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Perhaps one of the most singular cases, and at the same time one of the best authenticated on record, is that of Janet M'Leod, published in the Philosophical Transactions by Dr. Mackenzie.* She was at this time thirtythree years of age, unmarried, and from the age of fifteen had had various paroxysms of epilepsy, which had considerably shaken her frame, rendered the elevator muscles of the eyelids paralytic, so that she could only see by lifting the lids up, and produced so rigid a locked jaw that her mouth could rarely be forced open by any contrivance. She had lost very nearly her power of speech and deglutition, and with this, all desire either to eat or drink. Her lower limbs were retracted towards her body she was entirely confined to her bed, slept much, and had seldom any other egestions than periodical discharges of blood, apparently from the lungs, which was chiefly thrown out by the nostrils. During a very few intervals of relaxation she was prevailed upon with great difficulty to put a few crumbs of bread, comminuted in the hand, into her mouth, together with a little water sucked from her own hand, and in one or two instances a little gruel; but even at these attempts almost the whole was rejected. On two occasions also, after a total abstinence of many months, she made signs of wishing to drink some water, which was immediately procured for her. On the first occasion the whole seemed to be returned from her mouth; but she was greatly refreshed by having it rubbed upon her throat. On the second occasion, she drank off a pint at once, but could not be either prevailed upon or forced to drink any more, notwithstanding that her father had now fixed a wedge between her teeth, two of which were hereby broken out. With these exceptions, however, she seems to have passed upwards of four years without either liquids or solids of any kind, or even an appearance of swallowing. She lay for the most part like a log of wood, with a pulse scarcely perceptible from feebleness, but distinct and regular: her countenance was clear and pretty fresh; her features neither disfigured nor sunk; her bosom round and prominent, and her limbs not emaciated. Dr. Mackenzie watched her with occasional visits, for eight or nine years, at the close of which period she seems to have been a little improved. His narrative is very precisely as well as minutely detailed, and previously to its being sent to the Royal Society, was read over before the patient's parents, who were known to be persons of great honesty, as also before the elder of the parish, who appears to have been an excellent man; and, when sent, was accompanied by a certificate as to the general truth of the facts, signed by the minister of the parish, the sheriff-depute, and six other individuals of the neighbourhood, of high character, and most of them justices of the peace.

Yet even with the freest use of water, what can we make of such cases upon any chain of chemical facts at present discovered? What can we make of it, even in conjunction with the use of air? The weight and solid contents of the animal body are derived chiefly from that principle which modern chemists denominate carbon; yet neither water nor air, when in a state of purity, contains a particle of carbon. Again, the substance of the animal frame is distinguished from that of the vegetable by its being satu rated with nitrogen, of which plants possesses comparatively but very little; yet though the basis of atmospherical air consists of nitrogen, water has no more of this principle than it has of carbon; nor is it hitherto by any means established, that even the nitrogen of the animal system is in any instance derived from the air, or introduced by the process of respiration for the experiments upon this subject, so far as they go, are in a state of opposition, and keep the question on a balance-factis contraria facta.

Shall we, then, suppose with others, that the circle of perpetual mutation, which is imposed upon every other species of visible matter, is in these cases suspended, and that the different organs of the system are, so long as the anomaly continues, rendered incorruptible? But this is to suppose the intervention of a miracle, and without an adequate cause. Let us, then, rather con

*Vol. Ixvii. year 1777.

fess our ignorance than attempt to be wise upon the basis of conceit. All that we do know is, that bodies of every kind are reducible to a few elementary principles, which appear to be unchangeable, and are certainly invisible; and that from different combinations and modifications of these proceeds every concrete and visible form: hence, air itself, and water; hence mineral, vegetable, and animal substances. Air, therefore, and water, or either separately, may contain the rudimental materials of all the rest. We behold metallic stones, and of large magnitude, fall from the air, and we suppose them to be formed there we behold plants suspended in the atmosphere, and still, year after year, thriving and blooming, and diffusing odours: we behold insects apparently sustained from the same source; and worms, fishes, and occa sionally man himself, supported from the one or the other, or from both. These are facts, and as facts alone we must receive them, for we have at present no means of reasoning upon them. There are innumerable mysteries in matter as well as in mind; and we are not yet acquainted with the nature of those elementary principles from which every compound proceeds, and to which every thing is reducible. We are equally ignorant of their shapes, their weight, or their measure.

