Running within a tunnel, that's called the cochlear canal, the structures that form the cochlea are completely encased within the petrous temporal bone.
Seen from above, the axis of the spiral is perpendicular to the long axis of the petrous temporal bone, pointing almost horizontally, however it's common to describe the cochlea as if it was upright, with the apex at the top.
In this dissection, all the bone that surrounds the cochlear canal has been removed except for a thin shell, so we can see its overall shape.
The cochlear canal begins in the floor of the vestibule. It first runs downwards and toward us, as seen in this anterolateral view, then makes two and three-quarters spiral turns to end blindly here at the apex.
We'll be returning to this anterolateral view, but for a first look inside the cochlear canal we'll go to a dry bone specimen that's been divided in the horizontal plane, and look down on it from above. We'll flip our view so the apex of the cochlea points upwards.
The cochlear canal spirals around a hollow bony core, the modiolus. Also spiralling around the modiolus are two bony structures: the interscalar septum, which separates adjoining turns of the cochlear canal; and this projecting shelf, the spiral lamina, which supports the basilar membrane. At the apex, the cochlear canal ends in a small dome, the cupola. Beneath it, the two bony spirals end in interesting ways, as we'll see.
Along almost its whole length, two membranes divide the cochlea into three compartments. This is the all-important basilar membrane, which houses the organ of hearing. This slender membrane is Reissner's membrane.
Here are the same structures in a histologic cross-section: here's the basilar membrane, here's Reissner's membrane. The two membranes enclose the cochlear duct, which contains endolymph, separating it from the scala vestibuli above, and the scala tympani below, which both contain perilymph. The cochlear duct is also known as the scala media. The bony wall of the cochlear canal is lined by a layer of periosteum.
In this preserved specimen, we'll remove part of the bony wall, exposing the periostum; this is periosteum. These black dots are melanocytes, pigment cells, whose function in the inner ear is uncertain.
We'll remove some of the periosteum, so we can look inside. This is the cochlear duct. Here's Reissner's membrane, moving a little as it's touched with a probe. Removing part of Reissner's membrane, we start to see the basilar membrane beneath it. Here's the basilar membrane in full view. To understand it better we'll go back to the histological image.
This is the basilar membrane. Along its length it's suspended between these two structures: the spiral lamina on the inside, and the spiral ligament on the outside. The spiral lamina, which is formed partly of bone, projects from the modiolus. The spiral ligament is a thickening of the periosteum.
The vital feature of the basilar membrane is the organ of Corti, that's just visible in a dissection: it's this little bump.
This is the basilar membrane, which moves in response to sound vibrations. Overhanging it is the tectorial membrane, which doesn't move. Our sense of hearing depends on two sets of hair cells in the organ of Corti: this triple row of outer hair cells, which actively amplify movements of the basilar membrane, and this single row of inner hair cells, which translate these movements into sensory nerve impulses.
Going back to the view we had before, this is the spiral lamina, here's the edge of the spiral ligament, here's the basilar membrane, and these two thin lines indicate the inner and outer hair cells.
Here, we can see almost the whole length of the basilar membrane. It starts in the floor of the vestibule, and ends just short of the apex.
Beneath the membrane here is the round window. To see it we'll remove this part of the basilar membrane and spiral lamina. This is the round window. It's covered over by the round window membrane, which faces medially into the round window recess of the tympanic cavity. The flexibility of the round window membrane is what makes it possible for the stapes to move, and set up sound vibrations in the perilymph.
To see what happens at the apex of the cochlea, we'll look from behind at a dry bone specimen where we've exposed this much of the apical turn. The specimen is transilluminated. This is the apical turn, this is part of the second turn. This bony layer is the interscalar septum that forms the floor of the cochlear canal. It curls round beneath the cupola, twisting to end in a short free border, which joins the underside of the cupola to the tip of the modiolus. This is the spiral lamina. It curls around to end as a freestanding half-crescent, the hamulus, which supports the last part of the basilar membrane.
Here's the same area in a fresh specimen. This is the end of the cochlear duct. The hamulus is here. Between the curved free border of the hamulus and cochlear duct, and the interscalar septum, there's a circular opening, the helicotrema, where the scala vestibuli and the scala tympani become continuous.
Here's the apical region, in a specimen where almost all the bone has been removed. Here's the helicotrema, here's the hamulus, with nerve fibers fanning out from it to supply the most distal part of the basilar membrane. The basilar membane ends here, just short of the end of the cochlear duct, which is here.
The modiolus widens to a broad base, which faces medially into the end of the internal auditory canal. The modiolus is the bony conduit for the nerve fibers going to and from the basilar membrane. The nerve fibers emerge through the base of the modiolus into the internal auditory canal, forming the cochlear nerve.