The heart is a hollow muscular organ, divided by a longitudinal septum into a right and a left half, each of which is again subdivided by a transverse constriction into two compartments, communicating with each other, and named auricle and ventricle. Its general form is that of a blunt cone. Enclosed, as before said, in the pericardium, it is placed behind the sternum and the costal cartilages, the broader end, or base being directed upwards, backwards, and to the right, and extending from the level of the fifth to that of the eighth dorsal vertebra ; the apex downwards, forwards, and to the left.
In the living subject its stroke against the wall of the chest is felt in the space between the cartilages of the fifth and sixth ribs, a little below and to the inner side of the left nipple (3.25 inches from the middle line of sternum and 1.5 inch below the nipple): in the dead body the apex IB a little higher than during life. The heart, therefore, has a very oblique position in the chest, and projects farther into the left than into the right half of the cavity. Its position is affected to a certain extent by that of the body; thus it comes more into contact with the anterior wall of the chest when the body is in the prone posture or is lying on the left side. In inspiration, on the other hand, when the diaphragm sinks and the lungs expand, it recedes slightly from the chest-wall.
The heart is attached at its base to the great blood-vessels, and the serous layer of the pericardium is here continued on to it. Otherwise the heart is entirely free within the sac of the pericardium. The convex anterior surface looks somewhat upwards as well as forwards towards the sternum and costal cartilages: from these it is for the most part separated by the pleurae. The lungs also advance over it to some extent, and encroach still farther during inspiration, so as in that condition to leave only a triangular part, not more than two square inches in extent, uncovered. The posterior or under surface is flattened, and rests on the diaphragm. Of the two borders or margins formed by the meeting of the anterior and posterior surfaces, the right or lower border, called margo acutus, is comparatively thin, and is longer than the upper or left border, which is more rounded and is named margo obtusus.
A deep transverse groove, the auriculo-ventricular furrow, divides the heart into the auricular and the ventricular portions ; and on the ventricular portion two longitudinal furrows, situated one on the anterior, the other on the posterior surface, mark its division into a right and left chamber. They extend from the base of the ventricular portion, and are continuous one with the other a little to the right of the apex, which is thus formed entirely by the wall of the left ventricle. The anterior longitudinal furrow is nearer to the left, and the posterior furrow nearer to the right side of the heart, the right ventricle forming more of the anterior, and the left more of the posterior surface of the organ. In the furrows run the coronary arteries and veins with lymphatic vessels and nerves, embedded in fatty tissue and covered by the visceral layer of the pericardium.