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A D V E R T I S E M E N T
A D V E R T I S E M E N T
After entering the foramen magnum, the vertebrals give off anterior spinal branches, posterior inferior cerebellar arteries and posterior spinal arteries as they ascend around the medulla. The anterior spinal arteries supply the anterior 2/3 of the spinal cord. The vertebrals unite at the lower border of the pons to form the basilar artery. The basilar artery immediately gives rise to the anterior inferior cerebellar arteries. After giving off small pontine branches, the basilar gives off the superior cerebellar arteries at the upper border of the pons and then immediately bifurcates into its terminal branches, the posterior cerebral arteries. The latter are joined to the carotids by the posterior communicating arteries.
A D V E R T I S E M E N T
The circle of Willis is formed by the posterior communicating branches between the internal carotid and posterior cerebral arteries, and by the anterior communicating artery between the anterior cerebrals. The circle of Willis connects the anterior carotid to the posterior vertebrobasilar circulations, and joins the circulations of the two hemispheres.
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A D V E R T I S E M E N T
The circle of Willis encircles the optic chiasm, the pituitary stalk and the interpeduncular fossa. It takes the classic textbook form only in one third of individuals. One reference lists 83 variations. In 6% of individuals the anterior communicating is small enough to limit cross flow and in 3% of individuals one of the posterior communicatings is absent. More often one or more of the vessels is hypoplastic. The integrity of the circle determines the amount of collateral flow available to compensate for a vascular occlusion proximally.
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This page was last modified on 7/20/1999.
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