The arrangement of the brain's arteries into the circle of Willis is believed to create redundancy (analogous to engineered redundancy) for collateral circulation in the cerebral circulation. In another variation the anterior communicating artery is a large vessel, such that a single internal carotid supplies both anterior cerebral arteries this is known as an azygos anterior cerebral artery. In one common variation the proximal part of the posterior cerebral artery is narrow and its ipsilateral posterior communicating artery is large, so the internal carotid artery supplies the posterior cerebrum this is known as a fetal posterior communicating cerebral artery. Based on a study of 1413 brains, the classic anatomy of the circle is only seen in 34.5% of cases. Since they do not directly affect the circle of Willis, they are not dealt with here.Ĭonsiderable anatomic variation exists in the circle of Willis. The cortical branches are named for the area they supply. The central branches supply the interior of the circle of Willis, more specifically, the Interpeduncular fossa. The anterior communicating artery connects the two anterior cerebral arteries and could be said to arise from either the left or right side.Īll arteries involved give off cortical and central branches. The vertebral arteries arise from the subclavian arteries. The right and left posterior cerebral arteries arise from the basilar artery, which is formed by the left and right vertebral arteries. The anterior cerebral artery forms the anterolateral portion of the circle of Willis, while the middle cerebral artery does not contribute to the circle. The posterior communicating artery is given off as a branch of the internal carotid artery just before it divides into its terminal branches - the anterior and middle cerebral arteries. The left and right internal carotid arteries arise from the left and right common carotid arteries. The middle cerebral arteries, supplying the brain, are not considered part of the circle of Willis. Posterior communicating artery (left and right).Posterior cerebral artery (left and right).Internal carotid artery (left and right).Anterior cerebral artery (left and right).The circle of Willis is a part of the cerebral circulation and is composed of the following arteries:
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