to the head and upper extremities. Hence their large size and perfect development at birth. Returned from the upper extremities by the superior vena cava, the blood enters the right auricle again and, passing over the Eustachian valve this time, descends to the right ventricle, from which the greater part passes by the pulmonary artery and the ductus arteriosus to the descending aorta, though a small amount keeps on through the pulmonary artery to the lungs. In the aorta it mixes with the blood from the left ventricle and part goes to supply the lower extremities, though the greater part is carried back to the placenta through the two umbilical arteries. The fact that the greater part of the blood traverses the liver accounts for its large size at birth, while the lower extremities, which receive for the most part blood that has already circulated through the upper extremities, are of small size and imperfectly developed.
Arteries.—After birth the arterial blood for the general circulation leaves the heart by the aorta, the main distributing artery of the body. Through this and its branches it is carried throughout the body in what, with the return of the venous blood by the venae cavæ and other smaller veins, is known as the systemic circulation. The aorta ascends from the left ventricle and arches backward to the left over the root of the left lung to descend along the spinal column at the left to the fourth lumbar vertebra, about opposite the umbilicus, where, considerably diminished in size by the branches it has given off, it divides into the two common iliacs. For convenience its different parts are named, according to their position, the ascending aorta, the arch of the aorta, and the descending aorta, the last being subdivided into the thoracic and the abdominal aorta.
From the ascending aorta come off the coronary arteries which supply the heart muscle itself, as the coronary sinuses carry off the venous blood from the heart. From the arch are given off the left common carotid and left subclavian and the innominate, which
