Meninges

In mammals, the meninges consist of three layers: the dura mater, the arachnoid mater, and the pia mater. The primary function of the meninges and of the cerebrospinal fluid is to protect the central nervous system.

Dura mater is the outermost of the three layers of the meninges surrounding the brain and spinal cord. It is derived from mesoderm. The other two meningeal layers are the pia mater and the arachnoid mater. The dura surrounds the brain and the spinal cord and is responsible for keeping in the cerebrospinal fluid. The dura has been described as "tough and inflexible" and "leather-like".

The arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord. It is interposed between the two other meninges, the more superficial and much thicker dura mater and the deeper pia mater, from which it is separated by the subarachnoid space. The delicate arachnoid layer is attached to the inside of the dura and surrounds the brain and spinal cord. It does not line the brain down into its sulci (folds), as does the pia mater, with the exception of the longitudinal fissure, which divides the left and right cerebral hemispheres. Cerebrospinal fluid (CSF) flows under the arachnoid in the subarachnoid space. The arachnoid mater makes arachnoid villi, small protrusions through the dura mater into the venous sinuses of the brain, which allow CSF to exit the sub-arachnoid space and enter the blood stream.

Pia mater often referred to as simply the pia, is the delicate innermost layer of the meninges, the membranes surrounding the brain and spinal cord. The other two meningeal membranes are the dura mater and the arachnoid mater. Pia mater is a thin fibrous tissue that is impermeable to fluid. This allows the pia mater to enclose cerebrospinal fluid. By containing this fluid the pia mater works with the other meningeal layers to protect and cushion the brain. The pia mater allows blood vessels to pass through and nourish the brain. The perivascular space created between blood vessels and pia mater functions as a lymphatic system for the brain. When the pia mater becomes irritated and inflamed the result is meningitis.