The structure indicated is the azygos vein.
The azygos system of veins includes:
- Azygos vein
- Hemiazygos vein
- Accessory hemiazygos vein
The azygos vein runs in the posterior mediastinum, on the right side of thoracic vertebral column. It drains into the superior vena cava, and transports deoxygenated blood from the posterior walls of the thorax and the abdomen.
The azygos system forms important anastamotic connections, which serve to return blood to the heart via the superior vena cava, if the inferior vena cava becomes obstructed.