OF THE BLOOD VESSELS.
We are told by Dr Valli, that no contractions are excited by arming the blood vessels; but as he has not told us whether his experiments were made upon them while the blood still continued to flow through them, or after they had been deprived of their blood, I determined to make the following experiment.
Having laid bare, and separated from surrounding parts and from each other, the crural artery, and nerve, in the thigh of a full grown frog, I cut out the whole of the nerve between the pelvis and the knee. I then insinuated beneath the artery a thin plate of sealing wax, spread upon paper, and broad enough to keep a large portion of the artery completely apart from the rest of the thigh. The blood still continued to flow, through the whole course of the artery, in an undiminished stream. The artery, thus partially insulated, was touched with silver and zinc, which were then brought into contact with each other; but no contraction whatever was produced, in any muscle of the limb. This experiment was frequently repeated upon several different frogs, both in whom the nerve was, and in whom it was not, divided. The result was uniformly the same. But vivid contractions were produced in the whole limb, when an electrical spark, or even a full stream of the aura, was passed into the artery.
It, however, by no means follows from this experiment, that the sanguiferous system of animals bears no relation whatever to the influence discovered by Galvani. I have already shewn, that the heart may be affected by it, and have given reason to believe, that the smallest arteries of the body are not exempted from its action. Should it ever be proved to be an exclusive property of animals, it is not impossible but that even its origin may be traced to their sanguiferous system.
[11]. I have not been at the pains to inform myself, who first was the author of this doctrine; but its adoption by Caldani, by Haller, and by Fontana, and by all upon the faith of experiment, was certainly sufficient to give it currency, in opposition to that of Willis, Lower, Kaau, Boerhaave, Laghi, and even of the ingenious Whytt.
[12]. Mem. Med. Soc. Lond. vol. iii.
SECTION IV.
An attempt to investigate the Source from which the respective
Powers of Nerves, and of Muscles, are derived.
As yet, the question whence the nerves and muscles of animals derive their respective properties, remains in a state of doubt. By many, the brain has been considered as the source not only of the several energies exerted by nerves, whether appropriated to sensation, to the excitement of muscles subservient to the will, or distributed to organs exempted from its influence; but likewise of that unascertained power, by which muscles contract on the application of a stimulus.
By others again, these several properties are supposed to be derived from the arteries, which may either supply the materials and construction of that exquisite and peculiar organization, which fits nerves and muscles for performing their respective functions, or may furnish, from the blood, some subtile principle, such as that believed by M. Fontana, to exist there, or such as that we are now examining, which differently modified in different parts, may be the latent cause of all the phenomena exhibited by animals.
The advocates for the first opinion observe, that whenever the brain is considerably injured, or its free communication, by means of nerves, with moving parts is interrupted, a deprivation both of sense and motion is the uniform consequence: and, further, that the several organs, both of sense and of motion, appear to suffer detriment from the over strained exertions of the brain in thinking, equal to that which they experience from their own exertions.
The second opinion is countenanced by facts and observations not less important. From experiments of Haller; some which are recorded in one of the early volumes of the Philosophical Transactions, and others, it appears that a paralysis of the posterior extremities of animals was induced by tying their aorta.
Both Dr Monro and Dr Alexander of Halifax have remarked, that when all the blood vessels, supplying the posterior extremities of frogs, had been divided, and a solution of opium injected under the skin of these extremities, they became, in less than half an hour, both motionless and insensible; whereas, the fore part of the body was not observably affected six hours afterwards; and, in Dr Monro’s experiments, the frogs lived till the day following. Hence Dr Monro concludes, ‘that concomitant arteries, somehow or other, tune the nerves, so as to fit them to convey impression[[13]].’
On the other hand, where it is intended that nerves shall convey impressions with great accuracy, as in all the senses, and very remarkably in the part which some have amused themselves by considering as a sixth organ of sense, the distribution of blood vessels is more profuse than in almost any other equal part. It is likewise universally true, that increase of vascular action in a part is always attended with a proportional increase of sensibility there.
From the valuable experiments made by Mr Cruikshanks, and which have since received the fullest confirmation from those repeated by M. Fontana and others, it appears, that whatever may be the relation between brain and nerves, the latter may certainly be regenerated after excision, and have their functions fully restored. Now, in what manner this can be accomplished, unless by the agency of arteries, would, I imagine, be no easy task to point out.
