Heart
in human anatomy, the central organ of circulation, inclosed in a membrane, the pericardium, and lying between the two layers of pleura, the mediastinum, with the base directed upward and backward to the right shoulder, and the apex downward and forward between the fifth and sixth ribs, and to the left. The under side is flattened and rests on the diaphragm, the upper rounded and convex, formed by the right ventricle and partially by the left; above these are the auricles whose appendages project forward, overlapping the root of the pulmonary artery, the large anterior vessel at the root of the heart, crossing obliquely the commencement of the aorta. The right is the venous side of the heart, the left arterial. The right auricle is larger than the left, and more complex in structure; it has two valves, the eustachian and the coronary. There is not the same pyramidal form in the left ventricle as in the right; the apex of the heart is also the apex of the left ventricle, and therefore larger than the right. The valves of the right ventricle are the tricuspid and semilunar; of the left the mitral (bicuspid) and semilunar. The auriculoventricular opening connects the auricles and ventricles, and in connection with the ventricular valves we have the columnæ corneæ, of which there are three HEAT sets, and the chordæ tendineæ. There are three layers of fibers in the ventriclesthe external, middle, and internal-their peculiar spiral arrangement causing the tilting forward of the cardiac apex. The fibers of the auricles are in two layers - the external and internal; and the left auricle is thicker and more fleshly than the right. From the right ventricle arises the pulmonary artery, conveying the venous blood to be aërated in the lungs; the infundibulum is a prolongation of the anterior wall. The left auricle contains the four pulmonary veins returning the blood to the heart, thence to the left ventricle, and thence to the aorta, to be distributed to every part of the body, returning by the superior and inferior vena cava to the right auricle.
In the lowest animals we have no blood-vessels, every part absorbing nutritious fluid for itself; the lower Entozoa, and even the embryo in man in its early stage, are examples. Among the higher reptiles, we find the circulation approaching that in birds and mammals, till we get the double heart, as in man.