Meninges
The arachnoid mater is the middle layer of the meninges.  It is a thin, clear membrane that fits loosely over the pia mater.  This leaves a space in between these two layers called the "subarachnoid space."  ("Subarachnoid space" means "the space under the arachnoid.")  Cerebrospinal fluid flows through the subarachnoid space.