Anatomic space between the arachnoid membrane and pia mater.

It is occupied by spongy tissue consisting of trabeculae (delicate connective tissue filaments that extend from the arachnoid mater and blend into the pia mater) and intercommunicating channels in which the cerebrospinal fluid is contained.

see Subarachnoid cistern

Tarlov cyst is an abnormal expansion of the spinal nerve sleeve, and communicates with the subarachnoid space via a perineural fistula.

see Subarachnoid hemorrhage.