Meninges

The spinal cord and brain are enclosed by three connective tissue membranes, that is the meninges as shown in figure.

                                                                 Figure: The meninges.

The subarachnoid space, filled with cerebrospinal fluid, divides the pia mater and the arachnoid mater. The Superficial cerebral blood vessels, invested by the arachnoid mater, run throughout the subarachnoid space and their branches penetrate the brain becoming enclosed by a cuff of pia mater which extends as far as the capillaries. The perivascular space between the vessel wall and pia mater is continuous with the subarachnoid space. The passive exchange of water and solutes here across the pia mater keeps the CSF in equilibrium with the brain extracellular fluid. In the cerebral capillaries the pia mater is lost and the single layers of capillary endothelial cells, with their basement membrane, are enclosed by glial cells. The Expanded areas of the subarachnoid space are cisterns.

The dura mater is a tough thick outer layer with venous sinuses running through it. The small herniations of arachnoid mater known as the arachnoid villi protrude by the dura into the venous sinuses. The bulk flow of CSF into blood occurs via the mesothelial tubes in arachnoid villi which act as valves, closing when the pressure in the venous sinus exceeds that of the subarachnoid space to prevent reflux of blood into the CSF.

A subdural space is the potential space between the dura mater and the arachnoid mater. It is traversed by cerebral veins by entering the venous sinuses into the dura.