around the vessels, and accompany them through all their ramifications. By some anatomists, the arteries of the head, neck, thorax and abdomen, are conceived to be supplied from the great sympathetic, whilst those of the extremities are supplied from the nerves of the spinal marrow. It is not improbable, however, but that more accurate discrimination might trace the dispersion of the twigs of the great nervous system of involuntary motion on these vessels also.

This organization of the arteries renders them very tough and extremely elastic, both of which qualities are necessary to enable them to withstand the impulse of the blood sent from the heart, and to react upon the blood so as to influence its course in the mode to be mentioned hereafter. It is, likewise, by virtue of this structure, that the parietes retain their form in the dead body, which is one of the points that distinguish them from the veins.

The vitality of the arteries is inconsiderable. Hence their diseases are by no means numerous or frequent; an important desideratum, seeing that their functions are eminent and their activity incessant.

The capillary vessels are the vessels of extreme minuteness, formed by the termination of the arteries and the commencement of the veins; or rather they are the ultimate radicles of the arteries,become imperceptible by their successive divisions,-which give origin to the veins. It is this system of vessels which forms a plexus that is distributed over every part of the body, and which constitutes, in the aggregate, the capillary system. This admits of two great divisions, one of which is situated at the termination of the branches given off from the aorta, and is called the general capillary system; the other forming the branches of the pulmonary artery, the pulmonic capillary system.

Although the capillary system does not admit of detection by the sight, its existence is evidenced by injections, which can develope it artificially in almost every organ; and by inflammation, in which it is manifested by the increased redness. The parietes of the vessels cannot be distinguished from the substance of the organs, and the colour of the blood, or the matter of the injection alone indicates their course. In many parts, these vessels are so minute as not to admit the red particles of the blood to enter, as in the scrous membranes; whilst, in others, as in the muscles and in some of the glands, the red particles always circulate. This diversity has given rise to the distinction of the capillaries into red and white.

There are certain textures, which receive neither the one class nor the other, as the corneous and epidermeous.

The ancients were of opinion, that the arteries and veins are separated by an intermediate substance, consisting of some fluid effused from the blood, and which they called, in consequence, parenchyma. The communication between the arteries and veins, however, appears to be direct. The substance of an injection passes from one set of vessels into the other without any evidence of intermediate

extravasation. Besides, the blood has been seen passing, in living animals, directly from the arteries into the veins. LEEUENHOEK and MALPIGHI, on examining the swim-bladders, gills, and tails of fishes, the mesentery of frogs, &c.-which are transparent, -saw this distinctly. The artery and vein terminate in two different ways; sometimes directly, after the artery has become extremely minute, by sending off numerous lateral branches, as HALLER states he noticed in the swim-bladders of fishes; at others, they proceed parallel to each other, and communicate by a multitude of transverse branches. This communication takes place between both the red and the white capillaries and their corresponding veins.

The capillary vessels, as has been remarked, belong chiefly to the arteries, the venous radicles not arising imperceptibly from the capillary system, as the arteries terminate in it, but having a marked size, at the part where they quit this system, which strikingly contrasts with the excessive tenuity of the capillary arterial vessels. Between the capillary system and the arteries there is no distinct line of demarcation. The opinion of BICHAT was, that this system is entirely independent of both arteries and veins; and AUTENRIETH fancifully imagined, that the minute arteries unite to form trunks, which again divide, before communicating with the veins, so as to represent a system analogous to that of the vena porta.

Many speculations have been indulged, regarding the mode in which the vascular extremities of the capillary system open. BICHAT regarded it as a vast reservoir, whence originate, besides veins, vessels of a particular order, whose office it is to pour out, by their free extremity, the materials of nutrition,-vessels, which had been previously imagined by BOERHAAVE, and are commonly known under the appellation of exhalants. MASCAGNI supposed that the final arterial terminations are pierced, towards their point of junction with the veins, by lateral pores, through which the secreted matters transude. These are points, however, which will engage our attention under the head of secretion.

