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meninges(mĭnĭn`jēz), three membranous layers of connective tissueconnective tissue,
supportive tissue widely distributed in the body, characterized by large amounts of intercellular substance and relatively few cells. The intercellular material, or matrix, is produced by the cells and gives the tissue its particular character.
..... Click the link for more information. that envelop the brain and spinal cord (see nervous systemnervous system,
network of specialized tissue that controls actions and reactions of the body and its adjustment to the environment. Virtually all members of the animal kingdom have at least a rudimentary nervous system.
..... Click the link for more information. ). The outermost layer, or dura mater, is extremely tough and is fused with the membranous lining of the skull. In the brain it forms a vertical sheet that separates the cerebral hemispheres and a horizontal sheet that lies between the cerebrum and the cerebellum. The thin arachnoid membrane lies below and in close contact with the dura mater. The innermost layer, or pia mater, is in direct contact with the brain and spinal cord and contains the blood vessels that supply them. The pia mater and arachnoid membrane are separated by the subarachnoid space containing the cerebrospinal fluid, which carries nutrients, absorbs the impact of shocks, and acts as a barrier to disease organisms. Thus, the meninges provide a fluid-filled jacket for the protection of neural tissues and allow for the flexing and twisting of the vertebral column about the spinal cord.
the connective-tissue membranes enveloping the brain and spinal cord in vertebrates and man.
A primitive meninx develops in the embryo and differentiates into dura mater (adjoining the periosteum) and primary pia mater (adjoining the brain and forming the folds of the brain’s vascular plexuses). Three meninges are distinguished in adult mammals and man. In the brain region, dura mater, the outer membrane, forms the periosteum of the inner surface of the skull and passes longitudinal and transverse processes into the cranial cavity. The principal processes are the falciform, wedged between the two cerebral hemispheres (a rudiment is found in birds), and the tentorium cerebelli, which separates the cerebellum from the lower surface of the occipital lobes of the cerebrum. Sinuses form, when the dura mater splits, in certain places in the cranium; venous blood flows into them from the brain. In the spinal cord region, the dura mater consists of two layers, outer (periosteum) and inner. These are divided by the epidural space, which is filled with adipose tissue and venous plexuses.
The primitive pia mater in mammals splits into arachnoid and vascular or secondary, meninges. The arachnoid is internal to the dura mater. In the cranial cavity, it adjoins the surface of the gyri of the cerebral cortex, without entering the sulci or other depressions (thereby forming cisternae). In the spinal cord region, the arachnoid adheres firmly to the inner layer of the dura mater.
The innermost of the membranes is the meninx vasculosa, which adheres to the surface of the spinal cord and brain and extends into the brain’s sulci and depressions, containing the blood vessels that feed the brain. The subarachnoid space, between the arachnoid and the meninx vasculosa, is filled with cerebrospinal fluid. The arachnoid and the meninx vasculosa, which join near the cerebral cortex, are given the common name of “pia mater.”