The influence discovered by Galvani appeared to me an admirable test, by which something decisive might be ascertained relative to these important points in the physiology of animals, and as such I have employed it in the following experiments.
Considering, therefore, the brain on the one hand, and the sanguiferous system on the other, as the possible sources from which nerves and muscles might derive their power, I began by comparing the effects which result from interrupting their communication, first with the brain, and then with the arteries. This mode of procedure seemed to afford the best prospect of information with respect to every object which I had in view, but particularly with regard to the relations which this influence may bear to the several parts examined.
Before relating the experiments, I must observe that the comparison was instituted between the effects of only partially interrupted communication; since it must be obvious that a complete interruption, either of nervous or of arterious communication between any part of an animal, and the rest of its body, could not have been effected without so far injuring the animal, as to render the result fallacious.
Experiments in which the Sciatic Nerves of Frogs were divided.
EXPERIMENT I.
I divided the sciatic nerve, on one side only, in four large frogs. The division was made at the very top of their thighs, and before the nerve had given off the first large branch to the muscles of the thigh. This nerve lies immediately underneath the large crural artery, to which it is closely attached by a sheath of fine but very strong cellular membrane. A small nerve, which supplies some of the muscles on the under side of the thigh, was suffered to remain undivided. The legs, whose nerves had been divided, became completely paralytic below the knee, and very nearly so above it. These legs too, immediately after the division of their nerves, contracted vigorously when laid upon zinc, and excited by passing a rod of silver in contact with the under part of the knee till it touched the zinc; but the other legs which were suffered to remain in their natural state, in order that the contractility of one leg might all along be compared with that of the other, did not contract when the metals were similarly applied to them.
These frogs were all killed by cutting off their heads; the first, at the end of two days after dividing the nerve; the second, at the end of five days; the third, at the end of seven; and the fourth, at the end of nine. Their legs were carefully examined, in the manner I have described, four or five times every day after their heads had been taken off, so long as any contractions could be excited; but I could not perceive, in any one of these instances, that the contractile power continued either longer or more vigorous in the legs, in which the nerves were not divided than it did in those in which they were.
Both in these experiments, and in all my others, where a comparison was instituted between the two legs of the same frog, I divided equal portions of skin on both thighs, that there might be no unequal exposure of the muscles to the water, which would have occasioned a fallacy in the result.
On the 31st of March last, I divided, in two, a frog, in one of whose legs I had four months before excited inflammation, by laying bare the crural artery and nerve. The inflammation had been so violent and general, that the frog lost its cuticle in consequence of it, and, when compared with a healthy frog, its resperation was observed to be remarkably frequent. Three weeks after this, when the wound in its thigh had perfectly skinned over, I laid it open again, and divided the sciatic nerve. No general inflammation this time took place, nor did the wound again skin over; but for about a month before it was killed, a large ulcer had formed immediately over the division of the nerve, but had not proceeded down to it. The limb, at the time I killed the frog, was as destitute both of motion and of sensation, as at the first instant the nerve was divided, but contractions were excited in it, by touching the ulcer with zinc and silver. When the frog was dead, however, the contractions were found much more feeble in this than in the other leg.
The metals were now applied to the sciatic nerves within the abdomen. Vigorous contractions were excited in the sound leg, but none in that whose nerve had been divided. Hence it was plain, that no actual regeneration had taken place. On examining the nerve accurately at the part divided, I found the divided ends, which had receded considerably from each other, connected by a transparent gelatinous substance. From the upper end, which appeared elongated into a conical form, several red streaks projected into the interposed substance. The lower end was opaque, thickened, and rounded. No appearance of spiral bands could be detected, either in the interposed substance, or in the part of the nerve below the division, when these parts were examined with the assistance of a microscope. This substance had attained sufficient consistence to support the under part of the nerve, when the upper was raised with a pair of forceps. The leg, in which the nerve had been divided, continued to contract as long as the other, though much less vigorously, and the part, from which I could longest excite contractions, was the ulcer.
On the 14th of April last, I killed two other frogs, by dividing their hind extremities from their bodies. In one, the right sciatic nerve had been divided more than six weeks previous to its death. In the other, one of the sciatic nerves had been divided between three weeks and a month.
The legs of these frogs, examined by the metals both before and after their separation from the body, were found in a state very different from those before spoken of. The contractions were scarcely perceptible. The incisions made through the skin, in order to get at their nerves, had closed completely in less than a week after they had been made.
