These cisterns are filled with cerebrospinal fluid (CSF).

[1] Although the pia mater adheres to the surface of the brain, closely following the contours of its gyri and sulci, the arachnoid mater only covers its superficial surface, bridging across the gyri.

This leaves wider spaces between the pia and arachnoid and the cavities are known as the subarachnoid cisterns.

Rather, these subarachnoid cisterns are separated from each other by a trabeculated porous wall with various-sized openings.

Some major subarachnoid cisterns: It is clinically significant that cerebral arteries, veins, and cranial nerves traverse through the subarachnoid space, maintaining their meningeal covering until they exit the skull.