Meninges

The meninges (singular meninx) is the system of membranes which envelops the central nervous system. 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.

Anatomy

Dura mater

The dura mater (also rarely called meninx fibrosa, or pachymeninx) is a thick, durable membrane, closest to the skull. It consists of two layers, the periosteal layer, closest to the calvaria and the inner meningeal layer. It contains larger blood vessels which split into the capilliaries in the pia mater. It is composed of dense fibrous tissue, and its inner surface is covered by flattened cells like those present on the surfaces of the pia mater and arachnoid. The dura mater is a sac which envelops the arachnoid and has been modified to serve several functions. The dura mater surrounds and supports the large venous channels (dural sinuses) carrying blood from the brain toward the heart.

Arachnoid membrane

The middle element of the meninges is the arachnoid membrane, so named because of its spider web-like appearance. It provides a cushioning effect for the central nervous system. The arachnoid mater exists as a thin, transparent membrane. It is composed of fibrous tissue and, like the pia mater, is covered by flat cells also thought to be impermeable to fluid. The arachnoid does not follow the convolutions of the surface of the brain and so looks like a loosely fitting sac. In the region of the brain, particularly, a large number of fine filaments called arachnoid trabeculae pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater.

The arachnoid and pia mater are sometimes together called the leptomeninges.

Pia mater

The pia or pia mater is a very delicate membrane. It is the meningeal envelope which firmly adheres to the surface of the brain and spinal cord. As such it follows all the minor contours of the brain (gyri and sulci). It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells thought to be impermeable to fluid. The pia mater is pierced by blood vessels which travel to the brain and spinal cord, and its capillaries are responsible for nourishing the brain.

Spaces

The subarachnoid space is the space which normally exists between the arachnoid and the pia mater, which is filled with cerebrospinal fluid.

Normally, the dura mater is attached to the skull, or to the bones of the vertebral canal in the spinal cord. The arachnoid is attached to the dura mater, and the pia mater is attached to the central nervous system tissue. When the dura mater and the arachnoid separate through injury or illness, the space between them is the subdural space.