The subarachnoid space surrounds the brain and spinal cord and in certain locations it enlarges into expanded areas (subarachnoid cisterns). It contains cerebrospinal fluid (CSF) and blood vessels.
Cerebrospinal fluid is produced by the choroid plexus, primarily in the ventricles of the brain. It is a clear, colorless, cell-free fluid that circulates through the subarachnoid space surrounding the brain and spinal cord.