It appears from every experiment, which has been made in prosecution of Galvani’s discovery, that the nerves are very essentially concerned, in all the phenomena which it exhibits. It becomes, therefore, an object of inquiry, highly interesting, to ascertain if all the nerves of the body are equally subjects of this new influence, or if its effects are confined to those appropriated to muscles of voluntary motion. With this view, I surrounded with tin-foil the parvagum and intercostal nerves of several cows and sheep, while the auricles of their hearts were still contracting, and placed one end of a bent silver rod, at one time upon the heart itself, at another upon adjacent muscles, and sometimes upon the nerves; but all without producing the slightest perceptible variations, in the contractions of the heart, or a renewal of them when they had ceased.
I likewise included the caroted artery in the tin-foil; and, at another time, inserted the foil in longitudinal incisions made in the nerves, that it might be more immediately in contact with their substance; but still no contractions followed. I had as little success when I made similar experiments upon a dog, cats, rabbits, fowls, and frogs; yet, in all these animals, I could in general excite vigorous contractions, by arming the nerves of parts obedient to the will: I say in general, for in rabbits I have sometimes failed altogether; especially when they have been drowned in very cold water. Soon after making these experiments, I perceived from one of Dr Valli’s letters, published in the Journal de Physique, that he had made a similar one upon the heart of a dog, and with the same result. The heart, through the medium of its nerves, is not excitable, therefore, by the same means which are found efficacious in exciting other muscles to contraction. I confess I had not expected this result. It has been asserted indeed, by many physiologists of the first name[[11]], that the heart can in nowise be affected by the application of a stimulus to its nerves, or to the brain; but many considerations excited my doubts upon this subject, and some experiments which I made at this place, more than a year ago, tended to confirm me in an opposite opinion. That both the frequency, and the strength of the heart’s contractions are affected by passions of the mind, is a fact known to every one; but what is much more to the purpose, since we know so little either of mind or of its mode of influencing the body, we know that many derangements of the brain, such as apoplexy, hydrocephalus, phrenitis, &c. together with all kinds of mechanical injuries, (and what are these, but so many stimuli irritating the brain, and consequently the nerves sent to the heart?) affect the motions of the heart most materially and obviously. The contractions of the heart, so long as the brain remains entire, may be affected by a thousand different substances thrown into the stomach; but it appears from the experiment of Mr Kite, that this is by no means the case, when the functions of the brain are suspended by hanging, or drowning[[12]]. Dr Whytt’s experiment on this subject is one of the most decisive with which I am acquainted. He found, that opium operates much more slowly in destroying the heart’s motion in frogs, deprived of their brain and spinal marrow, than it does when these animals ate entire. Several of my own experiments, though not made expressly with this view, gave the same result with those of Dr Whytt. M. Fontana tells us, he has discovered the heart of the wheel polypus to be a voluntary muscle. It was probably this discovery which led him to try the effects of his will upon his own heart. For the success of his experiment, we have the testimony of his friend Dr Gerardi, Professor of Anatomy in the University of Parma, who, in a very learned little Dissertation on the Origin of the Intercostal Nerve, published in the Journal de Physique for September last, makes the following short mention of it; ‘Je ne dois point oublier de vous dire que M. Fontana a la faculté d’accélérer, ou de retarder à volonté son pouls, sans aucune contraction sensible des muscles.’
The direct experiments, by which I was first led to adopt the opinion that the heart might be affected by the mechanical irritation of its nerves, were made upon very young cats and rabbits; some with the assistance of my friend Dr Physick, now settled in Philadelphia; others in presence of several other gentlemen studying at this university. It appeared very decidedly from two or three of these experiments, that the contractions of the heart were quickened by irritating the brain at the origin of the spinal marrow. In others again, the result was by no means so clear. But it should be recollected that the evidence of one accurate, and positive experiment, is not in the least invalidated by twenty unsuccessful ones, especially upon animals of warm blood; where the irritability of their muscles is so very fleeting, and the result liable to variation from so many, as yet, unknown causes. The irritability of the arteries, for example, is now completely established, yet Haller’s experiments led him to deny it. And even those of the accurate Verschnir, to whom we are indebted for unquestionably the best series of experiments upon this subject, failed of success (as we are told by Dr Dennison, in an excellent Thesis confirming their truth,) when repeated before some of the Faculty here. Immediately, therefore, on discovering the superior powers of zinc, and molybdena, in exciting contractions, I began again to repeat with these metals the experiments on the nerves passing to the hearts of frogs; but for a long time without satisfying either myself, or others, whether any effect was really produced. At length, however, I was so happy as to succeed completely. On the 18th of March last, in presence of my friends, Mr Hunter and Mr Thomson, having dissected away the pericardium from a frog’s heart, which had an hour before ceased spontaneously to contract, I removed the muscles, and cellular membrane covering its nerves, and large blood vessels. I then placed one end of a rod of pure silver in contact with one side of these nerves, and blood vessels, and one end of a rod of zinc on the other, both of them at about the distance of the third part of an inch from the auricles of the heart. On bringing the opposite ends of these rods in contact with each other, the auricle first, and then the ventricle of the heart immediately contracted, and repeated their contractions as often as the ends of the metal rods were made to touch each other. When a stick of glass, wax, or wood, was made use of in place of one of the metals, no contraction took place. Contractions, however, were excited by irritating the heart itself with the point of a sharp instrument. The contractions were both more vigorous, and more constant when the metals were placed in contact with the heart itself, than when touching only its blood vessels and nerves. I have several times attempted to trace some of the nerves, which may be seen near the large blood vessels of the heart of a frog, into the heart itself, in order to arm them separated from other parts; but, partly on account of their minuteness, and partly on account of the weak state of my eyes, which does not permit me to look intently at minute objects, I have never been able to succeed.
