Anatomical Structure and Location of the Circle of Willis

Description

Centered on the ventral aspect of the brain, the arterial circle of Willis forms an anastomotic ring at the base of the diencephalon around the optic chiasm and infundibulum. The internal carotid arteries course superiorly to give rise to the anterior cerebral arteries, joined across the midline by the anterior communicating artery, while the posterior communicating arteries run posteriorly toward the posterior cerebral arteries. Posteriorly, the basilar artery ascends on the ventral pons and bifurcates into the posterior cerebral arteries, with adjacent brainstem, cerebellum, and inferior frontal and temporal lobes providing clear spatial context. Small perforators to the hypothalamus and midbrain are implied at the circle’s margins. Key landmarks. An anterior inferior view of the circle matters because it maps the major collateral pathway that buffers hemispheric perfusion when a proximal vessel narrows or occludes. Aneurysms cluster at branching points under hemodynamic stress, most classically at the anterior communicating artery and posterior communicating artery, and this orientation matches the surgical and angiographic mental model used when localizing third nerve palsy from a posterior communicating aneurysm or correlating subarachnoid hemorrhage patterns to a rupture site. Variants such as hypoplastic P1 segments or a fetal-type posterior cerebral artery can also be explained cleanly when the carotid and vertebrobasilar contributions are seen in one field. Use this illustration in neuroanatomy and cerebrovascular teaching when introducing the anterior and posterior circulations, collateral flow, and vascular territories, or in neurosurgery and neuroradiology materials that discuss aneurysm distribution and endovascular access planning. It also fits stroke education modules that link CT angiography findings to circle of Willis anatomy and common anatomic variants. Anatomical accuracy verified by SciePro's Medical Advisory Board.