Kitabı oku: «Experiments and Observations», sayfa 3
SECTION II.
Has Magnetism any concern in the Phenomena discovered by Galvani?
In answer to this question I have little to say, as the experiments which it suggested, and which I had an opportunity of making, have been but few.
I have repeatedly excited contractions, both with the natural and the artificial loadstone, but I could never observe any difference between them, and such as were excited by unmagnetised iron, or an ore containing an equal quantity of iron with the natural loadstone.
When the separated leg of a frog was laid upon a plate of iron, and a loadstone was brought in contact both with its nerve and the plate, no contraction was excited. I have often brought frogs, in every state of preparation, as nearly as possible to a very sensible magnetic needle, but no variation in its direction was in any case produced by the contractions of the frogs excited by the metals.
SECTION III.
What are the relations which subsistbetween the influence discovered by Galvani, and the muscles, the nervous, and the vascular systems, of animals?
In proposing to myself a question of this very extensive nature, it will hardly be imputed to me, that I ever entertained, for a moment, the idle expectation of being able completely to solve it. It is prefixed to the following experiments as the most commodious general head under which I could arrange, not only what I had further to say, upon the influence discovered by Galvani, but likewise upon the several physiological subjects, in the examination of which this influence was employed merely as a test.
OF THE MUSCLES
As I am acquainted with no criterion by which we can assure ourselves of the complete separation of muscular fibres from nerves, without rendering them objects too minute for accurate experiment; it can never be in our power, so far as I am able to judge, to satisfy ourselves, if this new influence can act immediately upon the muscular fibre. A doubt must always remain, whether nerve has not been present; and from this doubt will arise another still more difficult to solve, whether the influence produced or excited by the metals have passed through the nerve to the muscles? or if it have merely acted as a stimulus to the nerve, serving to rouse that unknown energy, by which nerves are known in certain circumstances to excite muscles to contraction.
The following experiments, made upon animals considered by anatomists, in general, as destitute of nerves, may to some appear decisive of this question, but to myself, I confess, they are by no means so. In by far the greater number of animals, we are precluded from the possibility of discovering nerves by their minuteness; yet the actions of these animals, not merely excited by mechanical irritation, but so obviously directed to the attainment of an end, oblige us to infer their existence even where our senses, aided by the best glasses, do not enable us to detect them.
Having laid some earth worms upon a plate of zinc, I tried to excite contractions in them, by passing a rod of silver over different parts of their length, till it came in contact with the plate; but for a long time without producing any effect. Application of the metals to a part recently divided seemed to produce as little effect. At length, I perceived one of them dart itself forwards, whenever the silver was passed under its belly near to a part which had been divided and rejoined. On repeating the experiment again, and with more care, I found, (as in the frog,) that when the animal was perfectly lively, and upon its guard, no contraction could in this way be excited; but that when a part had been rendered more sensible by previous disease, recent irritation, &c. or when the worm was taken unawares by hanging it over a probe, and lowering both upon the plate at the same instant; a sudden and involuntary motion seemed to dart through a great part of the worm’s length from the part touched towards the head; a direction contrary to that in which it takes place in other animals. I never could produce the same effect upon leeches. On varying the experiment, a most whimsical, but satisfactory phenomenon presented itself. I had laid a leech upon a crown piece of silver, placed in the middle of a large plate of zinc. The animal moved its mouth over the surface of the silver without expressing the least uneasiness; but having stretched beyond it and touched the zinc plate with its mouth, it instantly recoiled, as if in the most acute pain, and continued thus alternately touching and recoiling from the zinc, till it had the appearance of being quite fatigued. When placed wholly upon the zinc, it seemed perfectly at its ease; but, when at any time its mouth came in contact with the silver lying upon the zinc, the same expression of pain was exhibited as before.
With the earth worm, this experiment succeeded still more decisively. The animal sprang from the zinc in writhing convulsions; if, when the worm stretched itself forwards, one of its folds lit upon the zinc, it expressed little uneasiness in comparison of what it shewed when the point of its head touched the zinc.
These extraordinary effects were, however, considerably different from those produced by the metals upon the limbs of frogs, and other animals. They had not so much the appearance of involuntary, instantaneous convulsions, as long continued expressions of pain and disgust; such as are produced by applying zinc and silver to the tongue of a child.
A strong presumptive proof, in my humble opinion, that these animals are endowed with a most exquisite organ of sense, and, consequently, that they are not, as has been supposed, destitute of a nervous system.
Doubtful, therefore, if this influence can ever act upon the muscular fibre, except through the medium of nerves, I shall reserve what I have to say upon particular muscles, till I have related some facts relative to the nerves.
OF THE NERVES
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 name11, 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 drowning12. 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.
The sensation is most distinct when the tongue is of its ordinary temperature, and when the metals are of the same temperature with the tongue. When either the tongue, or the metals, or both, are heated or cooled, as far as can be borne without inconvenience, scarcely any sensation is produced. That this difference in the effect is owing to the alteration which has been produced in the state of the tongue, and not to that in the temperature of the metals, is evident from experiments which I have already related; from which it appears that neither the conducting, nor the exciting powers of metals are affected by differences of their temperature. But I have found it the uniform result of many experiments, that both the life and irritability of the most vigorous frogs is completely destroyed in a few minutes, by placing them in water heated to 106 degrees of Fahrenheit’s scale.
Cold, however, though it appears to affect the sensibility of the tongue nearly as much as heat, did not, in one or two instances in which I tried it, affect the irritability of the muscles of a frog. Some separated legs contracted equally well after they had lain upon a piece of ice for some hours, as they did before they had been in that situation.
