The arachnoid is the middle layer of the meninges which are the membranes covering the brain.
The arachnoid is characterized by the spider web like projections that extend between it and the pia mater (outer membrane of the meninges).
These projections are located in the subarachnoid space (space below the arachnoid). These projections contain the circulating cerebrospinal fluid or CSF.
The arachnoid extends into the dural sinuses through the arachnoid granulations in which the CSF is filtered and added to the blood for drainage from the brain.