Cerebrospinal fluid

Cerebrospinal fluid (CSF), Liquor cerebrospinalis, is a clear bodily fluid that occupies the subarachnoid space and the ventricular system around and inside the brain.

More specifically the CSF occupies the space between the arachnoid mater (the internal layer of the brain cover, meninges) and the pia mater (the most superficial layer of the brain).

It is an approximately isotonic solution with microglia and acts as a "cushion" or buffer for the cortex, providing also a basic mechanical and immunological protection to the brain inside the skull.