The brain is surrounded by three thin fibrous membranes. The dura mater is the outermost membrane (green) and is closely adherent to the inner surface of the skull. It is tough and inelastic. Below that is the arachnoid mater (blue), a thinner and more delicate layer. The pia is the innermost, very thin layer which is closely adherent to the surface of the brain and follows it’s folds and fissures.
In this coronal section of a normal childs head (from the front) the dura and arachnoid membranes surrounding the brain lie closely against each other. The potential space in between them is called the subdural space.
The space below the arachnoid membrane is called the sub arachnoid space and is filled with cerebrospinal fluid. This fluid acts as a shock absorber and helps to cushion the brain from injury.
In the center of the brain are the ventricles. These are a network of cavities which contain cerebrospinal fluid. Within the ventricular system is the choroid plexus which produces cerebrospinal fluid.