The structure indicated is the vertebral artery.
The vertebral arteries arise from the first part of the subclavian artery. Two other arteries are given off from the first part of the subclavian artery – the internal thoracic artery and the thyrocervical trunk. The vertebral and internal carotid arteries provide the arterial supply to the brain, forming the Circle of Willis at the base of the brain.
After the vertebral artery is given off from the subclavian artery, it passes through the transverse foramina of cervical vertebrae C1 to C6. It then enters the skull via the foramen magnum.
The vertebral arteries unite to form the basilar artery. Before forming the basilar artery, the vertebral arteries give off three branches. Two of these branches unite with their contralateral branches to form the anterior spinal artery and the posterior spinal artery. The third branch of the vertebral artery is the posterior inferior cerebellar artery.
The anterior spinal artery runs down the spinal cord in the anterior median fissure. The posterior spinal artery runs down the posterior surface of the spinal cord.
Learn more about the blood supply to the spinal cord in this tutorial.