The appearance of the muscles in the legs, whose nerves had been divided, was found to be precisely the same as in those where nothing had been done; but, notwithstanding this circumstance, even strong electrical sparks excited but very feeble contractions. On examining the nerves, the ends of that which had been longest divided were found connected by a substance not at all resembling nerve, but similar to that found in the former experiment, and evidently proceeding from the upper division. In the nerve which had not been so long divided, this circumstance was still more apparent, as the substance had not extended quite to the lower division. The cellular membrane surrounding these upper divisions had the appearance of innumerable vessels finely injected, and some red streaks were seen projecting, as if from the nerve itself, into the gelatinous production. In the sound nerves, the obliquely transverse lines of alternate opacity and transparency, or, as Fontana has called them, the white spiral bands of nerves, were seen distinctly at the first glance of the eye, and without the assistance of a glass; but no appearance of these could be found in the parts of the divided nerves below the division; these were uniformly opaque. Their bulk, however, was not in the least diminished. The organization of nerves long divided, therefore, undergoes a very evident alteration, although it is by no means so clear that the same change happens in the muscles, to which these nerves are distributed. Yet their susceptibility to the action of electricity, as well as to that of this new influence, was nearly lost. Some may consider this as an additional argument, that stimuli act upon muscles only through the medium of nerves.
I have before observed that muscles of frogs, from whom the skin has been stripped, become in a short time hard when exposed to the action of water. Wishing, therefore, to see if there would be any difference between these legs, whose nerves had been divided, and others, in this respect, I laid them in water, and examined them every ten minutes, but both became hard nearly at the same time. Mr Allen, a gentleman well versed in physiological pursuits, was with me when I examined the alteration which had taken place in one of these nerves, in consequence of its having remained long divided, and I had afterwards an opportunity of shewing it to Dr Rutherford. In all the frogs, whose nerves I have divided, I have observed that the divided extremities, though placed in most exact contact from each other, had after a time receded at least 1/12 of an inch from each other.
Experiments in which the Crural Arteries of Frogs
were tied as near to the Trunks of their Bodies,
as where the Nerves had been divided in the former Experiments.
EXPERIMENT I.
Both crural arteries of a full grown frog having been laid bare, one of them was tied. The leg, in which this was done, became instantly weaker than the other, and rather dragged when the animal was put into water. The frog, however, could still jump about with great agility. Four hours after this operation, it was killed by crushing its brain. It continued to move its legs spontaneously, when touched, during more than two days after this, and contractions were excitable by the application of the metals for two days longer. Sometimes it appeared rather doubtful, which leg contracted most vigorously, but, in general, the leg in which the artery remained free did so, and contractions could be excited in it, more than an hour after every means to excite them in the other leg had failed.
Ligatures were passed round the crural arteries of two other frogs, and one of them was suffered to live thirty six hours afterwards, before its head was crushed: the other four days. In these, the disproportion between the vigour and continuance of the contractions in the compared legs, was so much greater than in the preceding experiment, as to leave no doubt of the effects produced by tying an artery. The leg, whose artery had remained tied four days, never contracted near so strongly as its fellow, and contractions had ceased to be excitable in it, upwards of twenty hours before they had ceased in the leg, whose artery had not been tied.
From these experiments, it appears decidedly, that a much greater detriment to that condition of a limb, upon which contraction depends, is induced by interrupting its circulation, than by intercepting its communication with the brain.
But still, as the effects arising from the interception of the influence of the brain, and of the circulation, were not compared with each other in the same but in different animals, whose age, relative strength, &c. might possibly differ, I thought proper to repeat the comparison, in the following manner.
Experiments in which the Sciatic Nerve was divided on one side,
and the Crural Artery tied on the other.
EXPERIMENT I.
I divided the sciatic nerve of one leg, and tied the crural artery of the other, in a large frog. Scarcely any blood was lost in doing either. Two days after this, I strangled it. During the first 24 hours, the leg, in which the nerve had been divided, appeared to contract with most vigour; after this period, the difference between them became more doubtful; but the contractions were at no time stronger in the leg, whose artery was tied, than in that whose nerve was divided.
The same operations were performed upon a large female frog full of spawn. Four hours afterwards, she was observed covered by a male, who had been treated in a similar manner. I mention this circumstance, as it tends to prove that the pain occasioned by the operation was probably not so great as to produce much fallacy.
