The bones related to the organ of hearing, the tympanis and petrous bones, are very solid and dense in structure.
On its inner side is a small air chamber in the petrous portion of the temporal bone, called the cavity of the tympanum.
From the surrounding 794 mesoderm the petrous bone is formed by a process of chondrification and ossification.
The bony labyrinth presents a series of cavities which are channelled through the substance of the petrous bone.
By this means the posterior surface of the petrous bone can be exposed as far inwards as the internal auditory meatus.