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 as it moves with the craniosacral rhythm. The movement of the membranes reflect every pull or tension within the membrane system.