Periostium, in Anatomy, a nervous vascular membrane, ended with a very quick sense, immediately surrounding, in every part, both the internal and external surfaces of all the bones in the body, excepting only so much of the teeth as stand above the gums, and the peculiar places on the bones in which the muscles are inserted. It is hence divided into the external and internal periosteum; and where it externally surrounds the bones of the skull, it is generally called the pericranium. See Anatomy Index.