The branches of basilar and internal carotid arteries anastomose at the base of brain around the interpeduncular fossa forming a six-sided polygon of arteries called circulus arteriosus or circle of Willis. The circle of Willis lies in the interpeduncular subarachnoid cistern and contributes most of the arterial blood supply to the brain.
The circle of Willis is formed:
Anteriorly, by the anterior communicating and the anterior cerebral arteries.
Posteriorly, by the basilar artery dividing into two posterior cerebral arteries.
Laterally on each side, by the posterior communicating artery connecting the internal carotid artery with the posterior cerebral artery.

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