When we think of the structure of our ears, cartilage, rather than bones, comes to mind. Their absence is what gives our ears that slightly soft, flexible feel when we touch them, but to say that ears contain absolutely no bones whatsoever would be incorrect.
The reason a lot of people don’t realize that ears do in fact contain bones, albeit not many, is because they’re situated internally, along the middle ear. Collectively, these bones are known as the auditory ossicles, and they’re essential to the hearing process.
It sounds strange, I know. Such rigid matter seems like it would only hinder our hearing rather than help it, but these tiny little pieces of osseous matter are some of the key players in the functioning of our auditory networks. Today, I’ll be discussing their function in detail, but first, it’s important that we understand a little about what they are and how they’re structured.
What and Where Are the Auditory Ossicles?
You may have heard of the auditory ossicles before — perhaps they were an answer to a quiz on TV or at a bar. The reason they’re quite renowned is that they’re the smallest bones in the entire human body.
There are 6 of them in total, 3 in each ear. Individually, they’re known as the malleus, incus, and stapes; however, as they all have rather striking appearances, they’re commonly referred to as their shapes: hammer, anvil, and stirrup.
This tiny team of bones forms a chain connecting the inner and middle ear. The first in sequence is the malleus, which runs from the edge of the tympanic membrane, or in other words, the eardrum, through to the second bone in sequence, the incus. The incus travels along the middle ear, eventually meeting up with the last of the trio, the stapes, which runs through to the fenestra ovalis (oval window) at the threshold of the inner ear.
Structure of the Auditory Ossicles
Malleus
The malleus, or hammer, is the largest of the auditory ossicles. It’s composed of five distinct sections: the head, the neck, the handle, and two extensions known as the anterior and lateral processes.
The anterior extension terminates in a tiny gap in the skull. It’s both the fulcrum that facilitates the rotation of the malleus and incus connection, as well as a damper for loud low frequencies.
A larger cone-shaped extension, the lateral process, is situated between the base of the handle and the eardrum, held fast by three ligaments.
Incus
The incus only consists of two parts, or limbs: the short limb and the long limb. Attached to the rear wall of the eardrum by a ligament, the short limb provides the stability required of this central bone.
The long limb of the incus travels towards the inside of the head (a diversion known as the lenticular process) before meeting the stapes.
Stapes
Despite being the smallest of all three ossicles, the stapes has four distinct sections: the head (capitum), the base (footplate), and the two limbs (anterior and posterior).
The limbs of the stapes curve out from the head, traveling in tandem to the base, giving the bone its stirrup-like appearance.
Joints
Much like all the other larger bones in our bodies, the auditory ossicles are interconnected by joints, but these aren’t your garden variety ball and socket or hinge joints. These specialist connections are known as incudomalleolar joints, and the joint that connects the stapes to the oval window is called a tympanostapedial joint.
We can’t feel it, but these joints allow the auditory ossicles to shift or dance in response to signals picked up by our eardrums, although their movement is quite limited.
What Is the Purpose of the Auditory Ossicles?
Simply put, the auditory ossicles facilitate the transformation of sound waves to electrical signals that the brain can read. Here’s an overview of how it all works.
It All Starts with the Eardrum
Sound waves wriggle their way into our ear canals, which herds them like sheep towards our eardrums, which vibrate in response.
The Auditory Ossicles Take it From There
These vibrations are picked up by the spatulate process and handle of the malleus, which, in turn, starts to vibrate. The vibrations travel along the neck and head of the malleus, over to the incus via the incudomalleolar joint.
The incus then passes the sonic news on to the stapes, completing the chain of vibration.
Small Equals Loud!
There’s a reason the auditory ossicles get smaller in sequence. The smaller the bone, the more furiously the vibrations can shake it, amplifying the signal.
This means the vibrations reach peak strength at the stapes, just before hitting the oval window.
Peering Through the Window
Just beyond the oval window is the cochlear, a snail shell-shaped cavity filled with fluid and thousands of infinitesimal receptor cells.
When the oval window starts to move, it causes turbulence in the cochlear fluid, and the tiny ripples formed stimulate the receptor cells, thereby transferring the mechanical data that started at the ear canal, into electric signals.
These signals then embark on a short voyage to the brain via the vestibulocochlear nerve.
Possible Conditions of the Auditory Ossicles
Unfortunately, there are a number of things that can go wrong with this delicate network of bones, including natural anatomical variations. Roughly 1 person in 10,000 is born with a malformation of the auditory ossicles on one or both sides, limiting their ability to hear.
Other conditions develop over time, the most common of which is ossicular chain discontinuity, which means two or all of the bones are fused together or situated too far apart. Symptoms include tinnitus, impaired hearing, and in serious cases, deafness.
Conditions that can lead to ossicular chain discontinuity and other ossicular issues include…
- Middle ear infections
- Abnormal tissue growth (otosclerosis)
- Non-cancerous growths (cholesteatoma)
- Head trauma
- Weakened stapedius muscle
What Is the Function of the Auditory Ossicles? Summing Up
The auditory ossicles are a trio of bones in each ear that work together as a team to transfer sonic vibrations from the earlobe, all the way through to the oval window.
Not only do they act as conduits for sound, but their tapering structure amplifies it too, helping to trigger the receptor hair cells in the cochlear. These cells then send a message in the form of electrical signals that the brain can process, allowing us to hear and understand a sound.