The structure indicated is the descending aorta.
The aorta consist of the ascending aorta, the aortic arch, and the descending aorta. The descending aorta is the largest artery in the human body and consists of a thoracic part and an abdominal part. The descending aorta passes through the aortic hiatus of the diaphragm together with the azygos vein and thoracic duct to enter the abdominal cavity where it is then referred to as the abdominal aorta. The abdominal aorta terminates by dividing at the aortic bifurcation into the common iliac arteries at the level of the fourth lumbar vertebra.