Whatever has a tendency to blunt the sensibility of the tongue, as laudanum, a strong solution of opium in water, distilled spirits, acids, &c. diminishes the effect of the metals. Acids, I think, diminish it least.
On placing different metals in the meatus auditorius externus of both my ears, and establishing an insulated metallic communication between them, I felt, or fancied that I felt, a disagreeable jirk of my head. The metals used were a silver probe, a roll of tin-foil, and a common brass conductor belonging to an electrical machine. On withdrawing them from my ears, I experienced a feeling similar to that which one has after emerging from under water. I was not sensible of having hurt my ears by the experiment, nor had I any uneasy sensation after it; but, on getting out of bed next morning, I perceived both my pillow and my face stained with blood; and, on examining, found that it had come from one of my ears. An hæmorrhagy from this part had never happened to me before. From whatever cause this accident happened, (and it is highly probable that it arose from some hurt unperceived at the time), I need not say, that I have never repeated the experiment, and that I certainly never shall.
I never could perceive, that the senses, either of touch or of smell, were in the least affected by the metals; but the effect which they produce upon the eye is very remarkable. Having laid a piece of tin-foil upon the point of my tongue, I placed the rounded end of a silver pencil-case, against the ball of my eye, in the inner canthus, and suffered them to remain in these situations till the parts were so far accustomed to them, that I could examine the sensations produced; I then brought the metals into contact with each other, and, to my surprise, perceived a pale flash of light diffuse itself over the whole of my eye. My tongue was at the same time affected with a similar sensation to that produced when both the metals are in contact with it. On darkening the room, the flash became more distinct, and of a stronger colour. This sensation is not the effect of pressure upon the eye, as in Sir Isaac Newton’s experiment; for no pressure should be used. All that is required, is, that the silver lie between the lids of the eye, and in contact with any part of the ball. If the experiment be made with zinc and gold, instead of tin-foil and silver, the flash is incomparably more vivid. I had the disagreeable opportunity of trying this experiment upon one of my eyes, in a state of inflammation; and, in this case, found the flash much more strong than it was in the uninflamed eye. I tried it likewise upon a patient, affected with amaurosis; but the man was so stupid that I could not satisfy myself as to the precise result.
Recollecting that fine nervous twigs pass from the ciliary or ophthalmic ganglion, through the sclerotic coat of the eye, to the choroid coat, and to the uvea; and that this ganglion is in great part formed from a twig of the nasal branch, of the fifth pair of nerves, in conjunction with a branch of the third, I proceeded to try if, by insinuating a rod of silver, as far as possible, up my nose, and thus arming this nasal branch, I could, by bringing the silver into contact with a piece of zinc, placed upon my tongue, pass this new influence up the course of the nerve, and thus produce the flash in the eye. The experiment answered my most sanguine expectation. The flash, in this way produced, is, I think, if any thing, stronger than when the ball of the eye itself is armed. I now thought I had discovered a certain method, by which I could ascertain the effect of Galvani’s influence, upon a very important, involuntary muscle, the human iris. It occurred to me that the ingenious physiologist Dr Whytt, had been able, through the medium of the nasal branch of the fifth pair of nerves, to produce, at pleasure, dilatations of the contracted pupil of a boy, in the last stage of hydrocephalus, by applying aq. ammonia to his nostrils; and this instance of the affection, of an involuntary muscle, through the medium of its nerves, had, previously to making any experiments upon the subject, always operated with me as a strong presumptive argument, that the contractions of the heart might be influenced in a similar manner.
I therefore desired some of my friends to observe my pupil, while I repeated the experiment, which I have above described. When the external light was strong, they found some difficulty in determining, whether the pupil contracted or not; but when no more light was admitted, than what was just sufficient for discerning the pupil, they perceived a very distinct contraction, every time the metals were brought into contact with each other. This experiment requires some attention, in order that it may succeed satisfactorily; but although I have repeated it a great number of times upon the eyes of others, it has seldom failed, when made in a steady light, and when the silver has been passed far enough up the nose.
The dilatation of the pupil, instead of its contraction, on the application of a stimulus to its nerves, as in the case related by Dr Whytt, is, I apprehend, not so uncommon a circumstance, as it may at first be supposed. I have myself seen three instances of it in diseases of the head. One of these was in an epileptic patient, whose pupils, during the intervals of his fits, became suddenly dilated whenever his eyes were exposed to a strong light.
My friend, Mr George Hunter of York, while one day amusing himself with repeating some of these experiments, discovered that by placing one of the metals as high up as possible between the gums and the upper lip, and the other in a similar situation with respect to the under lip, a flash was produced as vivid as that occasioned by passing one of the metals up the nose, and placing the other upon the tongue. It differs, however, from the flash produced in any other way, in the singular circumstance of not being confined to the eye alone, but appearing diffused over the whole of the face. On attending to the concomitant sensations produced by this disposition of the metals, I perceived that a sense of warmth, at the instant they were brought into contact, diffused itself over the whole upper surface of the tongue, proceeding from its root to the point. Dr Rutherford, to whom Mr Hunter had communicated this experiment, remarked, on repeating it, that a flash is produced not only at the instant the metals are brought into contact, but likewise at the instant of their separation. While they remain in contact, no flash is observed.
This fact is precisely analogous to one already mentioned of contractions being produced in the leg of a frog, at the instant one of the metals in contact with the other metal is withdrawn from the leg.
