(3.46)

On each side, the sense organs for hearing and balance are contained within a complicated cavity in the petrous temporal bone that's shaped like this. The cavity is known as the bony labyrinth.

The bony labyrinth consists of a central chamber, the vestibule, the three semicircular canals, and the spiral cochlea.

The cochlea contains the sense organ for hearing; the vestibule and the semicircular canals contain the five sense organs for balance: two in the vestibule, one in each of these thickenings in the canals, the ampullae.

Inside the protective walls of the bony labyrinth, the sense organs are contained within a complex set of delicate membranous structures, known collectively as the membranous labyrinth. We'll see these structures in detail later; meantime we'll begin by seeing where the bony labyrinth lies within the petrous temporal bone, and how it relates to some visible features of the temporal bone.

Seen from directly above, the bony labyrinth is aligned with the long axis of the petrous temporal bone. It's also aligned with this continuous cavity that's formed by the auditory tube, the tympanic cavity, and the mastoid air cells.

This part of the anterolateral aspect of the bony labyrinth lies close to the medial wall of the tympanic cavity. In an intact skull, looking into the external auditory meatus, we can only see a small part of that medial wall. To see it better, we'll go to an isolated temporal bone, and then take away all the bone that's external to the tympanic cavity, leaving this upper surface intact.

This is the medial wall of the tympanic cavity. This is the oval window. We're looking through it, into the vestibule of the inner ear. This opening below the oval window leads to the round window, which opens out of sight, here. This thickening, the promontory, overlies the basal turn of the cochlea. Up here, some cancellous bone has been removed, to reveal the thick, hard bone that surrounds the lateral and superior semicircular canals.

Going round to the posteromedial aspect of the bony labyrinth, this part of the cochlea and this part of the vestibule are separated by thin bony partitions from the lateral end of the internal auditory canal. Looking at a skull that's been divided in the midline, here's the internal auditory meatus.

We'll remove some bone here, so we can get a better view of the lateral end of the canal, and see how it's related to the bony labyrinth. The basal turn of the cochlea passes beneath the end of the internal auditory canal. The vestibule is immediately lateral to the end of the canal.