(5.51)

To look inside the vestibule we'll make an opening in the hard bone that surrounds the vestibule and the semicircular canals.

This is the vestibule. Here, we've also exposed the wide ampullary ends of the lateral and superior semicircular canals as they enter the vestibule close together. Back here we're looking downwards into the ampullary end of the posterior semicircular canal: that's this one. Above it are two openings for the non-ampullary ends of the three canals: this one for the lateral canal, and next to it, this one for both the superior and posterior canals, which join together here before entering the vestibule.

If we remove some more bone down here, below where the oval window was, we can see the beginning of the cochlea, which passes forwards and medially from the lowest part of the vestibule. This pale structure, which we'll meet later, is part of the osseous spiral lamina. Down here, right below the base of the cochlea, is the round window, that looks out into the tympanic cavity.

Now let's get back to the oval window. To see the oval window on the inside, we'll remove the bone up here, so as to make an opening in the top of the vestibule here. Here's the oval window from the outside, here it is on the inside.

The oval window is only an open window in a dry bone specimen. In the living body it houses the footplate of the stapes. A ring of fibrous tissue, the anular stapedial ligament, attaches the stapes to the edge of the oval window, giving it a small range of movement. Vibrations of the tympanic membrane are transmitted by the malleus and incus to the stapes.

To see the footplate of the stapes from the other side, we've again made an opening into the top of the vestibule, so we can look straight down into it. Here's the footplate of the stapes. The movements of the stapes produced by sound vibrations, which in reality are much smaller than this, are transmitted to the cochlea.

The essential structures of the cochlea begin right below the oval window, in the floor of the vestibule. We're seeing down here the start of two of those structures, which we'll be looking at later, the osseous spiral lamina, and the basilar membrane.

The main parts of the membranous labyrinth that are inside the vestibule are the utricle and the saccule, each of which contains a sense organ, or macula, that responds to linear motion. Down here in the floor of the vestibule is the very first part of the coclear duct. Also contained in the vestibule are the small passages that connect these components: the endolymphatic duct with its branches to the utricle and saccule, and down here the ductus reuniens, connecting the saccule to the cochlear duct.

We can see most of these structures in this dissection, that's seen from an anterolateral viewpoint. This is the vestibule. The oval window was here. Entering the vestibule up here are the anterior and lateral semicircular ducts. Here's the start of the cochlea opening into the floor of the vestibule.

This is the upper part of the utricle. Contained within it is the sense organ, the macula of the utricle, which is one of the two otolith organs. If we make an opening in the utricle here, we can see the white layer of crystals, the otoliths, whose weight and inertia provide the stimulus for our response to linear motion.

The utricle is also a manifold chamber, into which the three semicircular ducts open. Their ampullary ends open into the utricle here, the non-ampullary ends here, with the superior and posterior ducts having first joined to form the common crus.

Back here is the lower part of the utricle. The ampullary end of the posterior semicircular duct opens into it down here. The other openings are out of sight back here.

This is the saccule. Its wall is so thin that it's very hard to see. The saccule is shaped like a bent teardrop, lying against the medial wall of the vestibule. The otolith organ, or macula, of the saccule is out of sight here, close to the bony wall.

In the floor of the vestibule the saccule tapers downwards into a narrow duct, the ductus reuniens, which connects the saccule to the start of the cochlear duct.