Chapter 11 The nervous systems and the eye/Fig 11.12.jpg

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Circle of Willis. The anterior part of the circle is derived from the internal carotid arteries which divide into the proximal portions of the paired middle and anterior cerebral arteries, and the posterior part of the circle is derived from the vertebral and basilar system which gives origin to the paired posterior cerebral arteries. The circle is completed anteriorly by the anterior communicating artery which joins the anterior cerebral arteries and posteriorly by the posterior communicating arteries, which join the posterior cerebral and internal carotid arteries.