It is situated in front of the vertebral column, partly within the thorax and partly in the abdomen. It commences at the left ventricle of the heart, with which it is intimately connected ; and after ascending a short distance within the thorax, arches over to the left side, and then descending along the vertebral column, passes through the diaphragm into the abdominal cavity, and ends opposite the fourth lumbar vertebra, by dividing into the right and left common iliac arteries. In this course the primary systemic artery forms a continuous undivided trunk, which gradually diminishes in size from its commencement to its termination, and gives off larger or smaller branches at various points. Nevertheless, different parts of the vessel have received particular names, derived from their position or direction. The following are recognized, viz., the arch of the aorta, the thoracic aorta, and the abdominal aorta. The short curved part, which reaches from the ventricle of the heart to the side of the third dorsal vertebra, is named the arch ; the straight part, which extends from that vertebra to the diaphragm, is called the thoracic aorta; and the remainder of the vessel, down to its bifurcation, is spoken of as the abdominal aorta. These three parts will be examined separately, the first part or arch being described immediately, and the other divisions in subsequent pages.