Final answer:
The two vertebral arteries merge to form the basilar artery in the brain, which supplies the brain stem and cerebellum. They, along with the internal carotid arteries, form the circle of Willis to maintain blood perfusion in the brain.
Step-by-step explanation:
The two vertebral arteries in the brain merge to form the basilar artery. The basilar artery then gives rise to branches that supply the brain stem and cerebellum. These arteries, along with the internal carotid arteries, form the circle of Willis, which is a confluence of arteries that helps maintain blood perfusion to the brain.