We will now turn to the left side of the heart, and notice the general arrangement of its great vessels. Four veins, called the pulmonary veins, open into the left auricle, two from each lung. These veins start from very minute vessels the continuation of the capillaries of the pulmonary artery. They form larger and larger vessels until they become two large veins in each lung, and pour their contents into the left auricle. Thus the pulmonary artery carries venous blood from the right ventricle to the lungs, as the pulmonary veins carry arterial blood from the lungs to the left auricle.

From the left ventricle springs the largest arterial trunk in the body, over one-half of an inch in diameter, called the aorta. From the aorta other arteries branch off to carry the blood to all parts of the body, only to be again brought back by the veins to the right side, through the cavities of the ventricles. We shall learn in [Chapter VIII.] that the main object of pumping the blood into the lungs is to have it purified from certain waste matters which it has taken up in its course through the body, before it is again sent on its journey from the left ventricle.

187. The Arteries. The blood-vessels are flexible tubes through which the blood is borne through the body. There are three kinds,—the arteries, the veins, and the capillaries, and these differ from one another in various ways.

The arteries are the highly elastic and extensible tubes which carry the pure, fresh blood outwards from the heart to all parts of the body. They may all be regarded as branches of the aorta. After the aorta leaves the left ventricle it rises towards the neck, but soon turns downwards, making a curve known as the arch of the aorta.

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.

Fig. 72.—Left Cavities of the Heart.