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Arachnoid mater

Recommended video: Meninges of the brain [11:40]
Meninges and superficial vessels of the brain.

The arachnoid mater is the middle layer of the meninges enveloping the brain and spinal cord. It lies closely against the dura mater, however, it is not attached to it.

The arachnoid mater is named for its spiderweb-like appearance and is a thin transparent membrane encasing the brain and spinal cord like a loosely fitting sac. It is separated from the pia mater, the innermost layer of the meninges, by the subarachnoid space containing the CSF and the brain's blood vessels, including the cerebral arterial circle (of Willis).

In certain areas along the superior sagittal sinus, the arachnoid mater shows small protrusions into the inner layer of the dura mater. These protrusions are also called arachnoid granulations or arachnoid villi and function as one-way valves to allow the outflow of cerebrospinal fluid (CSF). Structurally, these granulations are composed of clusters of arachnoid villi which consist a fibrous core with bundles of collagen and elastic fiber cells arranged around it. 

Terminology English: Arachnoid mater

: Arachnoidea mater
Definition The arachnoid mater is the second layer of the meninges enveloping the brain and spinal cord.

Learn more about the cranial meninges in the following study unit:

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