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The muscular fibres of each auricle are in part continuous with those of the other, and partly separate; and the same remark holds true for the ventricles. The fibres of the auricles are, however, quite separate from those of the ventricles, the bond of connection between them being only the fibrous tissue of the auriculo-ventricular openings.

The muscular fibres of the heart, unlike those of most of the involuntary muscles, are striated; but although, in this respect, they resemble the skeletal muscles,

they have distinguishing characteristics of their own. The fibres which lie side by side are united at frequent intervals by short branches (fig. 92). The fibres are smaller than those of the ordinary striated muscles, and their striation is less marked. No sarcolemma can be discerned. The muscle-corpuscles are situate in the middle of the substance of the fibre; and in correspondence with these the fibres appear under certain conditions subdivided into oblong portions or "cells," the offsets from which are the means by which the fibres anastomose one with another (fig. 93).

b

α

Fig. 93. Muscular fibre cells from the heart. (E. A. Schäfer.)

Endocardium.-As the heart is clothed on the outside by a thin transparent layer of pericardium, so its cavities are lined by a smooth and shining membrane, or endocardium, which is directly continuous with the internal lining of the arteries and veins. The endocardium is composed of connective tissue with a large admixture of elastic fibres; and on its inner surface is laid down a single tesselated layer of flattened endothelial cells. Here and there unstriped muscular fibres are sometimes found in the tissue of the endocardium.

Valves of the Heart.-The arrangement of the heart's valves is such that the blood can pass only in one direction (fig. 94).

The tricuspid valve (5, fig. 89) presents three principal cusps or subdivisions, and mitral or bicuspid valve, because it has two such portions (6, fig. 90). But in both valves there is between each two principal portions a smaller one; so that more properly, the

tricuspid may be described as consisting of six, and the mitral of four, portions. Each portion is of triangular form, its base is continuous with the bases of the neighbouring portions, so as to form an annular membrane around the auriculo-ventricular opening, and is fixed to a tendinous ring which encircles the orifice between the auricle and ventricle and receives the insertions of the muscular fibres of both. In each principal cusp may be distinguished a central part, extending from base to apex, and

including about half its width. It is thicker, and much tougher than the border-pieces or edges.

While the bases of the cusps of the valves are fixed to the tendinous rings, their ventricular surface and borders are fastened by slender tendinous fibres, the chorda tendinea, to the internal surface walls of the ventricles, the muscular fibres of which project into the ventricular cavity in the form of bundles or columnsthe columnæ carneæ. These columns are not all alike, for while some are attached along their whole length on one side, and by their extremities, others are attached only by their extremities; and a third set, to which the name musculi papillares has been given, are attached to the wall of the ventricle

by one extremity only, the other projecting, papilla-like, into the cavity of the ventricle (5, fig. 89), and having attached to it chordæ tendineæ. Of the tendinous cords, besides those which pass from the walls of the ventricle and the musculi papillares to the margins of the valves, there are some of especial strength, which pass from the same parts to the edges of the middle and thicker portions of the cusps before referred to. The ends of. these cords are spread out in the substance of the valve, giving its middle piece its peculiar strength and toughness; and from the sides numerous other more slender and branching cords are given off, which are attached all over the ventricular surface of the adjacent border-pieces of the principal portions of the valves, as well as to those smaller portions which have been mentioned as lying between each two principal ones. Moreover, the musculi papillares are so placed that, from the summit of each, tendinous cords proceed to the adjacent halves of two of the principal divisions, and to one intermediate or smaller division, of the valve.

The preceding description applies equally to the mitral and tricuspid valve; but it should be added that the mitral is considerably thicker and stronger than the tricuspid, in accordance with the greater force which it is called upon to resist.

The semilunar valves, three in number, guard the orifices of the pulmonary artery and of the aorta. They are nearly alike on both sides of the heart; but the aortic valves are altogether thicker and more strongly constructed than the pulmonary valves, in accordance with the greater pressure which they have to withstand. Each valve is of semilunar shape, its convex margin being attached to a fibrous ring at the place of junction of the artery to the ventricle, and the concave or nearly straight border being free, so that each valve forms a little pouch like a watch-pocket (7, fig. 90). In the centre of the free edge of the valve, which contains a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, and from this and from the attached border fine fibres extend into every part of the mid substance of the valve, except a small lunated space just within the free edge, on each side of the corpus Arantii. Here the valve is thinnest, and composed of little more than the endocardium. Thus constructed and attached, the three semilunar valves are placed side by side around the arterial orifice of each ventricle, so as to form three little pouches, which can be separated by the blood passing out of the ventricle, but which immediately afterwards are

pressed together so as to prevent any return (7, fig. 89, and 7, fig. 90). This will be again referred to. Opposite each of the semilunar cusps, both in the aorta and pulmonary artery, there is a bulging outwards of the wall of the vessel: these bulgings are called the sinuses of Valsalva.

Structure of the Valves.-The valves of the heart are formed essentially of thick layers of closely woven connective and elastic tissue, over which, on every part, is reflected the endocardium.

II. The Arteries.

Distribution. The arterial system begins at the left ventricle in a single large trunk, the aorta, which almost immediately after

its origin gives off in the thorax three large branches for the supply of the head, neck, and upper extremities; it then traverses the thorax and abdomen, giving off branches, some large and some small, for the supply of the various organs and tissues it passes on its way. In the abdomen it divides into two chief branches, for the supply of the lower extremities. The arterial branches

wherever given off divide and subdivide, until the calibre of each subdivision becomes very minute, and these minute vessels pass into capillaries. Arteries are, as a rule, placed in situations protected from pressure and other dangers, and are, with few exceptions, straight in their course, and frequently communicate (anastomose or inosculate) with other arteries. The branches are usually given off at an acute angle, and the area of the branches

of an artery generally exceeds that of the parent trunk; and as the distance from the origin is increased, the area of the combined branches is increased also.

After death, arteries are usually found dilated (not collapsed as the veins are) and empty, and it was to this fact that their name was given them, as the ancients believed that they conveyed air to the various parts of the body. As regards the arterial system of the lungs (pulmonary system) it begins at the right ventricle in the pulmonary artery, and is distributed much as the arteries belonging to the general systemic circulation.

Structure.-The walls of the arteries are composed of three principal coats, termed (a) the external or tunica adventitia, (b) the middle or tunica media, and (c) the internal or tunica intima.

(a) The external coat or tunica adventitia (figs. 95 and 96 a.), the strongest and toughest part of the wall of the artery, is