A Guide to the Auditory Ossicles: the Bones of the Middle Ear

Malleus, Incus, and Stapes

White noise, conceptual artwork
Science Photo Library - VICTOR HABBICK VISIONS / Getty Images

The auditory ossicles are a chain of small bones in the middle ear that transmit sound from the outer ear to the inner ear as a mechanical vibration. The names of these three bones are taken from Latin. The malleus is also known as the "hammer," the incus is also called the "anvil," and the stapes is known as the "stirrup." How small are these auditory ossicles? They fit into an area no bigger than an orange seed.

What Do the Auditory Ossicles Do?

The purpose of the auditory ossicles (also called the "ossicular chain") is to transmit sound from the outer ear to the inner ear. What these little bones do is form a chain that connects the eardrum to the inner ear, where the cochlea is.

The first one in the chain is the malleus which connects the eardrum to the incus.The incus connects the malleus to the stapes. The stapes connects the incus to the oval window, which is a membrane covered opening to the inner ear.

When the eardrum vibrates from sound input, it makes the auditory ossicles vibrate too. The vibrations move through the chain, and the stapes will transmit the sound through the oval window and into the fluid of the cochlea. Then the rest of the hearing mechanism takes over, and sound waves are converted to nerve signals and processed by the brain.

The ossicles act in two ways to increase the sound level to accommodate the transition from air to fluid.

They act as a lever, and there is a change in surface area from the large eardrum to the small stapes that actually increase sound by about 30dB. 

How Do the Auditory Ossicles Protect Hearing?

The auditory ossicles may have a protective effect when exposed to continuous loud sounds. The stapedius muscle and tensor tympani muscles work to tighten up the surface of the eardrum and pull back on the stapes to reduce the force of the sound to the inner ear.

This decreases that amount of amplification that would normally take place in the middle ear. It should be noted that this a slow process that will not protect the ear from sudden loud sounds, and this protective effect may become less as age increases. If you will be around loud noise, hearing protection use is recommended.  

What Disorders Can Have an Impact on the Auditory Ossicles?

Here are some conditions that can impact the auditory ossicles and cause hearing loss:

  • Otosclerosis: One of the auditory ossicles becomes stuck in place and unable to vibrate, impairing hearing in that ear.
  • Disarticulation of the ossicular chain: the bones become dislocated due to fracture, perforated eardrum, or chronic infection.
  • Cholesteatoma: a skin cyst behind the eardrum, often due to chronic otitis media.
  • Eardrum perforation: caused by trauma or ear infections.
  • Middle ear fluid: often caused by ear infections.
  • Genetic syndromes that impact middle ear development

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