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Posterior-inferior cerebellar (posterior-inferior “cerebral”) artery is a branch of the vertebral artery that supplies the cerebellum and medulla but does not participate in the anastomotic ring of the circle of Willis. In contrast, the anterior cerebral, posterior communicating, and (although not part of the circle’s ring itself) the middle cerebral arteries arise from the internal carotid system that feeds directly into the circle [Kardile 2013, PMID 24551604].