It does not line the brain down into its sulci (folds), as does the pia mater, with the exception of the longitudinal fissure, which divides the left and right cerebral hemispheres.

The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges (lepto, from the Greek root meaning "thin" or "slender").

[4][clarification needed] The arachnoid mater in the rat is composed of approximately 10 layers of fibroblast cells.

Cerebrospinal fluid is produced by the choroid plexus (inside the ventricles of the brain, which are in direct communication with the subarachnoid space so the CSF can flow freely through the nervous system).

[7] Another source states that the "arachnoid membrane" was discovered and named by Gerardus Blaes (Blasius) in 1664, and that Ruysch adopted the term in 1692.