Cranial meninges

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Separating the brain and spinal cord from their surrounding bony enclosures are three membranes known as the meninges. The outermost layer is the dura mater (also known as pachymeninx) which consists of a double layer of thick, dense irregular connective tissue. The middle layer is the arachnoid mater, so-called for its spider web-like appearance, while the innermost layer is the thin and delicate pia mater. The arachnoid and pia mater can also be collectively referred to as the leptomeninges due to their common embryological and cellular structure.

These layers delimitate three clinically important spaces: the epidural (or extradural), subdural, and subarachnoid spaces. The epidural space is located between the bones of the cranium and outer (periosteal) layer of the dura mater, while the subdural space lies between the inner (meningeal) layer of the dura mater and arachnoid mater. Both of these are potential spaces, meaning that under normal circumstances they are closed. The subarachnoid space, located between the arachnoid and pia mater, is a fluid-filled space that contains CSF, as well as cerebral arteries and veins. Finally, separating the pia mater from the surface of the brain is a thin space known as the subpial space. 

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