The white arrow on the black card marks the site of a ruptured berry aneurysm in the circle of Willis. This is a major cause for subarachnoid hemorrhage at the base of the brain. As the weak wall of the artery, which lacks an internal elastic lamina and a media, expands to form the aneurysm, there may initially be leakage of blood that produces headaches, but there is risk for sudden rupture to produce a severe headache. The blood irritates the arteries to produce vasospasm and promote cerebral anoxia. [Image contributed by Jeannette J. Townsend, MD, University of Utah]

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