The meninges are layers of connective tissue that surround the BRAIN and SPINAL CORD; the singular noun is meninx. Around the brain are three layers: the DURA MATER (Latin, hard mother) is a relatively thick, tough but flexible layer of connective tissue that surrounds the brain and other meningeal layers. The outer surface of the dura adheres to the inner surface of the SKULL. Beneath the dura is the SUBDURAL SPACE, through which blood vessels pass, and below is the ARACHNOID MEMBRANE (from Greek, arachne: spider—the arachnoid membrane is like a spider’s web). The arachnoid is thinner and softer than the dura. It gives rise to the ARACHNOID TRABECULAE (Latin, trabecula: a little beam) which descend through the SUBARACHNOID SPACE to contact the PIA MATER (Latin, pious mother) a meningeal layer thinner still than the arachnoid, which contains small blood capillaries and is attached directly to the surface of the brain, following faithfully the GYRI AND SULCI. All three meningeal layers cover the brain and spinal cord but the arachnoid is not present around the SPINAL NERVES and CRANIAL NERVES, which are covered by a lining that is in effect a fusion of the dura and pia mater. The spaces between the meningeal membranes is filled with CEREBROSPINAL FLUID (CSF), which gains access to the subarachnoid space from the cerebral VENTRICLES (where it is synthesized) via three openings from the fourth ventricle: the FORAMINA OF LUSHKA (one in each hemisphere) and the FORAMEN OF MAGENDIE.
The function of the meninges is effectively to suspend the brain in fluid, supporting it and absorbing shocks. The fact that it is suspended in fluid effectively decreases the weight of the brain. Moreover, the fact that it is supported in a triple layer of membranes, lubricated by fluid, means that shearing forces generated following sudden impact can be absorbed. Were it not for this, sudden blows to the head would cause tissue to shear—to tear and fragment.
The terms dura mater and pia mater are curiosities: mater really does derive from mater, Latin for mother, and should, properly, not be referred to as matter, as is often done. Carlson (1998) explains the origins of these terms. A Persian physician, Ali ibn Abbas, in the tenth century used the Arabic term al umm to describe the meninges. Literally this does mean mother, but also described covering material, there being no term for membrane. The more solid dura was called al umm al djafiya, the softer pia al umm al rigiga. A century later these were unsympathetically translated literally into Latin as hard mother and soft mother, pious being used in the sense of soft rather than having religious connotation.
