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EXPERIMENTS ON THE STRUCTURE OF ARTERIES.

of blood) in consequence of mechanical irritation and exposure to the atmosphere.

Schwann performed a very neat experiment, showing the latter quality more exactly than had previously been done; though, of course, the contraction of blood vessels under the influence of cold has been known to the Medical Profession from time immemorial. On one very hot day, furnished with some very cold water, he brought the mesentery of a living frog into the field of a microscope, and adjusted his focus to an artery of about 1 inch diameter. He then let the cold water fall drop by drop on the membrane, and presently found the artery contracted to a third of its previous diameter. He desisted from the application, and in half an hour the artery had resumed its former size. He then again reduced it by cold; and he continued for some time thus alternately making the artery contract and letting it dilate, just as you may do with your iris in alternately turning your eye to light or shade. The experiment is a good one, because it is so precise.

The brothers Weber have demonstrated that the muscular coat of arteries is susceptible to electro-galvanic stimulation. By passing a current through small arteries (diameter to inch) in the mesentery of frogs, they have found that within a few seconds a contraction ensues, which reduces the canal of the artery to half its previous capacity; and that a continuation of the same stimulus would by degrees completely obliterate the vessel. This contraction confines itself exactly to the portion of the artery stimulated, not extending itself (if the operation be neatly performed) beyond so much of the artery as lies between the wires. The contraction does not begin suddenly at the moment of completing contact, but gradually a few seconds afterwards, and may go on augmenting itself for some time after the contact has been broken. After a little while (provided the stimulation have not been excessive), the artery recovers its former capacity, and is liable to present again the same phenomena, on a repetition of the experiment; but if the current have been too powerful or too long continued, the artery will for a while have lost its muscular tone, and will not only refuse to contract again under the current, but will yield to the pressure of the circulation, so as to present an aneurismal bulging.

Professor Paget, in the admirable lectures which he recently delivered at the College of Surgeons, drew attention to a case in which the irritability of arteries and veins may still more easily be demonstrated in warm-blooded animals. He stated that, under a very slight amount of gentle friction, any small artery and vein in the web of the bat's wing would gradually contract at the irritated point till their canals became quite obliterated, and would then gradually expand again, as in Weber's experiment, to their former or to still greater dimensions. By his kindness, I have been enabled to verify this; and I have likewise been able to observe similar changes in the mesenteric vessels of the frog.

2. With respect to nervous influence-there is unquestionable evidence that the nervous centres affect and modify the movements

EXPERIMENTS IN PROOF OF THIS INFLUENCE.

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of the heart; and that parts receive more or less than their usual supply of blood, under influences which are purely nervous.

First, we know that the heart may be affected through the nerves. The most familiar illustrations are those of a mental affection, quickening the pulse; but there are instances of an opposite kind, equally conclusive in their way.

It is quite true that you may cut out the heart of a living frog, and that it will continue beating as rhythmically in a glass of water* as just before it did in the pericardium; but this does not alter the fact that when the heart is in its natural relations, and when its ganglia have their natural commissural connections with other parts of the nervous system, its pulsations are accelerated or retarded by influences of a purely nervous origin.

For instance, Professor Weber finds (and the observation is one of the most striking with which I am acquainted) that if he sends a galvanic current through the medulla oblongata of a frog, the heart's action is instantaneously arrested, or, if not arrested, is retarded to a very remarkable degree; sometimes the one, and sometimes the other. One of his experiments stands as follows: Having cut a frog asunder, so that the spinal cord was removed below the cervical region, while the heart and lungs remained in connection with the head and medulla oblongata, he counted the beats of the heart, they were 36; he passed the current transversely through the medulla oblongata during a minute, the pulsations fell to 8; he stopped the current, they again rose to 36; and, on his once more completing the circuit, they again fell to 8, from which they subsequently rose to the original 36.

It is likewise beyond doubt that partial irregularities in the distribution of blood are produced under the influence of the nervous system. The flushing of the face with mental emotion is an instance of this; the same emotion may have excited the heart, but (as I have already explained to you) no excitement of the heart can cause partial distributions of blood. There must be some cause peculiar to the part, to account for its receiving more than its share of blood; if the heart were the sole agent in producing the effect, it ought to be produced in all parts of the body equally; we should blush all over. Erection of the penis is another phenomenon of local hyperæmia, notoriously dependent on nervous influences; and in this case the immediate importance of the nervous system to the production of the hyperemia has been shown by experimental evidence; for after division of the pudic nerves (according to Müller), the penis loses its faculty of erection.

