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4.11.5 Auditory ossicles

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(4.51)

Now we’ll put the three auditory ossicles back into the picture. They're the stapes, the incus and the malleus. We'll start with the tiny stapes, the smallest bone in the body.

The stapes consists of a head which articulates with the incus, an arch that's formed by the posterior crus, and anterior crus, and an oval base or footplate, which occupies the vestibular window.

Here's the vestibular window. We'll add the stapes to the picture. The edge of the footplate is attached to the inside of the window by a membrane that allows it to move. Movement of the stapes sets up sound vibrations in the perilymph of the inner ear.

The tendon of the tiny stapedius muscle, which we'll add to the picture, is attached to the head of the stapes from behind. Here's the tendon of stapedius. Its muscle belly is enclosed in bone back here. The stapedius muscle tilts the stapes backwards.

The head of the stapes articulates with the incus, which we'll add to the picture. Here's the incus. The incus moves the stapes, and is in turn moved by the malleus.

The incus has a body, a short crus, and a long crus. The long crus curves medially, ending at the lenticular process, wich articulates with the stapes. The short crus points backwards. The tip of the short crus is tethered to the wall of the tympanic cavity here, by the posterior ligament of the incus.

On the front of the body of the incus there's a saddle-shaped joint surface, at which the incus articulates with the malleus. Here's the joint surface.

We'll add the malleus to the picture, together with the ligaments that hold it in place, and the bone those ligaments are attached to.

We've already seen that this part of the ...

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(4.51)

Now we’ll put the three auditory ossicles back into the picture. They're the stapes, the incus and the malleus. We'll start with the tiny stapes, the smallest bone in the body.

The stapes consists of a head which articulates with the incus, an arch that's formed by the posterior crus, and anterior crus, and an oval base or footplate, which occupies the vestibular window.

Here's the vestibular window. We'll add the stapes to the picture. The edge of the footplate is attached to the inside of the window by a membrane that allows it to move. Movement of the stapes sets up sound vibrations in the perilymph of the inner ear.

The tendon of the tiny stapedius muscle, which we'll add to the picture, is attached to the head of the stapes from behind. Here's the tendon of stapedius. Its muscle belly is enclosed in bone back here. The stapedius muscle tilts the stapes backwards.

The head of the stapes articulates with the incus, which we'll add to the picture. Here's the incus. The incus moves the stapes, and is in turn moved by the malleus.

The incus has a body, a short crus, and a long crus. The long crus curves medially, ending at the lenticular process, wich articulates with the stapes. The short crus points backwards. The tip of the short crus is tethered to the wall of the tympanic cavity here, by the posterior ligament of the incus.

On the front of the body of the incus there's a saddle-shaped joint surface, at which the incus articulates with the malleus. Here's the joint surface.

We'll add the malleus to the picture, together with the ligaments that hold it in place, and the bone those ligaments are attached to.

We've already seen that this part of the malleus that hangs downwards, the handle or manubrium, is attached to the tympanic membrane.

In the dry bone, this is the manubrium. This is the head of the malleus. This joint surface, facing backwards, articulates with the incus. The malleus is suspended by two ligaments which are attached here behind, and here in front. This is the anterior ligament, this is the posterior one. The two ligaments are in line with each other.

The malleus makes a rotating movement through just a few degrees, around an axis of rotation that's in line with the anterior and posterior ligaments. There's very little movement at the joint between the malleus and the incus. The two bones move together.

The movement of the lenticular process causes a tilting movement of the stapes. Movement of the stapes is restrained by the action of the stapedius muscle.

Movement of the malleus is restrained in a similar way by a second small muscle, the tensor tympani. Here's the tendon of the tensor tympani. The tensor tympani muscle is enclosed in a bony tunnel here above and parallel to the auditory tube. Its tendon turns a corner as it emerges from the bony tunnel. The tensor tympani pulls the manubrium, and the tympanic membrane medially.

The stapedius and tensor tympani muscles act in response to loud noise. Their action helps to protect the inner ear from noise damage. Lastly we'll add to our picture of the tympanic cavity one highly unusual nerve, the chorda tympani.

The chorda tympani, a branch of the facial nerve, emerges from bone back here, passes between the malleus and the incus, and leaves the tympanic cavity up here on its way to join the lingual nerve. As we saw in a previous section, the chorda tympani conveys the sense of taste to much of the tongue.

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