Two key structures cross the subdural space to enter the superior sagittal sinus. One is the constricted passage-way through which CSF must pass to reach the interior of arachnoid granulations. The second is a group of bridging veins that empty blood from the surface of the hemisphere into the venous sinus. However if, in an accident, the skull strikes a hard surface (the dashboard of a car), the brain may shift position slightly within the cranial cavity and as a result the bridging veins may be torn. These veins pass through the subdural space, and that's where the blood accumulates. Since the the dura and arachnoid easily separate, the blood in this case spreads easily over the surface of the hemisphere.