connected with the heart are numerous. They have been partly men tioned in the preceding paragraphs. Two large veins open into the right auricle, one at the upper part—the superior vena cava—and the other at the lower part—the inferior vena cava. The superior cava brings blood from the head, neck, upper limbs, and chest, being formed by the union of two venous trunks, one from the right side of the body and the other from the left. In Fig. 136,E marks the junction of the right and left trunks (K, K) from which the vena cava descends. The in ferior cava is not shown in Fig. 136, being behind the parts there shown. It conies front below, and brings the blood from the lower part of the body. Thus these two large veins bring the blood from all parts of the body and pour it into the right auricle; and, as we have seen, this blood all passes down into the ventricle. From the right ventricle one large vessel arises—the pulmonary artery (the lung artery; Latin pubno, the lung), which, at a dis tance of not more than 2 inches from the heart, divides into two branches of nearly equal size, one for the right lung and another for the left. Each branch, having reached the lung for which it is destined, gives off branches, and these branches again give off others, and so the branch ing goes on till it ends in fine delicate vessels, capillaries, forming a rich net- work in every part of the lung. Thus the blood, driven out of the ventricle into the pulmonary artery, is by the artery distributed in fine streams through out the two lungs. Turn now to the left auricle. Four veins open into it, two coming from each lung—pulmonary veins, therefore. These veins arise in the lungs from the extremely fine vessels, which are the continuations of the ca pillaries of the pulmonary artery. The fine
vessels unite to form largel• vessels, and so on the process goes till two large veins are formed from each lung, which pass to the left auricle. The pulmonary veins, therefore, carry from the lungs to the left auricle the blood brought to Thus we see that the blood, collected from all parts of the body, enters the right side of the heart by the right auricle, and is poured down into the right ventricle, from which it is forced along the pulmonary artery through the lungs. It is there collected by the pulmonary veins and carried to the left auricle, poured into the left ventricle, and from it forced into the aorta, by the branches of which it is conveyed to every part of the body, only to be again brought back to the right side of the heart. As we shall see in Section XVI., a chief object of distributing the blood through the lungs is to have it purified from certain waste substances which it has re ceived in its course through the body, before it is again sent on its journey from the left ven tricle. [The various blood-vessels that have been mentioned are shown in Figs. 134 and 136.] The means by which the muscular substance of the heart is nourished remain to be con sidered. Close above the semilunar valves of the aorta there arise two arteries—the coro nary arteries, one of which passes to the right side of the heart and the other to the left. In Fig. 134 c. r. points to the right coronary artery, and c. 1. to the left. Between them these vessels carry sufficient pure blood to nourish the heart. After circulating through the heart substance the blood is collected by coronary veins, which pour it into the right auricle. Thus the heart gets its own share of the nourishment which it is its business to drive through the body.