AORTA, the main systemic artery arising from the heart. It receives the blood, which has been aerated in the lungs, from the left ventricle of the heart, and starts it on its way to all parts of the body. From the left ventricle the aorta arches first upward, then to the left and backward, and finally descends on the left of the spinal column through the thorax and abdomen to the fourth lumbar vertebra where it divides into two main branches, the common iliac arteries. From the aorta also arise the great arteries which supply the head and arms, and smaller arteries to the thoracic and abdominal organs. The diseases most commonly affecting it are arteriosclerosis and syphilis. By weakening its wall these diseases may cause an out-pouching of the aorta, called an aneurism, which sometimes ruptures, causing fatal haemorrhage.