In the brain, the<unk>  cavity<unk>  is the interval between the arachnoid mater and pia mater.
It is occupied by a spongy tissue consisting of trabeculæ (delicate connective tissue filaments that extend from the arachnoid mater and blend into the pia mater) and intercommunicating channels in which the cerebrospinal fluid is contained.
This cavity is small on the surface of the hemispheres of the brain. On the summit of each gyrus the pia mater and the arachnoid are in close contact, but in the sulci between the gyri, triangular spaces are left, in which the<unk>  trabecular tissue is found. Whilst the pia mater closely follows the surface of the brain and dips into the sulci, the arachnoid bridges across them from gyrus to gyrus.
At certain parts of the base of the brain, the arachnoid is separated from the pia mater by wide intervals, which communicate freely with each other and are named<unk>  cisternæ; in these the<unk>  tissue is less abundant. The<unk>  is the location of the interface between the vascular tissue and the cerebrospinal fluid and is active in the blood brain barrier.
The arachnoid mater continues down the spinal cord too, and the<unk>  layer with it. It serves a similar function in the spinal cord as it does in the brain.
Subarachnoid space
Subdural space
Epidural space
Danger space
Arachnoid mater
White matter
Cerebral aqueduct
Cerebral crus