| Medical Dictionary: arachnoid membrane |
A delicate fibrous membrane forming the middle of the three coverings of the brain and spinal cord, closely attached to the dura mater, from which it is separated only by the subdural cleft, but separated from the pia mater by the subarachnoid space.
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| Brain: Arachnoid mater | ||
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| The medulla spinalis and its membranes. | ||
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| Diagrammatic representation of a section across the top of the skull, showing the membranes of the brain, etc. | ||
| Gray's | subject #193 876 | |
| Part of | Meninges | |
| NeuroNames | ancil-561 | |
| MeSH | Arachnoid | |
The arachnoid mater is one of the three meninges, the membranes that cover the brain and spinal cord. It is interposed between the two other meninges, the more superficial dura mater and the deeper pia mater, and is separated from the pia mater by the subarachnoid space.
The delicate, spiderweb-like (hence the name) arachnoid layer, attached to the inside of the dura, surrounds the brain and spinal cord but does not line the brain down into its sulci (folds). Cerebrospinal fluid flows under this membrane in the subarachnoid space, which is full of the delicate fibres of the arachnoid extending down to attach to the pia mater.
The portions covering the brain and spinal cord are called arachnoidea encephali and arachnoidea spinalis, respectively.
The arachnoid and pia mater are sometimes considered as a single structure, the leptomeninx, or the plural version, leptomeninges. (Lepto- from the root meaning thin in Greek). Similarly, the dura in this situation is called the pachymeninx.
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Arachnoid is from a Greek root, and means cob web like. The mater designation (meaning mother in Latin) is borrowed from the dura mater and pia mater, which were Latin translations of Arabic terms. While mater does not technically belong with the arachnoid layer, it has nevertheless been adopted by it for uniformity with the other meninges, and arachnoid mater is currently the Terminologia Anatomica international standard.
 Meninges of the CNS |
 Diagrammatic section of scalp. |
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 | Medical Dictionary. The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Read more |
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