Epidural Hematoma

The brain is enclosed in the skull, which is a rigid, solid bone. Surrounding the brain is a tough, leathery outer covering called the dura (door-uh). The dura attaches to the brain, just beneath the skull bone. The dura normally protects the brain and keeps it nourished with blood and spinal fluid. When a person receives a severe blow to the head, the brain bounces within the cavity. This movement of the brain structures may cause shearing or tearing of the blood vessels surrounding the brain and dura. When the blood vessels tear, blood accumulates within the space between the dura and the skull. This is known as an epidural hematoma (epi-door-ul hem-a-to-ma), or blood clot at the covering of the brain.

http://www.muhealth.org/neuromed/epidural.shtml