This large vessel is a continuation of the arch of the aorta, and may be described as commencing opposite the lower part of the body of the third dorsal vertebra, and terminating opposite the fourth lumbar. Its commencement and termination are both on the left side of the spine, but that part of it which passes between the crura of the diaphragm approaches the middle line, so that in its entire course it forms a lateral curvature, the convexity of which is turned to the right side. In this respect the artery accommodates itself to the natural lateral curve which exists in the dorsal portion of the spine, the convexity of which is also directed towards the right side; in addition to this, the artery follows the curvature of the spine in the antero-posterior direction, and is therefore concave forwards in the thoracic region, and convex forwards in the abdominal. The descending aorta is divided into two portions, viz., the thoracic aorta, and the abdominal aorta. We shall first examine the thoracic, and then the abdominal portion.