The Arachnoid mater is one of the three layers of the meninges, interposed between the dura mater and the pia mater and separated from the pia mater by the subarachnoid space. The pia and arachnoid membranes are called leptomeninges (Orlando Regional Healthcare, 2004). The delicate, spiderweb-like arachnoid layer, attached to the inside of the dura, surrounds the brain and spinal cord but does not line the brain down into its sulci (folds). Cerebrospinal fluid flows under this membrane in the subarachnoid space, which is full of the delicate fibres of the arachnoid extending down to attach to the pia mater. The meninges (singular meninx) are the system of membranes that envelop the central nervous system. ... The dura mater (from the Latin hard mother), or pachymeninx, is the tough and inflexible outermost of the three layers of the meninges surrounding the brain. ... The pia mater (Latin: tender mother, itself a translation from Arabic) is the delicate innermost layer of the meninges - the membranes surrounding the brain and spinal cord. ... A sulcus (pl. ... Cerebrospinal fluid (CSF) is a clear bodily fluid that occupies the subarachnoid space in the brain (the space between the skull and the cerebral cortex—more specifically, between the arachnoid and pia layers of the meninges). ...

Reference

• Orlando Regional Healthcare, Education and Development. 2004. "Overview of Adult Traumatic Brain Injuries."