Auditory ossicles

At a glance

English

Auditory ossicles

Latin (TA2)

Ossicula auditus

System

Bone

Region

Head

In order

From the malleus to the stapes

About the auditory ossicles

Inside the middle ear, an air-filled space in the temporal bone, a short chain of three bones spans the gap between the tympanic membrane and the oval window of the inner ear. From outside inwards they are the malleus (hammer), the incus (anvil) and the stapes (stirrup), named after the objects they resemble. They are the smallest bones in the body, and they are linked to each other by small synovial joints and slung from the walls of the cavity by ligaments.

The malleus has its handle, or manubrium, fixed into the eardrum's inner face, with the head rising above into the upper recess of the cavity. The head of the malleus meets the body of the incus at the incudomallear joint. The incus has a short limb that points backwards to a ligament on the posterior wall and a long limb that hangs down beside the handle of the malleus, ending in the lenticular process. That process meets the head of the stapes at the incudostapedial joint. The two limbs of the stapes then reach the oval footplate, which is held in the oval window by the annular ligament.

The chain acts as a lever and transformer. Sound in air sets the large eardrum moving, the ossicles pass the motion across, and the much smaller footplate pushes on the fluid of the inner ear. This concentration of force onto a small area overcomes the loss that would occur if sound met fluid directly. Two small muscles damp the system in loud noise: tensor tympani, supplied by the mandibular nerve, pulls on the malleus, and stapedius, supplied by the facial nerve, pulls on the stapes.

The malleus and incus arise in the first pharyngeal arch, beside Meckel's cartilage, and the stapes in the second; in reptiles the bones that became the malleus and incus still form the jaw joint. Clinically, otosclerosis can fix the stapes footplate in the oval window and cause conductive hearing loss, treated by replacing all or part of the stapes with a prosthesis. Ossicles can also be fixed by scarring or eroded by chronic middle ear infection, and a facial nerve palsy that involves stapedius makes ordinary sounds seem too loud.