17. CIRCULATION.--The blood is in constant motion during life. From the heart, as a centre, a current is always setting toward the different organs; and from these organs a current is constantly returning to the heart. In this way a ceaseless circular movement is kept up, which is called the Circulation of the Blood. This stream of the vital fluid is confined to certain fixed channels, the blood-vessels. Those branching from the heart are the arteries; those converging to it are the veins. The true course of the blood was unknown before the beginning of the seventeenth century. In 1619 it was discovered by the illustrious William Harvey. Like many other great discoverers, he suffered persecution and loss, but unlike some of them, he was fortunate enough to conquer and survive opposition. He lived long enough to see his discovery universally accepted, and himself honored as a benefactor of mankind. {108} [Illustration: FIG. 27.--THE ORGANS OF CIRCULATION.] {109} [Illustration: FIG. 28.--THE HEART AND LARGE VESSELS. A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle. E, Aorta. F, Pulmonary Artery.] [Sidenote: 18. Office of the heart? Location of the heart? Its beat? Its shape? Protection to the heart? What else is said in relation to the heart?] 18. THE HEART.--The heart is the central engine of the circulation. In this wonderful little organ, hardly larger than a man's fist, resides that sleepless force by which, during the whole of life, the current of the blood is kept in motion. It is placed in the middle and front part of the chest, inclining to the left side. The heartbeat may be felt and heard between the fifth and sixth ribs, near the breast-bone. The shape of the heart is conical, with the apex or point downward and in front. The base, which is upward, is attached so as to hold it securely in its place, while the apex is freely moveable. In order that loss of power from friction may be obviated, the heart is enclosed between two layers of serous membrane, which forms a kind of sac. This membrane is as smooth as satin, and itself secretes a fluid in sufficient quantities to keep it at all times well lubricated. The lining membrane of the heart, likewise, is extremely delicate and smooth. {110} [Illustration: FIG. 29.--SECTION OF THE HEART. A, Right Ventricle. B, Left Ventricle. C, Right Auricle. D, Left Auricle. E, F, Inlets to the Ventricles. G, Pulmonary Artery. H, Aorta.] [Sidenote: 19. Formation of the heart? Right and left heart?]