LECTURE XIII.

ON THE CIRCULATION OF THE BLOOD, RESPIRATION, AND ANIMALIZATION.

THE progress of science is slow, and often imperceptible; and though in a few instances it has been quickened by an accidental discovery or an acci dental idea, that has given a new turn, or a new elasticity to the chain of our reasoning, still have we been compelled in every instance to follow up the chain, link after link, and series after series, and have never leaped forward through an intermediate space without endangering our security, or being obliged to retrace our career by a painful and laborious reinvestigation. It required a period of three thousand six hundred years to render, the doctrine of a vacuum probable, and of five thousand six hundred to establish it upon a solid foundation. For its probability we are indebted to Epicurus, for its certainty to Sir Isaac Newton. The present theory of the solar system was commenced by Pythagoras and his disciples five centuries before Christ, and only completed by Copernicus fifteen centuries after Christ. Archimedes was the first who invented the celebrated problem for squaring the parabola, which was upwards of two hundred years before the Christian era; yet an exact problem for squaring the circle is a desideratum in the present day. The simple knowledge of the magnet was familiar to the Romans, Greeks, and some of the oriental nations while in their infancy; it has been employed by the mariner for nearly six centuries in Europe, and for a much longer period by the Chinese, in their own seas; yet at this moment we are acquainted with only a very few of its laws, and have never been able to appropriate it to any other purpose than that of the compass.

The circulation of the blood in the animal system is our subject of study for the present lecture, and it is a subject which has laboured under the same difficulties, and has required as long a period of time as almost any of the preceding sciences, for its complete illustration and establishment. Hippocrates guessed at it; Aristotle believed it; Servetus, who was burnt as a heretic in 1553, taught it; and Harvey, a century afterward, demonstrated it. I shall not here enter into the various steps by which this wonderful discovery was at length effected; the difficulty can be only fairly appreciated by those who are acquainted with the infinitely minute tubes into which the distributive arteries branch out, and from which the collective veins arise; but every one is interested in the important fact itself, for it has done more

towards establishing the healing art upon a rational basis, and subjecting the different diseases of mankind to a successful mode of practice, than any other discovery that has emblazoned the annals of medicine.

In our last lecture we traced the action of the digestive organs: we beheld the food first comminuted by means of jaws, teeth, or peculiar muscles or membranes; next converted into a pulpy mass, and afterward into a milky liquid; and in this state drunk up by the mouths of innumerable minute vessels, that progressively unite into one common trunk, and convey it to the heart as the chief organ of the system, for the use and benefit of the whole.

But the new-formed fluid, even at the time it has reached the heart, has by no means undergone a sufficient elaboration to become genuine blood, or to support the living action of the different organs. It has yet to be operated upon by the air, and must for this purpose be sent to the lungs, and again returned to the heart, before it is fitted to be thrown into the general circulation. This is the rule that takes place in all the more perfect animals, as mammals, birds, and most of the amphibials ;* and hence these classes are said to have a double circulation. And as the heart itself consists of four cavities, a pair belonging to each of the two circulations, and each pair is divided from the other by a strong membrane, they are also said to have not only a double circulation, but a double heart-a pulmonary and a corporeal heart.

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The blood is first received into the heart on the pulmonary side, and is conveyed to the lungs by an artery which is hence called the pulmonary artery, that soon divides into two branches, one for each of the lungs; in which organs they still farther divide into innumerable ramifications, and form a beautiful network of vessels upon the air vesicles of which the substance of the lungs consists; and by this means every particle of blood is exposed in its turn to the full influence of the vital gases of the atmosphere, and becomes thoroughly assimilated to the nature of the animal system it is to support. The invisibly minute arteries now terminate in equally minute veins, which progressively unite till they centre in four common trunks, which carry back the blood, now thoroughly ventilated and of a florid hue, to the left side or corporeal department of the heart.

From this quarter the corporeal circulation commences: the stimulus of the blood itself excites the heart to that alternate contraction which constitutes pulsation, and which is continued through the whole course of the arteries; and by this very contraction the blood is impelled to the remotest part of the body, the arterial vessels continuing to divide and to subdivide, and to branch out in every possible direction, till the eye can no longer follow them, even when aided by the best glasses.

