Subarachnoid space
Anatomic space between the arachnoid membrane and pia mater,the two innermost layers covering the brain.
The subarachnoid space contains cerebrospinal fluid (CSF) and blood vessels.
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.
Tarlov cyst is an abnormal expansion of the spinal nerve sleeve, and communicates with the subarachnoid space via a perineural fistula.