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Circle of Willis
At the base of the brain, the carotid and
vertebrobasilar arteries form a circle of communicating arteries
known as the circle of Willis.
From this circle other arteries -- the anterior
cerebral artery (ACA), the middle cerebral artery (MCA), the
posterior cerebral artery (PCA) - arise and travel to all
parts of the brain. Posterior Inferior Cerebellar Arteries
(PICA), which branch from the vertebral arteries, are not
shown.
Because the carotid and vertebrobasilar arteries
form a circle, if one of the main arteries is occluded, the
distal smaller arteries that it supplies can receive blood
from the other arteries (collateral circulation).
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Stroke Education
