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The subarachnoid space is the interval between the arachnoid membrane and the pia mater. It is occupied by delicate connective tissue trabeculae and intercommunicating channels containing cerebrospinal fluid (CSF) as well as branches of the arteries and veins of the brain.
The cavity is small in the normal brain. Over the gyri, the arachnoid membrane and pia mater are in close contact. However, in the sulci, triangular spaces develop because the pia mater is closely applied to the cerebral cortex and the arachnoid membrane bridges the gap between adjacent gyri without descending into each sulcus.
At certain points, the pia mater and arachnoid membrane are separated by wide intervals which communicate freely with each other known as subarachnoid cisterns, in which the subarachnoid tissue is less abundant.
The subarachnoid space is also contiguous around the spinal cord.