The Circle of Willis
circulus arteriosus cerebri
Base of the brain — the interpeduncular and suprasellar cisterns
An anastomotic ring of arteries sitting in the subarachnoid space at the base of the brain, where the anterior and posterior cerebral circulations meet and join. Almost every artery that supplies the cerebral hemispheres leaves from this ring.
Explore the ring
Hover or tap any vessel to read what it supplies. The dashed amber line is the oculomotor nerve; the pulsing rings mark the three commonest sites for a saccular aneurysm.
How it is built
- 01Anterior inflow
- The two internal carotid arteries. Each gives an anterior cerebral artery, and the two are bridged across the midline by the single anterior communicating artery.
- 02Posterior inflow
- The two vertebral arteries fuse to form the basilar artery, which divides at the upper border of the pons into the two posterior cerebral arteries.
- 03The closing links
- On each side a posterior communicating artery runs back from the internal carotid to the posterior cerebral artery, closing the ring.
- 04Why the ring exists
- Collateral flow. If one feeding vessel narrows or occludes, blood can redistribute around the circle. In practice fewer than half of people have a complete, symmetrical ring — the variation is the norm.
Why the ring fails at its corners
At every branch point the tunica media thins and the internal elastic lamina is deficient — a weak spot where the arterial wall was never fully built. Decades of pulsatile stress at those bifurcations can push the wall outward into a saccular, or 'berry', aneurysm. Most arise in the anterior circulation: the anterior communicating artery, the origin of the posterior communicating artery, and the middle cerebral artery bifurcation. Because the circle lies within the subarachnoid space, bathed in CSF, a rupture delivers arterial blood at systemic pressure straight into that space — and the space is continuous over the convexities and down around the spinal cord.
Rosemarie is 25. She has a headache.
Somewhere in this structure, something has gone wrong. You cannot scan her and you cannot examine her — your only instrument is conversation. Take a history, and work out what the anatomy is telling you.
Scoring is unchanged from a standard MEKSI consultation — the same rubric, marked the same way. What changes is that each question you ask is also read as a probe of The Circle of Willis.