[TA] the roughly pentagonally shaped circle of vessels on the ventral aspect of the brain in the area of the optic chiasm, hypothalamus, and interpeduncular fossa; formed, sequentially and in an anterior to posterior direction, by the anterior communicating artery and by the two anterior cerebral, the two internal carotid, the two posterior communicating, and proximal segments of the two posterior cerebral arteries. SYN: circulus arteriosus cerebri [TA], arterial circle of cerebrum, circle of Willis.