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 venæ 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_. [Illustration: FIG. 44.--The aortæ and their branches. (Leidy.)] 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 divides into the _right common carotid_ and _right subclavian_. The _common carotids_ pass up the neck behind the sterno-cleido-mastoid muscles in a line from the sterno-clavicular joint to a point midway between the mastoid process and the angle of the lower jaw and divide opposite the upper border of the thyroid cartilage into the _internal_ and _external carotids_, of which the former with its branches supplies the anterior part of the brain, the eye and forehead, and the latter the neck and face.