The arterial blood having thus visited every portion of every organ, and supplied it with the food of life, is now returned, faint, exhausted, and of a purple hue, by the veins, as in the pulmonary circulation; it receives, a short space before it reaches the heart, its regular recruit of new matter from the digestive organs, and then empties itself into the right side or pulmonary department of the heart, whence it is again sent to the lungs, as before, for a new supply of vital power.

The circulation of the blood, therefore, depends upon two distinct sets of vessels, arteries and veins; the former of which carry it forward to every part of the system, and the latter of which return it to its central source. Both sets of vessels are generally considered as consisting of three distinct layers or tunics: an external, which in the arteries is peculiarly elastic; a middle, which is muscular in both, but whose existence is doubted by some physiologists; and an internal, which may be regarded as the common covering or cuticle. The projectile power exercised over the arteries is unquestionably the contraction to which the muscular tunic of the heart is excited by

* Cuvier seems to ascribe a double heart to the class of amphibia, without any limitation. See Lawrence's additional note E. chap. xii. of his translation of Blumenbach's System of Comparative Anatomy. Blumenbach himself has remarked, that many of the frogs, lizards, and serpents have a simple heart, consisting of a single auricle and ventricle, like that of fishes.--Sect. 162.

the stimulus of the blood itself; and which contraction would be permanent, but that the heart appears to become exhausted in a considerable degree of its muscular irritability by the exertion that produces the contraction, and hence speedily returns to its prior state of relaxation, exhibiting that alternating succession of systole and diastole which constitutes pulsation.*

In the venal system, however, we meet with even fewer proofs of muscular fibre than in the arterial, and no such force of the heart as to produce pulsation on a pressure of the finger; and hence, to this moment, we are in a greater degree of ignorance as to the projectile power by which this system is actuated. The theories that have been chiefly advanced upon the subject are, first, that of a vis à tergo, or an impetus given to the blood by the arterial contraction, which is supposed by its supporters to be sufficient to operate through the whole length of the venal canals; secondly, that of capillary attraction, the nature of which we explained in a former lecture; and lastly, a theory of a much more complicated kind than either, and which supposes the projectile power to result jointly from the impetus communicated by the heart and arteries, from the pressure of the surrounding organs, and especially from the elasticity of the lungs, and the play of the diaphragm, in conjunction with the natural irritability of the delicate membrane that lines the interior of the veins. It is unnecessary to enter into a consideration of any of these theories; for they all stand self-convicted of incompetency; and the last, which is the most operose of the whole, has been only invented to supply the acknowledged inefficacy of the other two. Whatever this projectile power consists of, it appears to have some resemblance to that of the vegetable system; and, like many of the vessels in the latter, is assisted by the artifice of numerous valves inserted in different parts of the venal tubes.

The most important process which takes place in the circulation of the blood is that of its ventilation in the lungs. It is this process which constitutes the economy of RESPIRATION, and has till of late been involved in more than Cimmerian darkness.

We see the blood conveyed to the lungs of a deep purple hue, faint and exhausted by being drained in a considerable degree of its vital power, or immature and unassimilated to the nature of the system it is about to support, in consequence of its being received fresh from the lacteal trunk. We behold it returned from the lungs spirited with newness of life, perfect in its conformation, more readily disposed to coagulate, and the dead purple hue transformed into a bright scarlet. How has this wonderful change been accomplished? what has it parted with? what has it received? and by what means has so beneficial a barter been produced?

These are questions which have occupied the attention of physiologists in almost all ages; and though we have not yet attained to any thing like demonstration, or even universally acceded to any common theory, the experiments of modern times have established a variety of very important facts which may ultimately lead to such a theory, and clear away the difficulties by which we are still encumbered.

These facts I shall proceed to examine into in language as familiar as I can employ: I must nevertheless presume upon a general acquaintance with the elementary principles and nomenclature of modern chemistry, since a summary survey of zoonomy is not designed to enter into a detail of its

Physiological experiments have sufficiently proved of late that the same alternation of contraction and dilatation does not take place in the arteries in a free or natural state; for where there is no resistance to the flow of the blood along their canals, there is no variation in their diameter; and that it is only the pressure of the finger or some other substance against the side of an artery that produces its pulse. Study of Med. ii. p. 16. Experimental Inquiry into the Nature, &c. of the Arterial Pulse, by C. H. Parry,

M.D. 1816.

