Meninges

The cranial and spinal meninges (singular: meninx) are three membrane layers in the central nervous system that protect and isolate parts of the central nervous system, both physically and against chemical and microbial damaging factors.

The hard outermost layer, closest to your skull is the dura mater which consists of two layers in the cranial space. These two layers ( endosteal and meningeal layers ) are closely fused except for some areas where they form dural venous sinuses. These sinuses collect venous blood.

The middle, weblike meninx, the arachnoid mater is separated from the pia mater by the subarachnoid space, which is filled with cerebrospinal fluid. Protrusions of the arachnoid mater through the dura mater are called arachnoid granulations at the site of the venous sinuses. These granulations are filled with cerebrospinal fluid, letting it be absorbed into the venous blood.

The innermost layer, the pia mater is delicate, and covers the brain and the spinal cord tightly, including all folds and spreading into even the deepest grooves.