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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