The accessory hemiazygos vein, which forms part of the azygos system, varies in origin and level of anastomoses. It is formed by and receives the left fourth to eighth posterior intercostal veins, receiving blood from the fourth to eighth intercostal spaces.  It descends on the left of the vertebral column to reach the body of the seventh thoracic vertebra where it usually traverses, posterior to the aorta, esophagus and thoracic duct, to join the azygos vein.  It may join with the hemiazygos vein to form a common trunk before draining into the azygos vein.