of the heart. Its cavity is generally greater than that of the left side, and its parietes not so thick, owing to their having to force the blood merely through the lungs. It communicates with the auricle by the auriculo-ventricular opening—the ostium venosum; and the only other opening into it is that which communicates with the interior of the pulmonary artery. The opening, between the auricle and ventricle, is furnished with a tripartite valve, called tricuspid or triglochin; and the pulmonary artery has three others, called sigmoid or semilunar. From the whole edge of the tricuspid valve, next the apex of the heart, small, round, tendinous cords, called chordæ tendineæ are sent off, which are fixed, as represented in figure 111, to the extremities of a few strong columnæ carneæ. These tendinous cords are of such a length as to allow the valve to be laid against the sides of the ventricle, in the distended state of that organ, and of its being pushed back by the blood until a complete septum is formed during the contraction of the ventricle. The semilunar or sigmoid valves are three in number, situated

Fig. 112.

around the artery. When these fall together, there must necessarily be a space left between them. To obviate the inconvenience, that would necessarily result from the existence of such a free space, a small granular body is attached to the middle of the margin of each valve; and these coming together at A, Fig. 112, when the valves are shut down, complete the diaphragm and prevent any blood from passing back to the heart. These small bodies are termed, from their reputed discoverer, corpuscula Arantii, and corpuscula Morgagnii; or from their resemblance to the seed of the sesamum, corpuscula sesamoidea. The valves, when shut, are concave towards the lungs, and convex towards the ventricle.

Immediately above them the artery bulges out, forming three sacculi or sinuses, called sinus Valsalvæ. These are often said to be partly formed by the pressure of the blood upon the sides of the vessels. The structure is doubtless ordained, and is admirably adapted for a specific purpose, viz. to allow the free edges of the valves to be readily caught by the refluent blood and thus to facilitate their closure. Within the right ventricle, and especially towards the apex of the heart, many strong eminences are seen, which are called columnæ carneæ, Fig. 111. These run in different directions, but the strongest of them longitudinally with respect to the ventricle. They are of various sizes, and form a beautiful reticulated texture. Their chief use probably is, to strengthen the ventricle and prevent it from being overdistended; in addition to which they may tend to mix the different products of absorption.

The corporeal, left, aortic, or systemic heart, called, also the heart of red blood, has likewise an auricle and a ventricle.

The left auricle is considerably thicker and stronger but smaller than the right; and is likewise divided into sinus venosus and proper auricle, which form a common cavity. The columns, in the latter, are like those of the right auricle, but less distinct. From the under part of the auricle, a circular passage, termed ostium arteriosum, or auricular orifice, leads to the posterior part of the base of the cavity of the left ventricle. The left auricle receives the blood from the pulmonary veins.

The left or aortic ventricle is situated at the posterior and left part of the heart. Its sides are three times thicker and stronger than those of the right ventricle, to permit the much greater force which it has to exert; for, whilst the right ventricle merely sends its blood to the lungs, the left ventricle transmits it to every part of the body. It is narrower and rounder, but considerably longer, than the right ventricle, and forms the apex of the heart. The internal surface of this ventricle has the same general appearance as the other, but differs from it in having its columnæ carneæ larger, more numerous, firmer, and stronger.

In the aperture of communication with the corresponding auricle, there is here, as in the opposite side of the heart, a ring or zone, from which a valve, essentially like the tricuspid, goes off. It is stronger, however, and divided into two principal portions only; the cordæ tendineæ are also stronger and more numerous. This valve has been termed mitral, from some supposed resemblance to a bishop's mitre.

At the fore and right side of the mitral valve, and behind the commencement of the pulmonary artery, a round opening exists, which is the mouth of the aorta. There are three semilunar valves, with their corpuscula Arantii, exactly like those of the pulmonary artery, but a little stronger; and, on the outer side of the semilunar valves, are the sinuses of Valsalva, a little more prominent than those of the pulmonary artery.

The structure of the two hearts is the same. A serous membrane covers both, which is an extension of the inner membrane of the pericardium.

