Middle ear

The middle ear is the part of the ear found just inside the eardrum and just outside the oval window of the cochlea, which is part of the inner ear. It plays an important role in helping us hear.

Inside the middle ear, there are three tiny bones called ossicles: the malleus, incus, and stapes. These bones move when the eardrum vibrates and turn those vibrations into waves in the fluid and membranes of the inner ear. The space in the middle ear is called the tympanic cavity and is surrounded by part of the skull known as the tympanic part of the temporal bone.

The middle ear also has a tube called the auditory tube, also known as the Eustachian tube, which connects to the back of the nose (nasopharynx). This helps keep the pressure the same between the middle ear and the throat. The main job of the middle ear is to help move sound energy from the air outside, through the eardrum, and into the fluid inside the cochlea, so we can hear sounds clearly.

Structure

The middle ear has three tiny bones called the malleus, incus, and stapes. These bones help move sounds from the eardrum to the inner ear. The stapes is found in all animals with four limbs, while the malleus and incus changed from jaw bones in reptiles.

Two small muscles can make these bones move less when there are very loud sounds. This helps protect the inner ear from being hurt by loud noises. The middle ear also has important parts of the facial nerve that pass through it. If these nerves are hurt during surgery, it can affect the face or the sense of taste on one side of the tongue.

Function

Sound transfer

The middle ear helps move sound from the air to the fluids inside the inner ear. When sound waves hit a liquid, most of the sound bounces back. But the middle ear makes it easier for sound to travel into the inner ear by matching the sound's "impedance," which means making the sound work better with the inner ear's fluids.

The middle ear has special parts that help increase how well sound travels. One part, the eardrum, is much larger than the part that touches the inner ear. This helps push the sound more strongly into the inner ear. Another part works like a lever, which also helps boost the sound. Together, these parts make the sound about 26 times stronger.

The middle ear can also protect the inner ear from very loud noises by tightening up when the sound gets too loud. This helps keep the inner ear safe.

Clinical significance

The middle ear has a hollow space called the tympanic cavity and a tube called the Eustachian tube. When you go up high or dive underwater, the pressure inside the middle ear can change. This pressure difference can hurt the eardrum if it isn’t relieved. The Eustachian tubes connect the middle ear to the back of the nose and help keep the pressure balanced. These tubes are usually closed to avoid getting filled with mucus, but you can open them by moving your jaw, which is why yawning or chewing can help when your ears feel full on an airplane. If the Eustachian tubes get blocked, fluid can build up in the middle ear and cause trouble hearing. Otitis media is when the middle ear becomes inflamed.

Injuries

The middle ear is safely hidden inside the body, so small outside injuries usually don’t affect it. However, it can be hurt by pressure changes, which is called barotrauma.

Infections

Scientists have found that a special kind of virus might affect the middle ear. They discovered pieces of this virus in samples from healthy middle ears, which means this area of the ear could sometimes be affected by the virus.

Diversity and evolution

The middle ear in animals that live on land, called tetrapods, shares similarities with a special opening in fish heads called the spiracle. In fish, this opening connects to the throat, but in land animals, it often becomes the eardrum. The part connected to the throat becomes the eustachian tube.

In reptiles, birds, and early land animals, there is usually one small bone called the columella that helps with hearing. This bone comes from a part of fish that supported the skull.

Amphibians, like frogs, often have a middle ear similar to reptiles, but some lack a full middle ear cavity. In these cases, the bone that helps with hearing connects directly to the skull.

Mammals, including humans, are special because they have three small bones in the middle ear: the malleus, incus, and stapes. These evolved from bones that were once part of the jaw in ancient animals. Over time, these bones changed to help hear higher sounds better. In many mammals, the middle ear is protected by a bony cavity called the auditory bulla, which is not found in other animals. Fossils show us steps in how these ear bones changed over millions of years.

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