Meninges
The arachnoid mater also has an important job in the blood-brain barrier.  Blood runs through the dura mater, the last layer of the meninges.  The arachnoid mater keeps blood from getting through into the cerebrospinal fluid.  This is important, because if blood gets through the arachnoid mater into the cerebrospinal fluid, the brain and spinal cord can get irritated and infected.  Like all parts of the body, the arachnoid mater needs blood and oxygen to survive.  Small blood vessels bring these things to the arachnoid layer.  However, the arachnoid layer prevents the blood from getting through to the cerebrospinal fluid.