With respect to the manner in which these irregularities of distribution are produced, there are no conclusive experiments evincing visibly that the muscular coat of the artery is under the direct con

There is a fallacy, too, in the above experiment; or, rather, it does not at all prove that the heart is independent of nervous influence; for the ganglia are in immediate connection with the heart; and if this organ, when removed, be cut across, it will be found that the basial half (still in connection with its ganglia) will continue to contract rhythmically, while the other half (now quite dissevered from nervous influence) will remain motionless.

60 INFLUENCE OF NERVOUS CENTRES ON NUTRITION OF PARTS.

trol of the nervous system. Valentin's experiment (in which the thoracic aorta appeared to contract while the thoracic ganglia of the sympathetic were undergoing stimulation) has not had its results confirmed by other observers. Analogy would justify us perhaps in assuming it as probable, since nervous fibrils are distributed to the tissue in question, that it (like all other specimens of the same tissue distributed in the body) should be under direct nervous influence; but there is a total want of evidence as to the nature and extent of that influence. In the lower animals, one may occasionally see something that seems likely to bear on the subject; in the leech, for instance, when you expose the dorsal vessel, you see it pulsating-and not only it, but the branches likewise which are derived from it, with a regular systole and diastole; and if you snip across the vessel at two points half an inch asunder, you see the intermediate bit of vessel, though now quite empty of blood, continue its former rhythm of movement uninterruptedly; but if you detach the vessel from its lateral connections, even without cutting it across, if you isolate it completely from the ganglia which supply it with nerves, every trace of contraction ceases at once and forever. Still, in attempting to apply this analogy to the functions of higher animals, we must not neglect two points which materially affect the conditions of such analogy; first, that the muscular structure of the leech's dorsal vessel is of a much higher development than that which exists in the arteries of the vertebrata; and secondly, that from this reason, and from the absence of a true heart in that animal, we ought perhaps to compare the contractile vascular system of the leech to the heart, rather than to the arteries, of more highly-organized animals.

There is some evidence making it probable that the nutrition of parts depends, for its quantitative variations, on impressions conveyed to the nervous centres by centripetal or sensitive nerves, and responded to by changes (whatever those changes may be) which induce active hyperæmia. The disorders of nutrition, which arise as consequences of disease or injury operating on the trunk of a sensitive nerve, seem to me an indirect but forcible testimony to this point. It almost seems, in such cases, as though Nature formed a misapprehension of the state of the part in consequence of wrong intelligence being transmitted through the diseased nerve; and as though she acted on that misapprehension according to her normal course of proceeding. A sensitive nerve conveys to the centre, where it terminates, a representation of the state of those parts where its fibrils commence, and can convey no other impression; a portion of the ulnar nerve commences in the integuments of the two innermost fingers, and terminates in the brain; it conveys impressions corresponding only to those fingers; and, whether you excite the nerve where it begins in the hand, or where it terminates in the brain, or at any intermediate inch of its line of telegraph, you can produce in the consciousness of the individual no other impression than that of sensation in his two fingers. Hit your "funny-bone," and you can hardly divest yourself of the conviction that the fingers themselves have been struck. Gradually, we become accustomed to this illusion;

NEUROPATHIC THEORY OF INFLAMMATION.

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we recognize it as a misapprehension, inseparable from the arrangements of our nervous anatomy. Yet, strange to say, Nature will sometimes act on that misapprehension; and the distant uninjured part will become the seat of hyperæmia. Several years ago, I was consulted, at King's College Hospital, by a man who some months previously had torn his ulnar nerve at the inner condyle; his two inner fingers had become swollen and livid with vascular injection. Two years ago, I had a female patient here with disease in a large portion of the lumbar and sciatic plexus of nerves on one side, causing paralysis and anesthesia of the limb; neuralgia was referred especially to the vicinity of the knee; and at this spot (when I first saw the patient) ulceration had occurred. It has long been known that injuries or diseases of the ophthalmic division of the fifth nerve lead to alteration in the nourishment of parts where the radicles of that branch arise to injection of the conjunctiva, to opacity and subsequent ulceration of the cornea, and often to final destruction of the globe of the eye. In a case of the kind, which I saw with Mr. Dixon, some years ago, (and which he has reported in a very instructive form in the Medico-Chirurgical Transactions for 1845,) there was likewise ulceration at the cutaneous source of the nerve at the inner canthus of the eye, and at the ala nasi. Similarly, it is known that the nutrition of the lung is gravely affected by injuries or diseases of its sensitive nerve, the pneumogastric; experimental division or bruising of these nerves (if the animal survive the operation) never fails to induce true pneumonia that is to say, not mere congestion of the lungs, but the development of new products.

