558 HEART ring from which they originated, or, by fleshy columns, to the tendinous chords of the ven- tricular valves. Besides its muscular tissue, the heart is covered externally by a serous and fibrous membrane, the pericardium, and is lined internally by another serous mem- brane, the endocardium ; the latter of which is continuous with the lining membrane of the blood vessels. At each of the great orifices of the heart are membranous valves, composed of a thin and flexible but strong fibrous tissue, so arranged as to allow the blood to pass free- ly onward in its course, but to prevent its flowing back in the opposite direction. Those situated at the auriculo-ventricular orifices are the ventricular valves; those at the orifices of the great arteries are the arterial valves. Each set, on the right and left side, has also received a special name corresponding to certain pecu- liarities of form or position. The right ven- tricular valve is composed of a broad fibrous FIG. 8. Horizontal Section of the Human Heart, at the level of the Ventricular Orifices. A. Orifice of pulmonary artery. B. Orifice of aorta. C, C, C. Tricuspid valves. D, D. Mitral valves. sheet with three main points or divisions, whence it is called the tricuspid valve. Its floating edges are easily displaced from the auricle toward the ventricle, thus allowing the blood a free passage in this direction ; but on being forced backward, from the ventricle to- ward the auricle, its free edges come in con- tact with each other and are firmly held in this position by a number of tendinous chords running from its edge and under surface to the inner wall of the ventricle below, which are now put upon the stretch. Thus the passage from the ventricle to the auricle is completely closed, and it can be opened only in a forward direction. The left ventricular valve is similar to the right, except that it consists of only two main pointed sheets, like the two halves of a bishop's mitre ; it is therefore called the mitral valve. It is supported in position by tendinous chords, as in the former instance (fig. 3). At each of the arterial orifices are three valves, having the form of semilunar bags, with their concavities toward the wall of the artery to which they are attached, and their convexities toward the cavity of the ventricle. Thus when the stream of blood passes from the ventricle into the artery, the valves are flattened against the wall of the vessel, and offer no resistance to its current ; but when distended by a back- ward impulse they fill with blood, and, their edges coming in contact, they close the orifice of the ventricle in this direction. Both sets of arterial valves, from the similarity of their figure, are called semilunar valves ; but they are also known as the pulmonary or the aortic valves, according to the particular vessel in which they are situated. They are usually de- scribed in connection with the heart ; but it is evident from what has been said above that they belong in reality rather to the arterial system. The action of the heart consists in an alternate contraction and relaxation, by which at one instant it receives the blood from the veins and at another propels it into the arte- rial system. In this process the two auricles, right and left, contract simultaneously; and the two ventricles subsequently contract, also at the same instant with each other. Still the auricular and ventricular contractions are not distinctly and separately alternate with each other, to the same extent as the strokes of the two pistons of a force pump. The action of the heart appears rather to consist in a sin- gle continuous contraction, which begins at the auricle and thence runs forward to terminate at the ventricle. The ventricular action is much more powerful than that of the auricle ; and of the two ventricles the left is so much the more important in size and strength, that mainly to the action of this part are due the changes m form and position of the heart at the instant of contraction. The contraction of the ven- tricles is almost instantly followed by their re- laxation ; and while in this condition they are gradually filled by the blood flowing steadily into them from the veins and through the qui- escent auricles. Then comes the contraction of the auricles, which completes the distention of the ventricular cavities ; and this distention is at once followed by the vigorous contraction of the ventricles, discharging their blood in great abundance, to be followed again by a pe- riod of relaxation. This is the succession of the phenomena which present themselves when the heart is seen in activity during life. At every ventricular contraction the substance of the heart becomes harder ; it twists slightly upon itself from left to right ; and its point strikes the walls of the chest, in the human subject, in the fifth intercostal space, a little to the left of the edge of the sterntfm. Tlie induration of the organ at the moment of its activity is due to its muscular nature, since the same tension and momentary induration can be felt in any of the voluntary muscles in con- traction. Such a muscle, when in activity, becomes swollen and at the same time harder
