The middle membrane of the three mammalian meninges covering the brain and spinal cord. [TFD][VHOG]

In fishes, the meninges consist of a single membrane, the primitive meninx, wrapped around the brain and spinal cord. With the adoption of terrestrial life, the meninges doubled. In amphibians, reptiles, and birds, the meninges include a thick outer dura mater derived from mesoderm and a thin inner secondary meninx. (...) In mammals, the dura mater persists, but division of the secondary meninx yields both the arachnoid and the pia mater from ectomesoderm.[well established][VHOG]

An arachnoid mater that surrounds a brain.

arachnoidea mater cranialis

cranial arachnoid mater

brain arachnoid matter

arachnoidea mater encephali

uberon

UBERON:0005397