A delicate fibrous membrane forming the middle of the three coverings of the central nervous system. In life, its smooth external surface is closely applied (but not attached) to the internal surface of the dura mater, with only a potential space (subdural space) intervening. It is held against the dura by the pressure of the cerebrospinal fluid (CSF) which occupies the subarachnoid space which lies immediately deep to the arachnoid, intervening between it and the pia mater. Thus, in a spinal puncture, dura mater and arachnoid are penetrated simultaneously as if a single layer. In the absence of CSF pressure in the cadaver, the arachnoid falls away from the dura, the subdural space becoming an artifactual true space. The arachnoid is named for the delicate, spider-web-like filaments that extend from its deep surface, through the CSF of the subarachnoid space, to the pia mater. See Also: leptomeninges. Syn: arachnoidea, arachnoides [NA], arachnoid mater, arachnoid membrane, parietal layer of leptomeninges.
Origin
arachnoid of brain
arachnoid mater
arachnoid mater cranialis
arachnoid mater encephali
arachnoidea mater spinalis
arachnoid of spinal cord