Which arteries contribute to the posterior circulation by joining to form the basilar artery?

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The key idea is that the posterior circulation comes from the vertebrobasilar system, which is formed when the two vertebral arteries fuse to make the basilar artery. Each vertebral artery runs up from the subclavian arteries, ascends through the foramina of the cervical vertebrae, and enters the skull. They join at the lower part of the pons to form the basilar artery, which then travels along the ventral surface of the brainstem and gives off branches to the brainstem, cerebellum, and occipital lobes. The internal carotid arteries contribute to the brain’s posterior circulation only indirectly via the posterior communicating arteries, but they do not join to form the basilar. External carotid arteries supply extracranial structures and are not part of the brain’s posterior circulation.