Circle of Willis Anatomy

Vessel connections and territories in the table follow this circle of Willis reference summary.

The circle of Willis arteries form a ring at the base of the brain, linking the internal carotid and vertebrobasilar supplies. Learn the ring in order: locate it, name each vessel, then check which arteries branch away.

The ring is one small part of wider brain anatomy, and it makes most sense once you can place it against the structures around it.

Location at the base of the brain

The circle of Willis sits at the base of the brain, in the subarachnoid space surrounding the pituitary gland and optic chiasm, according to a neurovascular reference page. Those two landmarks are the quickest way to find the ring on any inferior view.

1. Turn the brain over. Picture or open an underside (inferior) view, so the base faces you.
2. Find the landmarks. Locate the pituitary gland and the optic chiasm near the midline.
3. Trace the arteries around them. The ring wraps these structures rather than lying on top of the cerebral surface.

The Paris Brain Institute describes the circle as connecting the inner carotid arteries to the vertebral-basilar arteries. That is why it is best treated as a junction between two supply routes, not as a self-contained loop.

Arteries that form the ring

The ring is made up of the right and left internal carotid arteries, the right and left anterior cerebral arteries, the anterior communicating artery, the right and left posterior communicating arteries and the right and left posterior cerebral arteries, as this reference lists them. Read as a loop, it has a front, two sides and a back.

• Front: the anterior communicating artery joins the two anterior cerebral arteries across the midline.
• Sides: each posterior communicating artery runs from an internal carotid artery back to a posterior cerebral artery.
• Back: the posterior cerebral arteries arise from the basilar artery.

Textbook summaries often list the basilar artery and the middle cerebral artery alongside the ring. The Picmonic summary notes that the middle cerebral artery is technically not part of the circle but an extension of the internal carotid artery. For exams, this is the distinction worth keeping: the MCA belongs with the ring’s neighbours, not its segments.

Carotid and vertebrobasilar source vessels

Two supply systems meet in the ring. The carotid system feeds it from the front through the internal carotid arteries, which are supplied by the common carotid arteries. The vertebrobasilar system feeds it from behind: the basilar artery originates from the confluence of two vertebral arteries between the medulla and the pons.

This split explains the ring’s design. The anterior half draws on the carotid supply and the posterior half on the vertebrobasilar supply, and the communicating arteries connect the two halves. The Paris Brain Institute’s description of the circle as a link between the inner carotid and vertebral-basilar arteries states the same arrangement in one line.

Branches and the areas they supply

The arteries that leave the ring carry blood to the cerebral cortex. The communicating arteries do a different job, linking vessels rather than supplying territory.

• Anterior cerebral artery: supplies the motor and sensory cortex that controls the lower limbs.
• Middle cerebral artery: supplies the motor and sensory cortex for the upper limb and face. It continues from the internal carotid artery rather than forming part of the ring.
• Posterior cerebral artery: bifurcates from the basilar artery and supplies the occipital, or visual, cortex.

These territories are drawn from the same reference summary. When a question asks about the “branches” of the circle, give these three outflow arteries, then add that the anterior and posterior communicating arteries are connectors.

Branching points matter clinically as well. The Paris Brain Institute notes that intracranial aneurysms often develop in the circle’s bifurcations, so the junctions between these vessels are worth learning as carefully as the vessels.

Collateral flow and normal variation

The ring provides redundancy: if an artery is blocked, blood flow can be redirected by another route, according to the Paris Brain Institute. The communicating arteries are what make that rerouting possible, because they connect the carotid and posterior supplies and the two sides of the front.

The same source adds that an incomplete circle increases the risk of damage in the event of arterial occlusion. A complete ring should therefore not be assumed, and the anatomy literature records variation. Radiology Key’s account of the cerebral arterial circle reports a “normal” anterior configuration in 60% of cases and a normal posterior configuration in 55%. It also reports the posterior communicating artery as absent or very small on one side in 10%. Those figures describe particular segments, so they are not a single “complete circle” rate.

The same account explains that these anomalies can be combined in different individuals, producing many different types. One example is the fetal-type pattern, in which the posterior cerebral artery receives its blood supply primarily from the internal carotid artery, as the neurovascular reference describes. For study, learn the textbook ring first and treat variants as deviations from it.

How to remember the circle of Willis

The simplest memory aid is to walk the ring as a route instead of memorising a list. Start at the front and move around in one direction.

1. Begin at the anterior communicating artery. It is the front crossbar between the anterior cerebral arteries.
2. Follow one anterior cerebral artery back to the internal carotid artery on the same side.
3. Take the posterior communicating artery from that internal carotid artery to the posterior cerebral artery.
4. Reach the basilar artery where the two posterior cerebral arteries arise, then cross over and repeat on the other side.
5. Add the middle cerebral artery last as the extension of each internal carotid artery.

Sketch the route from memory once from an inferior view, and label each vessel with its territory. Drawing the ring from the front, then the back, helps fix which arteries are sources and which are connectors.

Troubleshooting common mix-ups

• Counting the middle cerebral artery as a ring segment. It is an extension of the internal carotid artery and sits outside the loop.
• Confusing the two communicating arteries. The anterior communicating artery bridges the right and left anterior cerebral arteries. The posterior communicating artery connects an internal carotid artery to a posterior cerebral artery on each side.
• Drawing a symmetrical, always-complete ring. The ring varies between people, so treat the textbook diagram as a reference pattern.
• Mixing up the supply routes. The internal carotid arteries come from the common carotid arteries, while the basilar artery comes from the vertebral arteries.

Once the ring is secure, try neighbouring topics for contrast. The corpus callosum anatomy guide covers another brain structure, and brachial plexus anatomy shows a network arranged outside the skull.

Frequently asked questions