Circle of Willis

The Circle of Willis (also called Willis’ Circle, cerebral arterial circle, and Willis Polygon) is a circle of arteries that supply blood to the brain. It is named after Thomas Willis (1621–1675), an English physician.

The Circle of Willis comprises the following arteries:
Anterior cerebral artery (left and right)
Anterior communicating artery
Internal carotid artery (left and right)
Posterior cerebral artery (left and right)
Posterior communicating artery (left and right)

The basilar artery and middle cerebral arteries, supplying the brain, are also considered part of the circle.

Physiologic significance

The arrangement of the brain’s arteries into the Circle of Willis creates redundancies or collaterals in the cerebral circulation. If one part of the circle becomes blocked or narrowed (stenosed) or one of the arteries supplying the circle is blocked or narrowed, blood flow from the other blood vessels can often preserve the cerebral perfusion well enough to avoid the symptoms of ischemia.

Anatomic variation

Considerable anatomic variation exists in the Circle of Willis. Based on a study of 1413 brains, the classic anatomy of the circle is only seen in 34.5% of cases. 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. In another variation the anterior communicating artery is a large vessel, such that a single internal carotid supplies both anterior cerebral arteries.

Origin of arteries

Cerebral angiogram showing an anterior/posterior projection of the vertebrobasilar and posterior cerebral circulation, the posterior aspect of the Circle of Willis and one of its feeding vessels.

An anterior view of major cerebral and cerebellar arteries.

The left and right internal carotid arteries arise from the right and left common carotid arteries.

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 anterior cerebral artery forms the anterolateral portion of the Circle of Willis, while the middle cerebral artery does not contribute to the circle.

The right and left posterior cerebral arteries arise from the basilar artery, which is formed by the left and right vertebral arteries. The vertebral arteries arise from the subclavian arteries.

The anterior communicating artery connects the two anterior cerebral arteries and could be said to arise from either the left or right side.

All arteries involved give off cortical and central branches. The central branches supply the interior of the Circle of Willis, more specifically, the Interpeduncular fossa. The cortical branches are named for the area they supply. Since they do not directly affect the Circle of Willis, they are not dealt with here.

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