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Artery: Circle of Willis
Schematic representation of the circle of Willis, arteries of the brain and brain stem.
The brain and the arteries of the base of the brain. Circle of Willis is formed near center. The temporal pole of the cerebrum and a portion of the cerebellar hemisphere have been removed on the right side. Inferior aspect (viewed from below).
Latin	circulus arteriosus cerebri
Gray's	subject #147 574
MeSH	Circle+of+Willis

The circle of Willis (also called the cerebral arterial circle or arterial circle of Willis) is a circle of arteries that supply blood to the brain. It is named after Thomas Willis (1621-1673), an English physician.^[1]

• 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, though they supply the brain, are not considered part of the circle.^[2]

The arrangement of the brain's arteries into the Circle of Willis creates redundancies 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.^[3]

Considerable anatomic variation exists in the circle of Willis. The "textbook version" of the circle, based on a series of 1413 brains, is only seen in 34.5% of cases.^[4] 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.

The redundancies that the circle of Willis introduce can also lead to reduced cerebral perfusion.^[5][6] In subclavian steal syndrome, which results from a proximal stenosis (narrowing) of the subclavian artery (a vessel that is supplied by the same vessel (the Aorta) that eventually feeds the circle of Willis via the Common carotid artery), blood is "stolen" from the circle of Willis to preserve blood flow to the upper limb.

Cerebral angiogram showing a transverse projection of the vertebrobasilar and posterior cerebral circulation, i.e. the posterior aspect of the circle of Willis and one of its feeding vessels.

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.

1. ^ Uston C. Dr. Thomas Willis' famous eponym: the circle of Willis. J Hist Neurosci. 2005 Mar;14(1):16-21. PMID 15804755. Free Full Text.
2. ^ Moore KL, Dalley AR. Clinically Oriented Anatomy, 4th Ed., Lippincott Williams & Wilkins, Toronto. Copyright 1999. ISBN 0-683-06141-0.
3. ^ "Spect measurements of regional cerebral perfusion and carbondioxide reactivity: Correlation with cerebral collaterals in internal carotid artery occlusive disease.". J Neurol. PMID 17063318. 
4. ^ Bergman RA, Afifi AK, Miyauchi R, Circle of Willis. Illustrated Encyclopedia of Human Anatomic Variation, URL: http://www.anatomyatlases.org/AnatomicVariants/Cardiovascular/Text/Arteries/CircleofWillis.shtml. Accessed on November 6, 2005.
5. ^ Klingelhofer J, Conrad B, Benecke R, Frank B. Transcranial Doppler ultrasonography of carotid-basilar collateral circulation in subclavian steal. Stroke. 1988 Aug;19(8):1036-42. PMID 3041649.
6. ^ Lord RS, Adar R, Stein RL. Contribution of the circle of Willis to the subclavian steal syndrome. Circulation. 1969 Dec;40(6):871-8. PMID 5377222.