Auditory Ossicles: The Bones of the Middle Ear

The auditory ossicles. medicalRF.com/Getty Images

What are the auditory ossicles?

The auditory ossicles are a chain of small bones in the middle ear. Their names 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? One source says they are 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. 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 so 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 impact the auditory ossicles?

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

Sources:

How We Hear. American Speech-Language-Hearing Association. http://www.asha.org/public/hearing/How-We-Hear/. Accessed May 2011.

I Love What I Hear. National Institute on Deafness and Other Communication Disorders. http://www.nidcd.nih.gov/health/education/teachers/hearing.html. Accessed May 2011.

Stevens, S. S., & Warshofsky, Fred,eds. Sound and Hearing, Time-Life Books, NY, 1965. Excellent illustration of inner ear and discussion of inner ear process.

DeBonis, D.A. and Donohue, C. L., Survey of Audiology: Fundamentals for Audiologists and Health Professionals, Boston: Pearson Education, Inc., 2004.

Taylor, T. (n.d.) Bones of the ear. Inner Body. Retrieved 12/15/2015 from http://www.innerbody.com/anatomy/skeletal/head-neck/bones-ear#full-description

Updated by Melissa Karp, Au.D. 

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