by Dr. Gary Farr on 24 June 2002
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The meninges, (singular meninx), are three membranous envelopes—the pia mater, the arachnoid, and the dura mater—that surround the brain and spinal cord. Cerebrospinal fluid fills the ventricles of the brain and the space between the pia mater and the arachnoid. The chief function of meninges and cerebrospinal fluid is to protect the central nervous system.
The pia mater is the meningeal envelope that firmly adheres to the surface of the brain and spinal cord. It is a very thin membrane composed of fibrous tissue covered on its outer surface by a sheet of flat cells believed to be impermeable to fluid. Pierced by blood vessels that pass through it to reach the brain and spinal cord, the pia mater follows these blood vessels into the depth of the brain.
Over the pia mater and separated from it by a space called the subarachnoid space is a second meningeal envelope of the central nervous system, the arachnoid, a thin membrane remarkable for its delicacy and transparency. It is composed of fibrous tissue and, like the pia, is also believed to be impermeable to fluid. The arachnoid does not follow the irregularities of the surface of the brain, and so looks like a loosely fitting sac with a variable distance, in certain regions relatively large and in others minute, between its walls and the surface of the nervous system.
In the region of the brain, particularly, a large number of fine filaments pass from the arachnoid through the subarachnoid space to blend with the tissue of the pia mater. These filaments are called arachnoid trabeculae. They are remnants from embryological life of the common origin of the arachnoid and pia mater, and they have the frail structure characteristic of these two of the meninges. The pia mater and arachnoid are called collectively the leptomeninges.
The outermost of the three meninges is the dura mater (or pachymeninx), a strong, thick, and dense membrane. It is composed of dense fibrous tissue. It can be regarded as a sac that envelops the arachnoid and that has been modified to serve several functions.
Within the skull the dura mater surrounds and supports the large venous channels (dural sinuses) carrying blood from the brain toward the heart. It also is prolonged into several partitions, or septa, which lend support to the brain. One of these, the falx cerebri, is a sickle-shaped partition lying between the two hemispheres of the brain. Another, the tentorium cerebelli, provides a strong, membranous roof over the cerebellum. A third, the falx cerebelli, projects downward from the tentorium cerebelli between the two cerebral hemispheres. The outer portion of the dura mater over the brain serves as a covering, or periosteum, of the skull bones' inner surfaces.
Within the vertebral canal the dura mater splits into two sheets. These are separated by a space, called the epidural space, which is filled with fat and thin-walled veins. The outer of these two sheets constitutes the periosteum of the vertebral canal. The inner sheet is separated from the arachnoid by the narrow subdural space, which is filled with fluid.
In a few places, however, the subdural space is absent, and the arachnoid is intimately fused with the dura mater. The most important area of fusion between these two meninges is in the walls of the large venous channels of the dura mater. There elongations of the arachnoid, like fingers, penetrate the dura mater and project into the veins. These fingerlike processes of the arachnoid, which are referred to as arachnoid villi, or arachnoid granulations, are involved in the passage of cerebrospinal fluid from the subarachnoid space to the dural sinuses.
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