4.11.4 Tympanic cavity (upper part)
Now that we've seen the auditory tube we'll come back to the tympanic cavity. In it we'll see the three small bones, the auditory ossicles, that conduct sound vibrations from the tympanic membrane to the inner ear.
So far we've just had a preview of this lower part of the tympanic cavity. To see the whole of the tympanic cavity we'll remove the bone that lies above and behind the external auditory meatus. Now if we look up from below we can see the full extent of the tympanic cavity.
With the auditory ossicles in place the picture is rather busy. We'll remove them for now, along with the bone here, and here, to give ourselves a clear look at the medial wall of the tympanic cavity.
These two openings in the medial wall both lead to the vestibule of the inner ear. The oval one above, the vestibular window, is occupied by the stapes. This round one below it, the cochlear window, is closed off by an inactive membrane.
This bulge, the promontory, is formed by the basal turn of the cochlea. The facial nerve runs here in the facial canal, just beneath the bony surface. In front, as we've seen, the tympanic cavity is continuous with the auditory tube.
Up here behind, it's continuous with a collection of air-filled spaces, the mastoid air cells, which we’ll look at in a dry specimen. Here’s the tympanic cavity. In this skull we’ve made an opening in the upper part of the mastoid process to expose the mastoid air cells. Here are the air cells. The tympanic cavity is through here. The mastoid air cells don’t go anywhere: collectively they’re a dead end.