Since making this last experiment, I have repeated it upwards of twenty times. In order to its complete success, it is necessary that the spontaneous contractions of the heart should nearly, if not altogether, have ceased; and, when in this state, the experiment is rendered still more satisfactory by removing the heart from the body of the frog, and laying it upon a plate of zinc. We are then sure that its contractions cannot have been excited, by any mechanical irritation, arising from the contractions of the muscles of the thorax.
For want of sufficient leisure, and convenient opportunities, I have neglected to make this experiment upon any animals of warm blood, except cats and rabbits. A few days after I had discovered the possibility of exciting the heart to contraction by means of zinc, and silver applied to its nerves, I procured an ordinary sized cat, and drowned it in water, as nearly as possible, of its own temperature. Four minutes after immersion, it was taken out of the water and dryed. Its thorax was immediately laid open, but no contractions were observed in any part of its heart, except in the right auricle, and even these were very slight. A plate of zinc was then placed in contact with the parvagum, and intercostal nerves, on one side of the trachea, and a half crown piece in contact with those of the other; both at the distance of about one third of an inch from the auricles. Every time the zinc and silver were brought into contact, complete contractions of the right auricle, and sometimes slight ones of the left were produced, but none in the ventricles. The contractions were observed to become stronger, in proportion as the metals were approached to the heart, and were strongest when one or both was in contact with the auricle. I think the contractions were fully as strong when molybdena, as when silver was used. No contractions could be excited, by arming any of the nerves of voluntary muscles, in this cat.
The next experiment was made upon a female cat, far gone with young. She was drowned in very cold water, and although her thorax was opened the instant she had ceased to struggle, which was in less than four minutes after immersion, her heart had ceased to contract; nor could its contractions be renewed, either by the application of the metals in the way described, in the last experiment, or by pricking or otherwise irritating its surface: but the diaphragm, the intercostal muscles, the fore legs, and the ears, continued to contract long and vigorously, when the metals were as usual applied to their nerves. On cutting into the uterus, however, and taking out one of the young, I found both auricles and ventricles of its heart, contracting most vigorously, though the mother had now been dead upwards of twenty minutes.
An opportunity, not to be neglected, now presented itself, of trying if it were possible to transmit this influence from the mother to the fœtus, through the medium of the umbilical chord. I therefore applied the two metals in the manner I before described, 1st, to the uterus of the mother, and to the cotyledans; afterwards to several different parts of her; but neither uterus nor fœtus were in any instance affected. As little was the fœtus affected, by arming the chord itself. As the hearts of the kittens continued their spontaneous contractions, for more than an hour after they were taken from the mother, I had repeatedly the pleasure of observing, and pointing out to Mr Thomson, and Mr Simpson, who obligingly lent me their assistance in these experiments, the effects of the metals when in contact with the parvagum, and entercostal nerves, both of quickening the repetition of the hearts contractions, while they continued spontaneous, and of exciting them anew when they had ceased to be so. This experiment, repeated upon a kitten a few days after birth, succeeded, but not quite in so satisfactory a manner as the foregoing, although the heart continued contracting for more than an hour and an half after the thorax was opened. Its contractions were quickened, and rendered vibratory by the slightest mechanical touch of its surface; so that it was difficult to determine the precise share which the application of the metals had in their production.
When these had ceased, I did not find that I could revive them by the application of the metals. In the hearts of some young rabbits, upon which I tried this experiment, the contractions appeared to be still more decidedly, occasioned by the application of the metals, than even in the cats.
Having ascertained this important fact, that one muscle, not subjected to the influence of the will, might be made to contract by the application of zinc and silver to its nerves; I proceeded to examine whether the same were the case with respect to all involuntary muscles. I could not, however, observe that any contractions were produced in the stomach or intestines, by placing the metals near the stomachic flexus and semilunar ganglion in a cat. I next proceeded to examine the effects of the metals upon the different organs of sense.
M. Volta’s discovery of the sensation produced upon the end of the tongue, by coating its upper and under surfaces with different metals, led me to compare this sensation with that produced by electricity. I found a very considerable difference between them. Both, indeed, are subacid, but as unlike to each other, as the taste of vinegar is to that of diluted vitriolic acid. That occasioned by the metals is accompanied with what is familiarly called the metallic taste; and differs according to the metals employed. With the greater number of metals it is scarcely perceptible. With zinc and gold, I think, it is strongest; next so with zinc and silver, or molybdena, and insufferably disagreeable with any of them.