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 helps 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.

Two small muscles can make these bones move less when sounds are very loud. This helps protect the inner ear. The middle ear also has parts of the facial nerve that pass through it.

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.

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.

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 space called the tympanic cavity and a tube called the Eustachian tube. When you go up high or dive underwater, the pressure in the middle ear can change. This can hurt the eardrum if the pressure isn’t balanced. The Eustachian tubes connect to the back of the nose and help keep pressure even. These tubes are usually closed to keep out mucus, but you can open them by moving your jaw. That’s why yawning or chewing can help when your ears feel full on an airplane. If the Eustachian tubes get blocked, fluid can build up and make hearing hard. Otitis media is when the middle ear gets inflamed.

Injuries

The middle ear is protected inside the body, so small injuries from outside usually don’t harm it. But it can be hurt by sudden changes in pressure, which is called barotrauma.

Infections

Scientists have found that a special kind of virus might affect the middle ear. They found pieces of this virus in samples from healthy middle ears. This shows that the middle ear can sometimes be affected by the virus.

Diversity and evolution

The middle ear in land animals, called tetrapods, is similar to 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, 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 how these ear bones changed over millions of years.

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