accessory hemiazygos vein

formed by the union of the fourth to seventh left posterior intercostal veins, passes along the side of the bodies of the fifth, sixth, and seventh thoracic vertebrae, then crosses the midline behind the aorta, esophagus, and thoracic duct, and empties into the azygos vein, sometimes in common with the hemiazygos vein. Syn: vena hemiazygos accessoria [NA], vena azygos minor superior.