The large vessel which opens out of the left ventricle of the HEART and carries blood to all of the body. It is about 45 cm (1••• feet) long and 2·5 cm (1 inch) wide. Like other arteries it possesses three coats, of which the middle one is much the thickest. This consists partly of muscle ﬁbre, but is mainly composed of an elastic substance called elastin. The aorta passes ﬁrst to the right, and lies nearest the surface behind the end of the second right rib-cartilage; then it curves backwards and to the left, passes down behind the left lung close to the backbone, and through an opening in the diaphragm into the abdomen. There it divides, at the level of the navel, into the two common iliac arteries, which carry blood to the lower limbs.
Its branches, in order, are: two coronary arteries to the heart wall; the brachiocephalic, left common carotid, and left subclavian arteries to the head, neck and upper limbs; several small branches to the oesophagus, bronchi, and other organs of the chest; nine pairs of intercostal arteries which run around the body between the ribs; one pair of subcostal arteries which is in series with the intercostal arteries; four (or ﬁve) lumbar arteries to the muscles of the loins; coeliac trunk to the stomach, liver and pancreas; two mesenteric arteries to the bowels; and suprarenal, renal and testicular arteries to the suprarenal body, kidney, and testicle on each side. From the termination of the aorta rises a small branch, the median sacral artery, which runs down into the pelvis. In the female the ovarian arteries replace the testicular.
The chief diseases of the aorta are ATHEROMA
and ANEURYSM. (See ARTERIES, DISEASES OF; COARCTATION OF THE AORTA.)