Subarachnoid space (intracranial)

Space between the arachnoid mater and pia mater in the brain filled with cerebrospinal fluid (CSF) and arachnoid trabeculae and septa. Normally, the space is quite small over the gyri, since the arachnoid and pia mater are in close contact. However, in the sulci the space widens, since the pia descends into the groove while the arachnoid bridges the gap without descending into the sulcus.