The main vessel of the systemic arterial circulation arising from the left ventricle of the heart; it is divided into three parts for convenience only. The first portion, the ascending aorta, passes upward under the pulmonary artery; the coronary arteries arise at the base of the ascending aorta behind the aortic valves. The second part, or aortic arch, curves over the hilum of the left lung, giving off the innominate, left carotid, and left subclavian arteries, which supply the neck, head, and forelimbs. The third portion, or descending aorta, continues downward in the thorax on the left side of the vertebral column to the diaphragm, giving off small arteries to the bronchi, esophagus, and other adjacent tissues. Below the diaphragm this vessel, known as the abdominal aorta, descends to the level of the fourth lumbar vertabra where it bifurcates into the two common iliac arteries supplying the hindlimbs.