Cranium anatomy, computer artwork. At bottom is the brain, which is enclosed in three membranes (grey) known as the meninges. From inner to outer, the meninges are the pia mater, the arachnoid mater, and the dura mater. Above these are the skull (yellow), the periosteum (bone-lining membrane, grey), and an outer skin layer (pink) that includes scalp hairs. Also shown are arterial (red) and venous (blue) capillaries, part of the blood-brain barrier.