On the day following, she had spawned, and on the sixth day from the operations, she was strangled. When laid upon a plate of zinc, and excited by means of a rod of silver, the contractions were found extremely feeble in the leg whose artery had been tied, and ceased altogether in about twenty-two hours after her death. In the leg, whose nerve had been divided, they appeared as vigorous as they usually are in legs to which no injury has been previously done, and continued excitable upwards of two days after they had ceased to be so in the other.
Having tied the crural artery on one side, and divided the sciatic nerve on the other, on three full grown male frogs, I strangled them all on the sixth day following. My motive for killing the frogs, subjected to such experiments, either in this manner or by crushing their heads, will be obvious. It was of consequence to preserve their circulation as entire as possible, and, at the same time, avoid the continuance of pain, which by exhausting all the parts of the body, whose communication with the brain was not interrupted, might considerably have affected the result of the experiments.
The contractions excited by means of the metals, were, in all these instances, likewise as much more strong and durable in the legs, whose nerves had been divided, than what they were in the legs, whose arteries had been tied, as what I had found them to be in the preceding experiment.
Having thus found, that a diminution of the circulation of a part, was accompanied with a proportionable diminution of the respective powers of nerves and muscles in that part, I next proceeded to examine if an increased circulation would be attended with a proportionable increase of these powers. That this is actually the case, with respect to the nerves, the few facts which I have related of the eye, in a state of inflammation, have a tendency to prove; and we all know how much the sensibility of every part of the body is increased, by an increase of vascular action. That a similar relation subsists between an increased action of the arteries, and the contractile power of muscles, is, I think, proved by the following experiment.
Experiments made with a view of ascertaining
some of the Effects of Inflammation.
EXPERIMENT I.
I have before said that if a living and entire frog be set upon a plate of zinc, contractions can very seldom be produced in any part of its body by passing a rod of silver over it, so that the silver, the frog, and the zinc, may be all in contact with each other. But, I have found in upwards of twenty experiments, that when inflammation had been excited in one of the hind legs of a frog, by irritating it with a brush, contractions uniformly took place in that leg when the metals were applied to it, although none had been produced in it before it was inflamed, nor could still be produced in the other leg which remained in its natural state.
Having previously excited inflammation, by means of a brush, in the foot and leg of a healthy and large frog, I cut off its head. The contractions excited by the metals in the inflamed leg were in vigorous and instantaneous jirks; those in the sound leg more languid and difficultly excited. Spontaneous motions continued at this time nearly the same in both. Till the end of the second day, after this frog’s head had been taken off, the contractions excited in the inflamed leg continued uniformly, and beyond all comparison more vigorous than what I could by any means excite in the sound leg. But, after this time, the inflamed leg became hard as a piece of wood; probably in consequence of the effusion to which the inflammation had given rise.
The event of five similar experiments was so nearly the same, that I should be thought unnecessarily minute, were I to relate them in detail.
We are now perhaps prepared to account for the deficiency of contractile power in those legs, whose sciatic nerves had been divided, the one, between three weeks and a month, the other, six weeks, compared with its continuance in the leg, whose nerve had been divided upwards of three months. It appears, from the circumstances of those experiments, that some of the arteries, appropriated to the supply of the sciatic nerves of frogs, have the same course with the nerves themselves; since the deposition of new matter could in all be traced from the upper division of the nerves. It is obvious, therefore, that the part of the nerves below the division, must have been deprived of so considerable a portion of their usual arterial supply, as in time would occasion some alteration in their structure, and consequently in their powers. We accordingly find that such alteration of structure, and such deficiency of power, had actually taken place. It is further probable, that, in proportion as the supply from the arteries was restored, the powers of that nerve, which had been three months divided, had been likewise restored. This supposition is countenanced by every instance in which nerves are reproduced; as we find the functions of the parts in which they had been divided, are not immediately, but gradually restored.
M. Fontana seems too hastily to have adopted the opinion, that the sciatic nerves, when divided, are probably never reunited by truly nervous structure, because no reunion took place during the very short period which he suffered to elapse between their division, and their subsequent examination. In the experiments, which I have related, the progress towards reunion seems to have borne a very exact proportion to the time the nerves had remained divided; and, in an experiment related by Dr Monro, where the sciatic nerve of a frog had been divided a year previous to the death of the animal, the reproduction was advanced so far as to have the appearance of being perfect. Nor can I doubt, that both the sensibility and the motion of the limb would have been restored, had the animal been permitted to live a sufficient length of time. The following fact renders the supposition at least extremely probable.
