Brain membranes. Cutaway illustration of the top view of the human skull showing the membranes (meninges) over the brain. The convoluted brain is covered by three membranes. From right to left, the first (brown) is the pia mater lying directly against the brain and following the folds of its surface. The middle layer (white spotted) is the elastic and web-like arachnoid. Between the pia mater and arachnoid layers flows the cerebrospinal fluid. The outermost layer (pale blue) is the dura mater. This is a tough fibrous layer lining the inside of the skull. Beyond the meninges, the thick layer of skull bone (white, spotted) and the skin layers can be seen.