Now, in all these cases, observing that sensitive nerves are the seats of lesion, and considering the known functions of those nerves, it is difficult to avoid the conviction, which I have already suggested, that the modus operandi, by which such diseases and injuries are enabled to produce distant inflammatory changes, consist in a reflex operation on the blood vessels; that a false impression of the state of distant parts is conveyed to the nervous centres by the disease of their incident nerve-tubules; and that this false impression determines a change in the vascular supply of the part where the nervetubules take their origin.

Endeavours have been made to found an entire theory of active hyperemia on phenomena such as these. It has been argued, that the first impulse to its occurrence is in the form of a sensational change that the causes of inflammation act on the extremities of sensitive nerves in the parts exposed to injury-that a nervous centre is then excited-that a motional impulse of some sort or other is reflected to the muscular coat of the artery, and that thus the affected organ becomes the seat of a vascular repletion. It is admitted, I believe, by the holders of this view, that idiopathic changes in the same nervous centres may exist, as an explanation of such acts of hyperæmia as do not arise in exterior local impressions.

Now that the first change in the inflaming or overgrowing part is a nervous change, leading to the determination of blood, is a position which I may venture to deny; at least, as regards the modus

62

ARGUMENTS FOR AND AGAINST

operandi of all exterior irritants. It is notorious that paralyzed parts are susceptible of artificial irritation, and become inflamed. I have made many observations on the subject, and have noticed that the absence of a spinal cord, or the division of all the roots of the nerves, or the section of the lumbar and sciatic plexus, will make little or no difference as to the certainty with which an irritant applied to the web of a frog's foot will quicken the circulation there, and subsequently lead to its retardation and arrest.

Perhaps the neuro-pathologists might object here, that we do not accurately know what are the branches of nerves-and still less what are the nervous centres, determining muscular movements in the arteries; and that, in the experiments just cited, some essential element for the nutritive innervation of the part (i. e. for the supposed function of its artery) might have been left uninjured.

There is, however, little room for fallacy in the experiment which I am now going to tell you. A patient had complete anesthesia of the fifth nerve, dependent (as a post-mortem examination subsequently showed) on its organic disease; the conjunctiva, as well as the integument of the face, was utterly insensible; not only was the function of the nerve destroyed, but those reflected nutritive changes of which I have already spoken had taken place, and had exhausted themselves; showing that the nerve was spoilt for participation in the acts of nutrition (whatever they may be) no less than for its more obvious uses as a medium of conscious sensation; the cornea had undergone ulceration, and had healed again. The following experiment was carefully made: The lids being held open, a single granule of cayenne pepper was laid upon the insensible conjunctiva; in a few moments, it had become the centre of a very distinct circle of increased vascularity, the redness of which slowly became more and more distinct as long as the stimulus was suffered to remain; so that, on its removal, there was a very evident circumscribed erythema on the surface of the membrane. I consider myself justified in believing that this change occurred without any intermediate nervous excitement; not only because the history of the case would lead me to consider the fifth nerve as annihilated; not only because the experiment was totally unattended with sensation; but likewise because there was the very remarkable absence of that sympathetic phenomenon, which the faintest remnant of nervous excitability would have produced-namely, there was not the slightest trace of lachrymation.

Altogether we may, I think, take it as an established certainty, that the first change which occurs in an inflaming or overgrowing part, and which leads to its becoming loaded with blood, is not a reflex change operated through the nerves, but is a direct change operated by the living molecular structure of the part on the blood which traverses it, or on the vessels which convey that blood. “Ubi stimulus, ibi affluxus" will stand, then, as a law of reciprocal relation between the solids and fluids of the body, which does not of necessity require the intervention of a nervous system for its initiation.