In the first volume of the Edinburgh Medical Essays, the case of a Captain of a man of war is related, who entirely lost the use of his right arm, in consequence of a gun-shot wound received in his neck. The circumstances of the case are such as leave no reason to doubt, that the loss of the power of motion, in this gentleman’s arm, was owing to the division of the cervical nerves proceeding to the arm: yet both the full use, and strength of this arm, were restored, after a period of about two years and a half. A proof perfectly satisfactory that an actual regeneration of nerves had, in this case, taken place; and if in this, one sees no reason why it should not equally take place in any other part of the body.
It might be difficult to assign a satisfactory reason for the very speedy reproduction of the intercostal, parvagum, and recurrent nerves, when compared with the great length of time required for the reproduction of others. May it not be owing to the very profuse manner in which they are supplied with arteries, probably both in an ascending, and in a descending direction; from above, by the superior, and from below, by the inferior laryngeal arteries?
It appears upon the whole, therefore, tolerably certain, that the sanguiferous system contributes more immediately than the brain to the support of that condition of muscles and of nerves, upon which the phenomena of contraction depend; since that condition is much more injured by intercepting the influence of the former than of the latter.
Every experiment and observation, which has been made upon the subject of nutrition, and of the reproduction of parts, clearly demonstrates that nerves and muscles, in common with every other part of the body, derive their structure from the arteries; and it is evident, that upon this structure their several properties must in some measure depend. But M. Galvani’s discovery of a subtile influence, which may be transmitted apparently from one part of an animal to another through foreign media, may reasonably give rise to a conjecture that the phenomena exhibited by nerves and by muscles may perhaps depend more immediately upon some such influence; and reasons exist, which might induce some to suspect that even this is derived from the blood.
Experiments suggested by some opinions of M. Fontana.
From the greatest number of experiments, perhaps, ever made by one physiologist, M. Fontana has been led to conclude, that the venom of the viper, opium, and several other poisons, which he examined, produce no effects whatever, when applied immediately to nerves and muscles alone, but that they destroy life, by exerting their influence upon some subtile principle existing in the blood.
Independent of the experiments, published by M. Fontana, on this subject, his opinion respecting the existence of such a principle may be thought to receive no inconsiderable countenance, from the opinions of Harvey and of Mr Hunter, concerning the life of the blood, and from those experiments, by which Mr Hewson has demonstrated, that changes are instantaneously produced upon the coagulability of the blood, by passions of the mind, and whatever else affects the action of the heart and arteries. An experiment made by Dr Alexander of Halifax, and published at this place in the year 1790, in his excellent Thesis, ‘De partibus corporis quae viribus opii parent,’ may at first appear a sufficient refutation of M. Fontana’s opinion.
He found that thirty three drops of a strong solution of opium in water, injected into the jugular vein of a large rabbit, destroyed it, as in M. Fontana’s experiments, in four minutes and a half; whereas, the same quantity injected into the crural vein in each leg of another rabbit, with an interval of twenty six minutes between the two injections, although it rendered the animal sleepy and stupid for a few hours, did it no material or permanent injury. Hence, Dr Alexander concludes, that the opium, injected into the jugular vein, did not destroy the animal by acting upon the blood alone, since if it had, the same effect, should have been produced, by introducing an equal quantity into any other vein of the body; but a quantity double of that, which had occasioned death when introduced into the jugular vein, failed to occasion it when introduced into the crurals.
It is not, however, by one experiment, formidable as it must be allowed to be, that the innumerable hosts brought to the contest by M. Fontana ought to be combated. Besides, it might be objected even to this one, that the opium was introduced into veins, from which it must have been so much longer in passing to the arterial blood, than from the jugular vein, and consequently so much more diluted, and perhaps too altered in its nature before it got there, as might be sufficient to account for the difference of result in the two cases compared.
The opportunity afforded by M. Galvani’s discovery, of putting the truth of the opinion held by M. Fontana more fully to the test, and the possibility which presented itself, that if any such principle, as he supposes in the blood, should really be found to exist there, it might prove to be identically the same with that discovered by M. Galvani, induced me to make the following experiments.
Having selected two frogs as nearly as possible of the same size and vigour, I deprived one of its blood by opening, first, one of its crural veins, then, a crural artery, and last of all, the heart. To assure myself of the complete evacuation of its blood, I next injected water into its heart, and immediately afterwards forty drops of a strong aqueous solution of opium[[14]].
