The arachnoid mater is one of the three meninges, the protective membranes that cover the brain and spinal cord.
It is interposed between the two other meninges, the more superficial and much thicker dura mater and the deeper pia mater, from which it is separated by the subarachnoid space. The delicate arachnoid layer is attached to the inside of the dura and surrounds the brain and spinal cord. It does not line the brain down into its sulci (folds), as does the pia mater, with the exception of the longitudinal fissure, which divides the left and right cerebral hemispheres. Cerebrospinal fluid (CSF) flows under the arachnoid in the subarachnoid space. The arachnoid mater makes arachnoid villi, small protrusions through the dura mater into the venous sinuses of the brain, which allow CSF to exit the sub-arachnoid space and enter the blood stream.
The arachnoid mater is named after the Greek words "Arachne" ("spider") and suffix "-oid" ("in the image of"), and "mater" (the Latin word for mother), because of the fine spider web-like appearance of the delicate fibres of the arachnoid which extend down through the subarachnoid space and attach to the pia mater.
The arachnoid mater covering the brain is referred to as the "arachnoidea encephali", and the portion covering the spinal cord as the "arachnoidea spinalis". The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges. ("Lepto"- from the Greek root meaning "thin"). Similarly, the dura in this situation is called the pachymeninx.
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... The middle element of the meninges is the arachnoid mater, so named because of its spider web-like appearance ... The arachnoid mater is a thin, transparent membrane ... of fibrous tissue and, like the pia mater, is covered by flat cells also thought to be impermeable to fluid ...