of the breach of equilibrium.

II. From observing, that fishermen, in order to preserve their fish from putridity, crush their brains; and thus, by interrupting the medium between the external and internal surfaces of muscles, prevent these repeated discharges of the electrical fluid, which, according to Dr Valli, hastens their putridity.

III. From finding that in general, when the sciatic nerve on one side of a living frog was divided, the other being left entire, communicating with the brain, both armed and equally excited, the limb, in which the nerve had been divided, preserved its power of contracting longer than the other. From this well devised experiment, he concludes, likewise, that animal electricity is the principle of life. That, on the side where the nerve remained entire, it was withdrawn from the muscles, and deposited in the brain. That, from the impossibility of this taking place on the other side, where the nerve was divided, it had continued in the limb, and enabled it to contract.

If it were indisputably true, as I once believed, that contractions could be excited in a limb without the metals having any communication with it, except through the medium of a nerve; this circumstance would alone be a sufficient refutation of Dr Valli’s hypothesis: but, as I have already shewn, that contractions were not in this way produced in any experiment, which I have made, when no moisture, forming a communication between the metals and the muscles, had been left adhering to the surface of the nerve, it becomes necessary to have recourse to less dubious arguments.

The Dr should have recollected that, in cases of a breach of equilibrium in the distribution of the electrical fluid, all that is required, in order to restore equality of distribution, is, the interposition of a single conducting substance between the place in which it abounds, and that in which there is a deficiency. Whereas, in the phenomena, which he attempts to explain, two conducting substances are necessary to the effect.

When a separated limb is placed under water, one would naturally imagine, that from the perfect communication, which is then formed between the external surfaces of muscles and their nerves, no breach of equilibrium could possibly have place: yet we find Galvani’s phenomena even more readily produced in this situation, than when both muscles and nerves are free from surrounding moisture.

The following experiment was made with a view of rendering the equilibrium of the electrical fluid, in different parts of frogs, as perfect as possible.

The head of a frog having been separated from its body, the latter was laid upon a plate of zinc, held by a person sitting in an insulated chair, which communicated with the prime conductor of an electrical machine. The machine was put in action, and both the person and the frog were electrified positively. In these circumstances, no sparks could be drawn from the frog, by the person holding it: nor could any other electrical appearance take place between them. But, when a piece of silver was passed over different parts of the frog, and, at the same time, brought into contact with the zinc plate, contractions were uniformly excited, differing not in the least, either in strength or frequency, from those which are excited when no artificial electricity is present. The result was precisely the same, when the frog and the person holding it were negatively electrified. This experiment was often repeated. The following experiment was made, in order to see if the effect produced upon a frog, by the passage of artificial electricity from any part of its body, would be increased by employing two different metals as conductors.

A frog was laid, successively, upon a number of different metals, insulated upon glass, and positively electrified by communicating with the prime conductor of an electrical machine. The contractions produced in the frog, thus disposed, by drawing sparks from it, with metals different from those on which it was placed, were not in the least stronger, than those occasioned by drawing similar sparks from it, with conductors of the same metal.

In establishing a communication between two opposite electricities, as, for example, between the two sides of a charged phial, it is matter of indifference to which the conductor is first applied. But it is by no means so, in the case of muscles and armed nerves. For, if one branch of a conductor be applied to the tin-foil arming a nerve, before the other branch has been applied to the muscles, it frequently fails to excite contractions. If first applied to the muscles, this is very seldom the case.

As for the intervals of rest which alternate with the contractions, and which the Dr considers as employed by the nerves, in restoring the breach of equilibrium between the internal surfaces of muscles, and their external; these may possibly admit of a different explanation.

We find them alternating with contractions however excited. It is difficult to conceive, that violent contractions should not derange in some degree, however slight, the intimate organization of muscular fibres: and some time must necessarily elapse before their elasticity can have restored the organized particles, of which they are composed, to that relative situation with respect to each other, which will fit them for again contracting.

This explanation is drawn from observing the following facts. Hearts, taken from the living thorax, and exposed to the action of a strong stimulus, contract vividly for a time, and then cease to be effected by any further application. If they be then removed from the stimulus, and placed for a time either in cold water or in open air, they are observed to regain their susceptibility of the action of stimuli, and again contract. Mr Coleman, in his excellent dissertation on Suspended Respiration, makes an observation, which I have often had opportunity of verifying: that hearts distended with blood, and in which no contraction can be produced, by scratching their surface with a pointed instrument, contract spontaneously, if one of the large vessels, at some distance from them, be cut so as to evacuate some of the blood.

The organization, in this case, is suffered to recover by the removal of the stimulus, (distention) which had deranged it. Even, in the living and entire animal, the heart does not renew its contractions, on the first influx of blood. Some time must elapse, while it recovers from the derangement occasioned by the preceding contraction.

I have repeatedly excited, by means of zinc and silver, contractions in the leg of a frog, whose head had been divided from its body, upwards of three days before. The receptacle, for the electrical fluid, was in these cases removed. Now, either the nerves continued extracting it from the internal parts of muscles, or they did not. If they did, having no longer a receptacle, in which they could deposite their electricity, they must have remained positively electrified; and thus, being in the same state with the outer surface of the muscles, no contraction should, according to the hypothesis, have been excited by the application of the metals. But this is contrary to the fact.

If it be contended, on the other hand, that their pumping power had ceased; then the first application of the metals, which produced a contraction, having restored the equilibrium, which could not afterwards be broken, must have precluded the possibility of further contractions. But this too is contrary to fact.

This argument appears, to me, to do away all support, which the hypothesis may seem to derive from the experiment, before quoted, of applying the metals equally to both sciatic nerves, after one of them had been divided; I may however remark, that the pain necessarily excited by arming a nerve, whose communication with the brain was not interrupted, would fully account for the more rapid exhaustion of the muscles, to which it belonged, compared with such as had not been acted upon by so strong an additional stimulus. As fact, however, is always more satisfactory than argument, I shall relate the following accidental experiment, in proof of the relevancy of the foregoing observation.

Four days after I had divided the crural nerve of a female frog, full of spawn, I found her dead; she had been observed alive the night before. The application of the metals to the leg, whose nerve had not been divided, did not excite the slightest contractions, but on applying them to the leg, in which the nerve had been divided, tolerably strong contractions were excitable, for more than twelve hours after she was found. The spawning season had closed, upwards of a week before this happened, and, as this frog had long been without a male to assist her, it is probable, that her death had been occasioned by the retention of her spawn, as it was found in a very dissolved state. The pain, necessarily preceding such a death, could affect the different parts of the animal, only through the medium of its nerves; and hence the exemption of that part from its effects, to which the communication, by nerve, had been interrupted.

The same observation will apply to that argument, which Dr Valli has drawn, in support of his hypothesis, from the practice of fishermen. By destroying the brain, they take away all sense of pain, and, consequently, preclude that exhaustion which is so notorious for disposing to putridity.