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
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.
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.
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.
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.
Pathology
There are three types of
hemorrhage involving the meninges:
Other medical conditions which affect the meninges include meningitis (usually from fungal, bacterial, or viral infection) and meningiomas arising from the meninges or from tumors formed elsewhere in the body which metastasize to the meninges.
Additional images
References
