Meninges
The CNS is covered by 3 membranes called meninges: in direct contact with the CNS is the pia (mater), a thin membrane, next is the arachnoidea (=Latin "spider web"), also quite thin and rich in fine blood vessels and outermost the dura (mater), a tough, elastic membrane, attached to the skull. The spaces between these membranes are filled with fluid: cerebrospinal fluid (CSF).
GE Healthcare Glossary