Membranes
or Meninges
Surrounding the brain and spinal cord is a tough
waterproof balloon made up of three membranous layers called the meninges.

The meninges are closely attached to the bones of the
skull. The outer layer of the meninges called the Dura is so closely
attached to the bones of the skull that it forms what is called
a periosteum or inner lining to these bones.
Consequently, all the bones that attach the membranes
follow any motion exhibited by the membrane, expanding and contracting
in accordance with the membrane and reflecting every pull or tension
within the membrane system.
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