A subdural hematoma is a collection of blood under the dura mater, between the dura and the arachnoid layer. ("Subdural" means "under the dura.") Usually, subdural hematomas are caused by injuries to the head that tear veins. Because these veins are smaller, and because veins do not bleed as fast as arteries, subdural hematomas may not bleed as quickly as epidural hematomas.