Accessory hemiazygos v.

The accessory hemiazygos vein is a vein on the left side of the vertebral column that generally drains the fifth through eighth intercostal spaces on the left side of the body. 

The accessory hemiazygos vein varies inversely in size with the left superior intercostal vein. 

It receives the posterior intercostal veins from the 4th, 5th, 6th, and 7th intercostal spaces between the left superior intercostal vein and highest tributary of the hemiazygos vein. The left bronchial vein sometimes drains into it as well. 

It either crosses the vertebral column at the eighth thoracic vertebra to join the azygos vein or ends in the hemiazygos vein. 

When this vein is small or altogether absent, the left superior intercostal vein may extend as low as the fifth or sixth intercostal space.