The substance of the heart is essentially muscular. The fibres run in different directions, longitudinally and transversely, but most of them obliquely. Many run over the point, from one heart to the other, and all are so involved, as to render it difficult to unravel them. The cavities are lined by a thin membrane, forming, by its folds, the valves to which reference has been made. It differs somewhat in the two hearts; being in one a prolongation of the inner coat of the aorta; and in the other of the venæ cavæ. On this account, the inner coat of the left heart is but slightly extensible;easily ruptured, and considerably disposed to ossify; that of the right heart, on the other hand, is very extensible, not readily ruptured, and but little liable to ossify. The tissue of the heart is supplied with blood by the cardiac or coronary arteries—the first division of VOL II.

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the aorta; and their blood is conveyed back to the right auricle by the coronary veins. The nerves, which follow the ramifications of the coronary arteries, proceed chiefly from a plexus, formed by the pneumogastric nerves, and great sympathetic.

In both hearts, the auricles are much thinner and more capacious than the ventricles; but they are themselves much alike in structure and size. The observation that the right ventricle is larger than the left is as old as Hippocrates, and has been attempted to be accounted for in various ways. Some have ascribed it to original conformation; others to the blood being cooled in its passage through the lung, and therefore, occupying a smaller space when it reaches the left side of the heart.

Haller and Meckel assert, that it is dependent upon the kind of death; that if the right ventricle is usually more capacious, it is owing to the lung being one of the organs, that yields first; thus occasioning accumulation of blood in the right cavities of the heart; and they stale that they succeeded, in their experiments, in rendering either one or other of the ventricles more capacious, according as the cause of death arrested the circulation in the lung or in the aorta.

The experiments of Legallois, Meckel, and Seiler, especially of the former,—with mercury poured into the cavities,—on dogs, cats, Guinea-pigs, rabbits, in the adult, the child, and the stillborn factus, have shown, that, except in the fœtus, the right ventricle is more capacious, whether death has been produced by suffocation, in which the blood is accumulated in the right side of the heart, or by hæmorrhage; and Legallois thinks, that the difference is owing to the left ventricle being more muscular, and, therefore, returning more upon itself.

The two hearts, united together by a median septum, form, then, one organ, which is situated in the middle of the chest, (see Fig. 103,) between the lungs, and consequently in the most fixed part of the thorax.

The heart is surrounded by its proper capsule, called the pericardium—a fibro-serous membrane, which is composed of two layers. The outermost of these is fibrous, semitransparent, and inelastic; strongly resembling the dura mater in its texture. Its thickness is greater at the sides than below, where it rests upon the diaphragm; or above, where it goes along the great vessels, which communicate with the heart. The inner layer is of a serous character and lines the outer, giving the polish to its cardiac surface; it is then reflected over the heart, and adheres to it by cellular substance. Like other serous membranes, it secretes a fluid, which is termed the liquor pericardii, and lubricates the surface of the heart. It is always found in greater or less quantity after death; and a question has arisen regarding the amount, that must be considered morbid. This must obviously vary according to circumstances. It seldom, however, in the healthy condition, is above a tea-spoonful.

When its quantity is augmented, along with inflammation of the membrane, the disease hydropericarditis exists. The great use of the pericardium is probably to keep the heart constantly moist by the exhalation effected from it; and, also, to restrain the movements of the heart, which, under the influence of the emotions, sometimes leaps inordinately. If the pericardium be divided in a living animal, the heart is found to bound, as it were, from its ordinary position; and hence the expression—"leaping of the heart" during emotion—is physiologically accurate.

The arteries are solid, elastic tubes, which arise, by a single trunk, from the ventricle of each heart, and which gradually divide and subdivide, until they are lost in the capillary system. The large artery, which arises from the left ventricle, and conducts the blood to every part of the body,—even to the lungs, so far as regards their nutrition,—is, as we have seen, the aorta, and that, which arises from the right ventricle and conveys the venous blood to the lungs, is the pulmonary artery. Neither the one nor the other is a continuation of the proper tissue of the ventricles; the inner membrane is alone continuous; the muscular structure of the heart being united to the fibrous coat of the arteries, by means of an intermediate fibrous tissue.

