Meningeal coverings

The meninges (singular meninx) are the system of membranes that envelop the central nervous system. The meninges consist of three layers, the dura mater, the arachnoid mater, and the pia mater.

Anatomy

 * The pia or pia mater is a very delicate membrane. It is closest to the brain or the spinal cord. As such it follows all the minor contours of the brain (gyri and sulci). Within the pia mater are capillaries responsible for nourishing the brain.
 * The middle element of the meninges is the arachnoid mater, so named because of its spider web-like appearance.
 * The dura mater is a thick, durable membrane, closest to the skull. It contains larger blood vessels that split into the capilliaries in the pia mater.

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

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

Normally, the dura mater is attached to the skull in the head, 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.

Pathology
There are three types of hemorrhage involving the meninges:
 * A subarachnoid hemorrhage is acute bleeding under the arachnoid; it may occur spontaneously or as a result of trauma.
 * A subdural hematoma is a hematoma (collection of blood) located in a separation of the arachnoid from the dura mater. The small veins that connect the dura mater and the arachnoid are torn, usually during an accident, and blood can leak into this area.
 * An epidural hematoma similarly may arise after an accident or spontaneously.

Other medical conditions that affect the meninges include meningitis (a viral or bacterial infection). Meningiomas are tumors (generally benign) arising from the meninges. Malignant tumors formed elsewhere may also metastasize to the meninges.