Corpus: Auditory ossicle

The term auditory ossicles refers to the three articulated bones within the tympanic cavity that enable the transmission of sound from the eardrum to the inner ear and mechanically amplify the sound. They are the smallest bones in the human body.

The auditory ossicles include:

• Malleus (hammer)
• Incus (anvil)
• Stapes (stirrup)

The three ossicles are connected by two true joints. The articulatio incudostapedia is a small ball-and-socket joint that connects the incus to the stapes. Similarly, the second joint, the articulatio incudomallearis, connects the malleus with the incus (hammer-anvil joint). A syndesmosis, the syndesmosis tympanostapedia, also fixes the base of the stapes to the labyrinth capsule in the oval window.

The malleus and incus originate from the mesenchyme of the 1st gill arch. The stapes is derived from the tissue of the 2nd gill arch. Bone formation in the malleus begins between the 3rd and 4th foetal month with the formation of a periosteal bone cuff. This process begins somewhat later in the incus. Bone formation of the stapes usually only begins at the end of the 4th foetal month.

Sound is transmitted from the eardrum to the oval window via the ossicular chain. Leverage forces between the individual ossicles and the difference in surface area between the eardrum and the oval window amplify the sound by a factor of about 22.

Impedance conversion[Bearbeiten]

Through this amplification, the body solves a problem caused by the change in medium of the sound, namely from air conduction in the ear canal to fluid conduction in the cochlea. When the medium changes, a large proportion of the sound is normally reflected. The amplification by the ossicles is large enough to compensate for this effect and transmit sufficient sound energy to the inner ear.

• Image source podcast: ©Magda Ehlers / Pexels