Heart, hahrt, a hollow, muscular organ, which forces the blood through the veins and arteries and thus maintains the circulation. In mammals and birds the heart has four chambers or cavities, two auricles and two ventricles; but in most reptiles there are butthreechambers, and in some still lower orders there are but two. The heart in man is a coneshaped organ, situated in the chest behind the sternum and placed obliquely in an inverted position. The base points upward, inward and to the right, and the apex downward, outward and to the left. About two-thirds of the heart is to the left of the median line of the body. The heart is suspended in the chest by the large blood vessels attached to it and is surrounded by a serous membrane known as the pericardium. A similar membrane of finer texture, known as the endocardium, also lines the cavities. The heart is about 5 inches long, its greatest diameter is 3 1/2 inches and the average weight of the organ in an adult is from 9 to 10 ounces. The walls are constructed of muscular fibers, and the muscular partition called the septum separates the organ into right and left sides. The left side contains the left auricle and left ventricle, and the right side, the right auricle and right ventricle. The right side of the heart forces the blood through the lungs and is connected with the pulmonary circulation. Because of the functions of this part of the heart the muscular walls are much thinner and weaker than those of the left side, whose function it is to force the blood through the arteries in the systemic circulation. The auricles occupy the base of the heart and are formed by very thin walls. The right auricle receives the blood from the superior and the inferior vena cavae, and the left auricle receives it from the pulmonary veins. The ventricles occupy the apex and force the blood through the arteries. Between the auricles and ventricles are valves, composed of a thin, strong white membrane, whose folds are so arranged that they allow the blood to flow from the auricle into the ventricle, but prevent its return when the muscles of the heart contract. The valve on the right side is known as the tricuspid, and that on the left side is called the bicuspid, or mitral, valve. The openings into the arteries are also guarded by valves, which in shape resemble a half-moon and are called semilunar valves. The course of the blood through the heart is illustrated in the diagram. See CIRCULATION.