The veins have already been described under venous absorption.

Physiology of the Circulation.

The blood, contained in the apparatus just described, is in constant motion, and this always in one direction. The venous blood, brought from every part of the body, is emptied into the right auricle; the right auricle contracts and sends it into the corresponding ventricle; the latter projects it into the pulmonary artery, by which it is conveyed to the lungs, passing through the capillary system into the pulmonary veins. These convey it to the left auricle; the left auricle sends it into the corresponding ventricle; and the left ventricle sends it into the aorta, along which it passes to the different

organs and tissues of the body, through the general capillary system, which communicates with the veins; and these convey the blood back to the part whence we have supposed it to set out. This entire circuit includes both the lesser and the greater circulations.

It was not until the commencement of the seventeenth century, that any precise ideas were entertained regarding the general circulation. In antiquity, the most erroneous notions prevailed; the arteries being generally looked upon as tubes for the conveyance of some aerial fluid to, and from, the heart, whilst the veins conducted the blood, but whither or for what precise purpose was not understood. The names given to the principal arterial vessel—the aorta, and to the arteries themselves, sufficiently show the functions originally ascribed to them, both being derived from the Greek, anp, air, and pey, to keep, and this is farther confirmed by the fact, that the trachea or windpipe was originally termed an artery, the appia paxsia of the Greeks, the aspera arteria of the Latin writers.

In the time of GALEN, however, the arteries were known to contain blood; and he seems to have had some faint notions of a circulation. He remarks, that the chyle, the product of digestion, is collected by the meseraic veins and carried to the liver, where it is converted into blood; the supra-hepatic veins then convey it to the pulmonary heart; thence it proceeds in part to the lungs, and the remainder to the rest of the body, passing through the medium septum of the auricles and ventricles.

This limited knowledge of the circulation continued through the whole of the middle ages; the functions of the veins being universally misapprehended; and the general notion being, that they also convey blood from the heart to the organs; from the centre to the circumference. It was not until after the middle of the sixteenth century, that the lesser circulation, or that through the lungs, was comprehended, by the labours of MICHAEL SERVETUS,-who fell a victim to the persecutions and intolerance of CALVIN,-of ANDREW CAESALPINUS, and of REALDUS COLUMBUS. It has, indeed, been imagined, that they possessed some notion of the greater circulation. However this may have been, all nations unite in awarding to HARVEY the merit, if not of entire originality, of at least having first clearly described it. The honour of the discovery is, therefore, his; and by it his name has been rendered immortal; for its importance in the physiology and pathology of the animal fabric is overwhelming. How vague and inaccurate must have been the notions of the earlier pathologists regarding the doctrine of acute diseases, in which the circulation is always largely affected-diseases, which, according to the estimate of some writers, constitute two-thirds of the morbid states to which mankind are liable! It was in the year 1619, that HARVEY attained a full knowledge of the circulation; but his discovery was not promulgated until the year 1628; in a tract, under the title-"exercitatio anatomica de motu cordis et sanguinis,"

to which the merit of clearness, perspicuity and demonstration has been awarded by all. Yet so strong is the force of prejudice, and so difficult is it to discard preconceived notions, that it was remarked, according to HUME, that no physician in Europe, who had reached forty years of age, ever, to the end of his existence, adopted HARVEY'S doctrine of the circulation, and HARVEY'S practice in London diminished extremely from the reproach drawn upon him by that great and signal discovery.

