From the arch are given off the arteries which supply the head and arms with blood. These are the two carotid arteries, which run up on each side of the neck to the head, and the two subclavian arteries, which pass beneath the collar bone to the arms. This great arterial trunk now passes down in front of the spine to the pelvis, where it divides into two main branches, which supply the pelvis and the lower limbs. The descending aorta, while passing downwards, gives off arteries to the different tissues and organs. Of these branches the chief are the cœliac artery, which subdivides into three great branches,—one each to supply the stomach, the liver, and the spleen; then the renal arteries, one to each kidney; and next two others, the mesenteric arteries, to the intestines. The aorta at last divides into two main branches, the common iliac arteries, which, by their subdivisions, furnish the arterial vessels for the pelvis and the lower limbs. Illustration: Fig. 72.—Left Cavities of the Heart. A, B, right pulmonary veins; with S, openings of the veins; E, D, C, aortic valves; R, aorta; P, pulmonary artery; O, pulmonic valves; H, mitral valve; K, columnæ carnoeæ; M, right ventricular cavity; N, interventricular septum. The flow of blood in the arteries is caused by the muscular force of the heart, aided by the elastic tissues and muscular fibers of the arterial walls, and to a certain extent by the muscles themselves. Most of the great arterial trunks lie deep in the fleshy parts of the body; but their branches are so numerous and become so minute that, with a few exceptions, they penetrate all the tissues of the body,—so much so, that the point of the finest needle cannot be thrust into the flesh anywhere without wounding one or more little arteries and thus drawing blood. 188. The Veins. The veins are the blood-vessels which carry the impure blood from the various tissues of the body to the heart. They begin in the minute capillaries at the extremities of the four limbs, and everywhere throughout the body, and passing onwards toward the heart, receive constantly fresh accessions on the way from myriad other veins bringing blood from other wayside capillaries, till the central veins gradually unite into larger and larger vessels until at length they form the two great vessels which open into the right auricle of the heart. These two great venous trunks are the inferior vena cava, bringing the blood from the trunk and the lower limbs, and the superior vena cava, bringing the blood from the head and the upper limbs. These two large trunks meet as they enter the right auricle. The four pulmonary veins, as we have learned, carry the arterial blood from the lungs to the left auricle. Illustration: Fig. 73. A, part of a vein laid open, with two pairs of valves; B, longitudinal section of a vein, showing the valves closed.