meninges

the three CONNECTIVE TISSUE membranes that surround the SPINAL CORD ans BRAIN . The outermost layer,or meninx, is called the dura mater, which is fibrous, tough and inelastic, and also called the pachymeninx, closely lining the inside of the SKULL and helping to protect the brain. It is thicker than the middle layer, the arachnoid mater, which surrounds the brain. The innermost layer, the pia mater, is thin and delicate and lines the brain. CEREBROSPINAL FLUID circulates between it and the arachnoid mater, and both these inner layers are richly supplied with BLOOD VESSELS that supply the surface of the brain and skull. These two inner membranes are sometimes collectively called the pia-arachnoid or leptomeninges.