The brain is a very delicate and sensitive organ, it must be protected and cushioned to prevent collision against the bony skull. Collectively, the layers that protect the brain all join together to form a uniform, continuous layer called the meninges.
The meninges are a flexible sheet made up of 3 layers of membrane which surround and protect the brain and spinal cord. These layers are: Dura mater (the outermost layer), Arachnoid mater (the middle layer) and Pia mater (the innermost layer).
The Dura mater is a thick, durable membrane which is the closest to the surface of the skull. It surrounds and supports a network of blood vessels which carry blood towards the heart.
The Arachnoid mater gets it's name from it's spider-web like appearance. It connects the dura mater and the pia mater and forms a space containing the CSF. This layer acts as a shock absorber which reduces the forces exerted on the brain from sudden impacts and movement, preventing the brain from “colliding” with the skull.
The Pia mater lies directly on the surface of the brain and provides nutrients and oxygen to the brain. It also protects the brain from direct contact with CSF.
The CSF is a colourless fluid formed in the ventricles of the brain. It flows around the brain and acts as a cushion or padding, which reduces friction.
The brain weighs about 1.4kg. Because the brain floats in CSF, it's weight inside the skull is reduced to approximately 0.025kg! This "neat trick" helps to reduce pressure in the head. This is why when there is a tear in the meninges and CSF leaks out, the brain sags in the skull and causes "pressure headaches" especially when standing in an upright position.