The dura has been reflected back (with a small portion visible at the lower right) to reveal a subdural hematoma. Such a blood clot is usually the result of trauma with tearing of the cerebral bridging veins at the vertex. The venous source means that the bleeding may be slow and blood collects over a variable length of time, leading to neurologic signs and symptoms within hours to days or even weeks. [Image contributed by Jeannette J. Townsend, MD, University of Utah]