I then removed the sternum of the other frog, and having made an opening into the ventricle of its heart, injected into it likewise forty drops of the solution. Less blood was effused in doing this, than one would at first expect; for the ventricle contracts so strongly, immediately after the incision, as to prevent much blood from passing out, unless the incision be made, as it was in the other frog, purposely large.
The moment, at which each injection was made, was accurately noted, and the time expended in evacuating the blood from the first frog, was allowed for. The frog, from which the blood had been withdrawn, ceased to contract, when irritated, very nearly an hour before the other, even calculating not from the time of injection, but from the moment I began to bleed it; nor could I by means of the metals excite contractions in it, for upwards of a day before they had ceased to be excitable in the other frog.
As evacuating the blood from a living animal is rather a severe operation, and might have occasioned some fallacy in the last experiment, by subjecting the frog, in which this was done, to a greater degree of pain, and consequently of exhaustion, than what the other was subjected to, I crushed the brains of two other frogs before I proceeded, as in the former experiment, to withdraw the blood from one of them. Instead of forty, I injected no more than thirteen drops of the strong solution of opium, into each of the hearts of these frogs. The instant the injection had entered, both hearts became white, and ceased from contracting. Forty eight hours after the injection of the opium, the contractions excited by the metals in the frog, deprived of its blood, had become very slight, particularly in the limb whose vein and artery had been opened. The other frog still continued to contract with so much vigour, as to raise its body from the plate of zinc, upon which it was laid. Seventy two hours after the injection, no contractions could be excited in the frog, from which the blood had been withdrawn, except some very slight ones in the leg, whose artery and vein had not been opened. The contractions in the legs of the other frog, continued still so vigorous as to raise its body from the plate, and some were produced even by mechanical irritation.
Ninety six hours after the opium had been injected, (both the frogs having lain out of water all night,) that without blood was found quite putrid. In the other, the contractions, produced by exciting the legs, were sufficiently strong to move the feet: as the body, however, had become putrid and offensive, it was thrown away.
EXPERIMENT III.
The heads of two other full grown and lively frogs, having been crushed, their hearts were laid bare, and the blood was evacuated from one of them, as in the former experiments. A small portion of the skull of each then being removed, eight drops of the strong solution of opium was injected upon their brains. At least half the quantity seemed to return from the wound. Both frogs became instantaneously motionless after the injection, but, in about an hour, were considerably recovered.
Spontaneous motions continued during more than fifty hours, in the legs of that from which the blood had not been drawn, and contractions were excitable by the metals, upwards of 24 hours after they had ceased to be so, in that from which the blood had been drawn.
The following experiments may be deemed still more satisfactory, than the preceding, from the circumstance of the comparison having been instituted, between the effects of opium, upon different, but similar parts of the same frog, differently circumstanced.
One of the crural arteries of a frog having been included in a tight ligature, as near as possible to the body, I suffered four days to elapse, and then injected through a perforation in its skull, eight drops of the strong solution upon its brain, and in a direction towards its spinal marrow. This frog continued most violently convulsed for more than an hour, and, in two, was to all appearance dead. When laid upon zinc, and excited with silver, the contractions were not at first perceptibly stronger in one leg than in the other. After eight hours, however, they were evidently most strong in the leg whose artery remained free. After 21 hours, this difference became still more decided. At the end of 34 hours, scarcely any contractions could be excited in the leg whose artery had been tied; though they continued vigorous in the other; and, at the end of 46 hours, they had ceased altogether to be excitable, in the leg whose artery was tied. In the other, they continued during several hours afterwards.
EXPERIMENT V.
Having tied one of the crural arteries of another frog, I filled its stomach, immediately afterwards, with a saturated solution of opium in water. The difference between the strength, and the continuance of the contractions, excited by the metals, in the two legs of this frog, was not so great as in the former; yet still the difference was considerable in favour of that leg in which the artery remained free.
In two other frogs, in each of which a crural artery had been tied, and the solution of opium (without regard being paid to quantity), repeatedly injected underneath their skulls immediately after; the contractions appeared to be very little weaker in the legs, whose arteries were tied, than what they were in the legs in which they were not tied, and they continued excitable during an equal length of time in both.
