Experiments and Observations

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.

After this full detail of these curious phenomena, I hardly need remark, that they demonstrate the free communication, which subsists between the several branches of the fifth pair of nerves, and consequently give strong support, if not absolute confirmation, to the well known doctrine of nervous sympathy, or of the reciprocal influence, which different parts exert upon each other, through the medium of nerves.

If I might be allowed to hazard a conjecture, where we cannot have recourse to demonstration, I should say that the flash, observed in the above experiments, was the effect of contractions excited in involuntary muscles by the application of a stimulus to their nerves; or, in other words, that the effects of the application of the metals to the nasal branch of the first division of the fifth pair of nerves, had been propagated through the ciliary ganglion, along the ciliary nerves, and to the choroid coat, whose vessels it had excited into instantaneous action; and that their action again (as in the case of action excited by pressure, or a blow upon the eye,) had by stimulating the retina occasioned the sense of light.

This supposition is, I think, rendered probable by several considerations. I have already shewn that this influence can excite contractions in involuntary muscles, through the medium of their nerves. And certainly no reason can be assigned, a priori, why it should not act equally upon every description of involuntary muscles; upon those which make a part of the minutest vessels in the body, as well as upon the heart, or upon the iris.

That it excites to increased action the arteries of the tongue in the experiment, in which a sense of warmth is produced along its surface by the application of the metals to the lips, seems to be almost demonstrated; for it would be difficult to point out the presence of another cause competent to occasion the evolution of the heat, in this case, besides the increased action of the arteries: and that this cause is competent to the effect we know from numberless experiments, too familiar to need being particularized here.

Whether the metals, however, do or do not affect the action of the blood vessels, is a question which admits of solution by experiment. The following, I confess, was not quite satisfactory, and I have not yet found leisure and opportunity to repeat it with all the attention it requires.

I inspected the foot of a living frog with a microscope of very high powers. In fixing the foot so as to keep the web expanded, a considerable degree of inflammation was excited, notwithstanding every precaution to avoid it. The current of blood was seen distinctly in several vessels, now flowing rapidly, now slowly, and now in a direction contrary to that in which it was first observed, but with equal rapidity. A thin plate of zinc was introduced between the fleshy part of the foot and its supporter, and a silver probe was used as an excitor. To me, the circulation appeared very decidedly to be quickened several times when the metals were made to touch each other: but the gentlemen who assisted me could observe no change. To prevent the contractions in the muscles of the leg from producing any fallacy, the crural artery should be laid bare, and insulated from surrounding parts, by passing a thin plate of glass, or sealing wax, between it and them.

That the flash is the effect of such an increased action of the vessels, composing the choroid coat, might be somewhat more difficult to prove. It is however known to every one, that a blow, and that pressure upon the eye, are capable, as I have before observed, of producing a similar effect. And the following case, which Bonetus quotes from Hermannus Cummius, if it may be credited, affords an almost positive proof, that vision depends upon the stimulus given to the retina by the activity of blood vessels in some part of the eye. ‘Quando theologus, plaga dolorifica, a rupta instrumenti musici chorda accepta, nocte subsequenti jam adulta, e somno evigilans, cuncta clare, ac si de die esset, vidit, adeo, ut minimos picturarum et tapetum tractus observare, characteresque ex libro legere posset. Oculo vero læso clauso, tenebras densissimas adesse ille percepit, eodemque iterum aperto, conclave illustratum visum est, lucem tamen candelæ allatae solisque splendorem de die, ægre tulit oculus affectus, quod per aliquot dies duravit, tandemque sensim remisit.’

Haller speaks of such cases as by no means uncommon, and quotes the names of several authors, who have related similar ones.

The direction of this influence, when suffered to pursue its natural course, appears to be the same with that of most other stimuli, i. e. from the place at which it first affects a nerve, onwards to the part, in which that nerve terminates. I have repeatedly caused electrical sparks to be passed into my own ulnar nerve at its passage over the inner condyle of the humerus, but both the sensations and the contractions produced by them have been entirely confined to the hand and fore arm.

It appears too, both from the experiments of Dr Monro, and of Dr William Alexander of Halifax in Yorkshire, that when no communication is left between the trunk and posterior extremities of a frog, except by its sciatic nerves, a strong solution of opium, injected under the skin of its posterior extremities, deprives them both of their sensibility and of their contractile power; but that it does not in the least affect the trunk of the body. If, on the contrary, it be applied to the trunk, it exhausts both the trunk and the extremities.

M. Galvani is said to have observed the effects of the influence, which he discovered, diffused over the whole body of a frog, when the metals were applied to a nerve merely laid bare, without being either divided or separated from surrounding parts. If we are allowed to infer this diffusion of the influence from the restlessness expressed by the animal, M. Galvani’s observation may be just. If from the contractions produced, I suspect it is by no means so; since, in every experiment which I have made upon the subject, the contractions have been confined to those parts to which the nerve touched by the metals was distributed.

That this influence, however, may pass in a direction contrary to the course of nerves, is evident from some of the experiments which I have related relative to its effects upon the senses, but is still more clearly demonstrated by the following.

If, after having divided at the pelvis a frog recently killed, the sciatic nerves be freed from cellular membrane up to their origin from the spine, and all the parts below this, except themselves, be cut away, the muscles on each side of the spine, for some little way up, may be brought into contraction by touching the nerves alone with the two metals in contact. This experiment has not always succeeded with me, and never unless the frog had been recently killed. So long as the hind legs remain undivided from the nerves, it never succeeded; the only contractions produced being in the legs.

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.

SECTION IV.

An attempt to investigate the Source from which the respectivePowers 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 impression13.’

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 dividedEXPERIMENT 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.

EXPERIMENT II

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.

EXPERIMENT III

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.