The middle of the three layers of the meninges or the membrances covering the brain and spinal cord.

The smooth external surface of the arachnoidea is closely applied between two other meninges, the more superficial "dura mater", while the inner roughened surface is separated from the "pia mater" by the subarachnoid space.

The delicate, spiderweb-like, or arachnoid layer, attached to the inside of the dura, surrounds the brain and spinal cord but does not line the brain down into its folds. In some locations the space is crossed by fine trabeculae.