Having tied the crural artery of another frog, I immediately filled both its stomach and abdomen with a strong solution of opium. In an hour after this, it was to appearance quite dead. At the end of eight hours, the contractions, excited by the metals, had become very feeble in the leg whose artery was tied, in comparison of what they were in the other leg; and, at the end of twelve hours, no contractions could be excited in any part of the frog, except in the leg whose artery remained free. In this they continued excitable about an hour longer.
As it was possible, that the more speedy exhaustion of the legs, in which the arteries were tied, might have been owing in some measure to the pain, occasioned by that operation, I repeated the experiment with the following variation.
EXPERIMENT VIII.
I first divided the sciatic nerves, in both legs of two frogs, and then tied the crural artery in one leg of each. Eight drops of the solution of opium were immediately afterwards injected upon their brains. But the event of this experiment was precisely the same with the majority of those before related. The contractions excited by the metals, in the legs whose arteries were tied, were uniformly more feeble, and of shorter duration, than those excited in the other legs: yet it is evident, that, in all these experiments, the very reverse of this ought to have taken place, had it been true, as M. Fontana has asserted, that opium has no effect upon any part of the body, except through the medium of the blood.
The experiments however, which I am now to relate, may perhaps appear still more satisfactory.
EXPERIMENT IX.
Having laid equally bare both the sciatic nerves of a frog, at the upper part of its thighs, I passed a ligature round one of them, and drew it as tight as it was well possible, without dividing the nerve. I then removed a portion of its skull, and with a small brush, kept it constantly wet with laudanum during several hours. The frog soon became convulsed; and, during ten or twelve hours, continued in that state of exquisite sensibility, which opium never fails to produce in these animals. It may here be worth remarking, that, while they are in this state, the slightest touch of a feather, or even breathing upon them, excites instantaneous convulsions. The leg whose nerve was tied, remained paralytic during this time, but when it was laid upon zinc and excited with silver, it contracted as strongly as the other. After forty three hours, the contractions were very feeble in the leg whose nerve was not tied, but still vigorous in the other. After fifty three hours, no contractions could be excited in any part of the frog, except in the leg whose nerve was tied. In this they were sufficiently strong to move the foot, and continued so for more than an hour longer.
One of the crural nerves of another frog having been tied in a similar manner, eight drops of the strong solution of opium were injected upon its brain. The animal instantly became motionless, but, in less than an hour afterwards, was considerably recovered.
The contractions, excited by the metals, in the leg whose nerve was free, soon became more feeble than those excited in the leg, whose nerve had been tied. This disproportion, between them, continued increasing during ninety six hours, after the opium had been injected, when contractions could no longer be excited in the leg whose nerve remained free. In that, in which the nerve had been tied, they continued upwards of 4 hours afterwards.
EXPERIMENT XI.
Immediately after having divided the sciatic nerve, in one thigh only, of three other frogs, I injected as much of the strong solution of opium underneath their skulls, as could possibly be retained. The legs, in which the nerves had been divided, continued contractile several hours after the others had ceased to be so.
Hence, then, we see no reason for suspecting that the more speedy cessation of contractions in those legs, in which the crural arteries were tied, than in those on which no operation was performed, was owing to the pain occasioned by such operation, since even the more painful operations of tying or dividing the sciatic nerves, were attended with no such effect.
Upon the whole, therefore, it appears, that the conclusion which M. Fontana draws from his numerous experiments with opium, ‘That the circulation of the blood and humours in the animal machine, is the vehicle for opium, and that, without this circulation, it would have no action on the living body,’ is the very reverse of that which I am warranted to draw from the experiments I have just related; since the parts, most affected by the action of opium, were not those in which the circulation remained most entire, but those in which it had been almost altogether interrupted; and since in two parts where the circulation remained equal, and entire, the action of opium was rendered unequal, by interrupting the communication of one of them, by means of nerves, with the parts to which the opium was applied.
The existence, consequently, of any such principle in the blood, as that supposed by M. Fontana to exist there, is rendered far too problematical, even to allow me to expect that it can ever be proved: far less that it may turn out to be the same with that discovered by M. Galvani, or with that, whatever it may be, upon which the phenomena of nerves and of muscles may depend.
[13]. Essays Physical and Literary.
[14]. This solution, which is the same that I employed in all my subsequent experiments, was of the same strength with that used by Dr Alexander in the greater number of his, viz. an ounce of crude opium mixed in a mortar with two ounces and a half of water, and filtered through paper, after having remained twelve hours, in a close corked bottle, near a chamber fire.