The examination of this vessel may be deferred till its branches have been dissected. It is about five inches and a half or six inches in length, and extends from the aortic opening in the diaphragm to the left side of the fourth lumbar vertebra, or to the cartilage between the fourth and fifth: it may, however, extend to the fifth, or only as far as the second. The aortic opening in the diaphragm is oblique, and corresponds to the twelfth dorsal and part of the first lumbar vertebrae : its sides are formed by the two crura of the diaphragm; anteriorly and superiorly it is bounded by a tendinous arch which unites the two crura across the anterior aspect of the artery, and from the convexity of which arch some of the short fleshy fibres of the crura arise; and pos-teriorly by the anterior common ligament of the spine, which separates the vessel from the first lumbar vertebra. The posterior surface of the abdominal aorta rests on the spine, right crus of the diaphragm, which here sends an expansion in front of the lumbar vertebrae; on the receptaculum chyli, and left lumbar veins: the lumbar and middle sacral arteries arise from this surface of the vessel, and are therefore placed posterior to it. The anterior surface is covered from above downwards, first by the posterior edge of the liver, next by the union of the semilunar ganglia to form the solar plexus; by the aortic plexus of nerves, by the lesser omentum and stomach, then by the commencement of the vena portae and superior mesenteric artery, both of which separate it from the pancreas, which also crosses the anterior surface of the vessel; lower down, it is covered by the left renal vein, which separates it from the third portion of the duodenum; this intestine crosses the artery at a point corresponding to about the third lumbar vertebra: still lower, it is crossed by the transverse mesocolon and mesentery, and inferiorly by a single layer of the peritoneum, namely, the continuation downwards of the inferior or descending layer of the mesentery. Its left side corresponds to the left pillar of the diaphragm above, and below to the peritoneum. Its right side is separated from the vena cava superiorly by the Spigelian lobe of the liver, the right crus of the diaphragm, the vena azygos, and the thoracic duct; lower down it is nearly in contact with the latter vein. The sympathetic nerves also lie one at each side of the aorta, the left being in closer relation to it, and both on a plane posterior to the vessel.