It has lately been pretty clearly established, that by far the most active power in the return of the blood to the heart from the veins, is the comparative vacuum which takes place in the ventricles of the heart when exhausted of blood by the systole or alternating contraction of this organ; in consequence of which, the venous blood is, as it were, sucked up into the right ventricle from the vena cavæ, or venous system at large. So that the heart, upon this beautiful principle of simplification, becomes alternately a forcing and a suction pump. By its contraction it forces the blood into the arterial system, and by its vacuum it sucks it up from the venous. Sec Study of Med. ii. p. 19, 2d edit. 1825.

mere alphabet or rudiments, but to apply and harmonize detached facts that relate to it, and to condense the materials that have been collected by others into a narrow but regular compass.

The chief substance which has been ascertained to be introduced from the atmosphere into the air-vesicles of the lungs during the act of respiration, and from these into the blood, is oxygen, of which the atmosphere, when pure, consists of about twenty-eight parts in a hundred, the remaining seventy-two being nitrogen.

That this gaseous fluid enters into the lungs is rendered highly probable from a multiplicity of experiments, which concur in proving that a larger portion of oxygen is received by every act of inspiration than is returned by every correspondent act of expiration; and that it passes from the air-vesicles of the lungs into the blood we have also reason to believe from the change of colour which immediately takes place in the latter, and from other experiments made out of the body, as well as in the body, which abundantly ascertain that oxygen has a power of producing this change, and of converting the deep purple of the blood into a bright scarlet.

It is also supposed very generally, that a considerable portion of caloric or the matter of heat, in its elementary form, is communicated to the blood at the same time and in conjunction with the oxygen; but as this substance has hitherto proved imponderable to every scheme that has been devised to ascertain its weight, this continues at present a point avowedly undetermined. That an increase of sensible heat at all times accompanies an increase of respiration is admitted by every one; but since caloric may be obtained by other means, if obtainable at all, and since a denial of its existence as a distinct substance has of late years been as strenuously urged as it was in former times by the Peripatetic school, and upon experiments inaccessible to those philosophers, we are at present in a state of darkness upon this subject, from which I am much afraid we are not likely to be extricated very soon.

I have already observed that nitrogen, or azote, as it is also called, is the other gaseous fluid that constitutes the respirable air of the atmosphere. And from a variety of well-conducted experiments by Mr., now Sir Humphry, Davy, it appears also that a certain quantity of this gas is imbibed by the lungs in the same manner they imbibe oxygen, and that, like oxygen, it is also communicated from the lungs to the blood while circulating through its substance; for in the experiments adverted to he found that, as in the case of the oxygen, a smaller quantity was always returned by every successive act of expiration than had been inhaled by every previous act of inspiration.* The only gas that seems to have been thrown out from the lungs in the course of these experiments is carbonic acid; a very minute proportion of which appears also to be almost always contained in the atmospheric air, though altogether a foreign material, probably eliminated from the decomposition of animal and vegetable bodies, that is perpetually taking place, and certainly unnecessary to healthful respiration.

The general result of these experiments was as follows: the natural inspirations were about twenty-six or twenty-seven in a minute; thirteen cubic inches of air were in every instance taken in, and about twelve and threequarters thrown out by the expiration that succeeded.

The atmospheric or inspired air contained in the thirteen cubic inches,nine and a half of nitrogen, three and four-tenths of oxygen, and one-tenth of an inch of carbonic acid. The twelve inches and three-quarters of returned air contained nine and three-tenths of nitrogen, two and two-tenths of oxygen, and one and two-tenths of carbonic acid.

This inhalation, however, varies in persons of different-sized chests from 26 to 32 cubic inches, at a temperature of 55°; but these by the heat of the lungs, and saturated with moisture, become forty or forty-one cubic inches. Taking, therefore, 40 cubic inches as the quantity of air equally inhaled and exhaled about 20 times in a minute, it will follow that a full-grown per

* Priestley had before shown that nitrogen is absorbed. See Phil. Trans. 1790, p. 106.

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