The aorta, as soon as it quits the left ventricle, passes beneath the pulmonary artery, is entirely concealed by it, and ascends to form a curvature with the convexity upwards, the summit of which rises to within three-quarters of an inch or an inch of the superior edge of the sternum. This great curvature is called the cross or arch of the aorta. The vessel then passes downwards, from the top of the thorax to nearly as far as the sacrum, where it divides into two trunks, one of which proceeds to each lower extremity. In the whole of this course, it is applied to the spine, and gives off the various branches, that convey arterial blood to the different parts of the body. Of the immense multitude of these ramifications, an idea may be formed, when we reflect, that the finest pointed needle cannot be run into any part of the surface of the body, without blood,—probably both arterial and vencus,—flowing.

The larger arteries are all situated deeply, and are thus remote from external injury. They communicate freely with each other, and their anastomoses are more frequent as the arteries became smaller and farther from the heart. At their final terminations, they communicate with the veins and the lymphatics.

The branches of the aorta, when taken collectively, are of greater capacity than the parent trunk, and this inequality goes on augmenting; so that the ultimate divisions of an artery are of a much greater capacity than the vessel itself. Hence the arterial system has been considered, to represent, in the aggregate, a cone, whose apex is at the heart, and the base in the organs.

As all the minute arterial ramifications are not visible, it is obviously impracticable to discover the ratio between their united ca

pacity and that of the aorta at its origin; yet the problem has been attempted. Keill, by experiments made upon an injected subject, considered the ratio to be as 44507 to 1. J. C. A. Helvetius and Sylva as 500 to 1. Senac estimated not their capacities but their diameters, and he conceived the ratio of these to be as 118,490 to 90,000; and George Martine affirmed that the calibre of a parent arterial trunk is equal to the cube root of the united diameters of the branches.

The pulmonary artery strongly resembles the aorta. Its distribution has been already described as a part of the respiratory organs.

The arteries are composed of different coats in superposition, respecting the number of which anatomists have not been entirely of accord. Some have admitted five, others four, but, at the present day, three only are received;—first an external or cellular, called also nervous, cartilaginous, by Vesalius, and tendinous, by Heister, which is formed of condensed cellular substance, and has considerable strength and elasticity, so that if a ligature be applied tightly round the vessel, the middle and internal coats will be completely cut through, whilst the outer coat remains entire. Scarpa is not disposed to admit this as one of the coats. He considers that it only serves as an exterior envelope, to retain the vessel in situ.

The next coat is the middle, muscular, or proper coat, the character of which has been the subject of much discussion. It is composed of yellow, circular fibres, which do not appear individually to pass entirely round the vessel. This coat was, at one time, almost universally believed to be muscular. Such was the opinion of Hunter; and hence the muscularity of the arteries was invoked as an agent in the circulation. Careful examination does not, however, exhibit any of the characters of the muscular tissue. The latter is soft, extensible, contractile, and of a red colour; whilst the arterial tunic is firm, solid, elastic, easily ruptured, of a yellow colour, and does not exhibit the least irritability on the application of any mechanical or chymical irritant whatever. Nysten and Magendie applied the galvanic stimulus to it frequently but without effect; yet it is known, that this is the most sensible test of irritability. It appears to be a tissue of a peculiar character, the base of which is formed by the tissu jaune or yellow tissue of the later compara

tive anatomists.

The third or inner coat is smooth and polished, and is a continuation of the membrane which lines the ventricles. It is generally described as lubricated by a kind of serous exhalation, but of its nature and even existence doubts may be entertained.

The arteries receive the constituents that belong to every living part, arteries, vein, lymphatics, and nerves. The arteries proceed not from the vessels themselves, but from adjacent trunks, as we have remarked of the vasa vasorum, to which these really belong. The nerves proceed from the great sympathetic, form plexuses