The auditory ossicles form a mobile chain of small bones across the tympanic cavity from the tympanic membrane to the oval window (Latin fenestra vestibuli), an oval opening on the labyrinthine wall of the tympanic cavity leading to the vestibule of the bony labyrinth. These ossicles are the first bones to be fully ossified during development and are essentially mature at birth. The bone from which they are formed is exceptionally dense (hard). The ossicles are covered with the mucous membrane lining the tympanic cavity; but unlike other bones, they lack a surrounding layer of osteogenic periosteum.