On top of the surface of the brain tissue is a layer called the arachnoid. It looks like cellophane. It is clear in young individuals, can become a little opaque as we all get older. Between this layer and the brain is cerebrospinal fluid which is circulating around the brain tissue. Blood vessels are also along the surface of the brain. These are the blood vessels which can be injured from trauma. When they bleed, the blood will mix and spread with the cerebral spinal fluid along the surfaces of the brain. Since it is below the arachnoid, it is called a subarachnoid hemorrhage. Other relatively common causes of subarachnoid hemorrhage include rupture of cerebral aneurysms and arterial venous malformations. Sometimes communicating hydrocephalus may develop after subarachnoid hemorrhage.