azygous vein, one of the seven veins of the thorax. Beginning opposite the first or second lumbar vertebra, it rises through the aortic hiatus in the diaphragm and passes to the right of the vertebral column to the fourth thoracic vertebra, then arches ventrally over the root of the right lung, and ends in the superior vena cava. It receives numerous veins, such as the hemiazygous veins, several esophageal veins, and the right bronchial vein. In cases of obstruction to the inferior vena cava it is the principal vein that returns blood to the heart. Compare internal thoracic vein, left brachiocephalic vein, right brachiocephalic vein.
