AORTA, the name given to the great arterial trunk of the body. It springs from the left ventricle of the heart, arches backward to the vertebral column, and, descending in the back portion of the thorax, passes through the diaphragm into the posterior part of the abdo men, at the lower portion of which, opposite the fourth lumbar segment of the spinal col umn, it divides into the two common iliac arteries. Throughout its course it becomes smaller and smaller, and gives off a large num ber of branches that send blood to all parts of the body. It is not divided into any distinct divisions, but for conventional purposes of de scription anatomists describe three parts, the arch, the thoracic aorta and the abdominal aorta. The arch reaches to the lower border of the fifth thoracic vertebra; from this point to the opening in the diaphragm it is known as the thoracic aorta; from the diaphragm to its bifur cation into the iliacs, the abdominal aorta. At its very beginning at the top of the heart it gives off the coronary arteries that supply the walls of the heart with blood. The arch is then conventionally divided into three parts, the ascending, transverse and descending por tions. From the transverse portion of the arch the great vessels of the neck, head and arms are given off,— the innominate or brachio cephalic artery, that goes to the head and upper extremity of the right side, the left common carotid, to the head on the left side and the subclavian that supplies the upper extremity of the left side. There are really two large
arteries supplying each side, but on the right they arise from the one branch from the aorta, while on the left side the arteries going to the head and to the upper extremities are separate. The thoracic aorta lies close to the spinal column in the chest. It supplies arteries ex tending to the walls of the chest and to all the viscera in the thorax, lungs, etc., save the heart. The abdominal aorta supplies the diaphragm, the muscles of the walls of the abdomen, the liver, kidneys, spleen, stomach, pancreas su prarenals, the small and large intestines, the spermatic vesicles and a part of the pelvis. The two great branches into which it finally divides supply the pelvic walls, the organs in the pelvis, the external genitals and finally the lower limbs. There are a number of variations in the details of the distribution of the differ ent large vessels of the aorta, but these concern the anatomist. The walls of the aorta resemble in their microscopical structure those of the arteries of the body save in possessing more elastic fibrous tissues. This is necessitated be cause of the greater pressure on this part of the circulatory apparatus. See ARTERY; CIRCU LATION ; HEART.