Of the truth of the course of the blood, as established by HARVEY, we have numerous, incontestible evidences, which it may be almost a work of supererogation to adduce. We will briefly refer to some of the most striking. First. If we open the chest of a living animal, we find the heart alternately dilated and contracted; so as manifestly to receive and expel the blood in reciprocal succession. Secondly. The valves of the heart, and of the great arteries, which arise from the ventricles, are so arranged as to allow the blood to flow in one direction, and not in another: and the same may be said of the valves of the veins. These last are directed towards the heart; the tricuspid valve permits the blood to flow only from the right auricle into the corresponding ventricle; the sigmoid valves admit it to enter the pulmonary artery, but not to return; and as there is no immediate communication between the right and left sides of the heart, the blood must pass along the pulmonary artery and by the pulmonary veins to the left auricle. The mitral valve, again, is so situated, that the blood can only pass in one direction from auricle to ventricle; and, at the mouth of the aorta, the same valvular arrangement exists, as at the mouth of the pulmonary artery, permitting the blood to proceed along the artery, but preventing its reflux. Thirdly. If an artery and a vein be wounded, the blood will be observed to flow from the part of the vessel nearest the heart in the case of the artery; in that of a vein from the other extremity. The ordinary operation of blood-letting at the flexure of the arm affords us an elucidation of this. The bandage is applied above the elbow, for the purpose of compressing the superficial veins, but not so tightly as to compress, also, the deep-seated artery. The blood then passes along the artery to the extremity of the fingers, and returns by the veins, but its progress back to the heart by the subcutaneous veins being prevented by the ligature, they become turgid; and, if a puncture be made, the blood flows freely. If, however, the ligature be applied so forcibly as to compress the main artery; the blood no longer flows to the extremity of the fingers; there is none consequently, to be returned by the veins. They do not rise properly; and if a puncture be made no blood flows. This is not an infrequent cause of the failure of a bungling phlebotomist. If the bandage, under such circumstances, be slackened, the blood will resume its course along the artery, and a copious stream will issue from the orifice, which did not previously transmit a drop. This operation, then, exhibits the fact of the flow of blood along the ar

teries from the heart and of its return by the veins. From what has been said, too, it will be obvious, that if a ligature be applied to both vessels, the artery will become turgid above the ligature, the vein below it. Fourthly. The microscopical experiments of LEEUENHOEK, MALPIGHI, and SPALLANZANI have exhibited to the eye the passage of the blood in successive waves by the arteries towards the veins, and its return by the latter. Lastly. The fact is farther demonstrated by the effects of transfusion of blood, and of the injection of substances into the vessels; both of which operations will be alluded to in another place.

In tracing the physiological action of the different parts of the circulatory apparatus, we shall follow the order observed in the anatomical sketch; and describe, in succession, the circulation in the heart, in the arteries, in the capillary vessels, and in the veins; on all of which points there has been much interesting diversity of opinion; much room for ingenious speculation, and for farther improvement.

1. Circulation in the heart.-It has been already observed, that when the heart of a living animal is exposed, it is remarked to undergo alternate contraction and dilatation; the auricles, on each side, contracting, and the ventricles at the same time dilating. The latter, then enter into contraction, and the auricles dilate simultaneously; so that the blood is received into the two auricles at the same time, and is transmitted into the two great arteries synchronously.

In order that the heart shall receive blood, it is necessary that the auricle should be dilated. This movement is probably effected by virtue of the elasticity which it possesses in its structure. Let us suppose it to be once filled; the stimulus of the blood excites it to contraction, and the blood is thus sent into the corresponding ventricle. As soon, however, as it has emptied itself, the stimulus is withdrawn; and, by virtue of its elasticity, it returns to the state in which it was prior to contraction. An approach to a vacuum is thus formed in the cavity, and the blood is solicited towards it from the veins, until it is again filled and its contraction is renewed. When the right auricle contracts there are four channels by which the blood might be presumed to pass from it,-the two terminations of the venæ cave, the coronary vein, and the auriculo-ventricular communication. The constant flow of blood from every part of the body prevents it from returning by the venæ cave, whilst the small quantity, that, under other circumstances, might enter the coronary vein, is prevented by its valve. To the flow of the blood through the aperture into the ventricle, which is in a state of dilatation, there is. no obstacle, and accordingly it takes this course, raising the tricuspid valves.

It may be remarked, that physiologists are not entirely of accord regarding the reflux of blood into the vena cava. Some think that VOL. II.

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