The circle of Willis is a ring of arteries on the underside of the brain, at its base. Its links are the ends of the two internal carotid arteries, the first parts of the anterior and posterior cerebral arteries, and the anterior and two posterior communicating arteries. [1][2]
Its shape varies between people: the textbook ring is seen in only about one-third of people. [1]
Lit: the ring — both communicating links, the first part of each anterior and posterior cerebral artery, and each internal carotid from the skull base up. The model’s ring is complete; every other artery shows in a darker colour. [3][1]
The 3D brain needs JavaScript. The text below still says where the circle of Willis is and what it does.
Slice
Opens with the blood-supply layer on (about 0.8 MB) and a top (axial) slice through the base of the brain, so the ring is seen from above; the middle cerebral arteries leave it sideways but are not part of it. Tap a name under “What each artery supplies” to light that artery on its own. Drag to turn, pinch or scroll to zoom, tap any part to name it; .
It lies on the underside of the brain, in and around the interpeduncular fossa: the hollow bounded in front by the optic chiasm, with the mammillary bodies and the stalk (infundibulum) of the pituitary gland in its floor. [1][4]
The front of the ring is the short anterior communicating artery, which joins the left and right anterior cerebral arteries in front of the optic chiasm. [5]
Its sides are the posterior communicating arteries, each running back from the internal carotid to the posterior cerebral artery in the interpeduncular cistern, above and to the side of the pituitary gland. [6]
The back of the ring is the first part of each posterior cerebral artery, where the basilar artery, lying in the midline on the front of the pons, divides at the junction of the pons and midbrain. [7][8]
Which links are thin or missing differs from person to person: the two posterior communicating arteries often differ in size, and in about one adult in five a posterior cerebral artery still draws mainly on the posterior communicating artery rather than the basilar, as it does before birth. [6][8]
What it does
Its two sources meet in it: the left and right internal carotid arteries, which enter the skull through the carotid canals, and the basilar artery, formed by the two vertebral arteries, which enter through the foramen magnum. [2]
The links make the supply redundant: blood from the carotids and from the basilar, and from the left and right sides, can reach each cerebral artery by more than one route. [1][2]
The anterior cerebral artery forms the front-side part of the ring, while the middle cerebral artery, which also arises from the internal carotid, leaves next to it and does not contribute to the ring. [1][9]
Other names
Also called the cerebral arterial circle (Latin: circulus arteriosus cerebri). It is named after Thomas Willis (1621–1675), an English physician and anatomist. [1]
Size
The basilar artery that feeds its back ranges from 1.5 to 6.6 mm across; the two vertebral arteries run about 32 mm inside the skull before they join, and often differ in width: in one study the left was the wider in 54% of people, the right in 30%. [7][10]
On this model the ring’s own links (without the internal carotids, which the model draws from the skull base up) fit in a box about 26 mm front to back, 31 mm side to side and 20 mm top to bottom (the box they fit in, not their volume). [3]
Facts describe anatomy and normal function only. The 3D model is the adult male BodyParts3D brain (credits and licence); real brains vary in size and shape, so its measurements describe one model, not anyone’s brain.