Meninges

The meninges, the cerebrospinal membranes, invest the brain and spinal cord, the optic nerve, and also the first portions of the cranial and spinal nerve roots. There are three cerebrospinal membranes: pia mater, the innermost; arachnoid, the intermedial; dura mater, the outermost. All three cerebrospinal membranes consist of connective tissue. They are mostly of mesodermal origin, the pia and arachnoid covering the telencephalon and diencephalon are probably derived from the neural crest.