The middle ear

Remit

This short tutorial takes you through the basic knowledge required to understand and answer questions about the middle ear.

Anatomy

The middle ear is a space within the temporal bone that contains the ossicles, facial nerve, corda tympani and air.

It communicates with the nasopharynx via the Eustachian tube and with the mastoid air cells within the mastoid bone via the aditus.

It has three main regions and these are defined by their relationship with the eardrum. The area above the lateral process of the malleus is called the epitympanum, below the eardrum the hypotympanum and level with the drum the mesotympanum. The diagram below illustrates this and shows the other important structures together with short notes.

Attic or Epitympanum, above the level of the lateral process of the malleus
Meso - mesotympanum, medial to drum
Hypo - hypotympanum, below level of drum. It is small in this case but is often large and has 'stalagmites' of bone growing up from it.
VII - Facial Nerve. Its presence here makes it vulnerable in ear diseases.
Malleus. Notice that the majority of the malleus lies above the level of the drum in the epitympanum. It is only the handle of the malleus that is easily visible on otoscopy. This is the part attached to the fibrous layer of the ear drum.
I -Incus. As with the malleus, the body of the incus lies above the level of the drum and cannot be seen on otoscopy. One can often see the long process of the incus (LPI) through the drum. The incus lies posterior to the malleus.
Stapes - this is the smallest bone in the body. Its head can sometimes be seen through the eardrum.

Anatomic Relations

You should not get bogged down with the complex anatomy of the region but the following would be regarded as essential information. Most of the relationships on this list are common sense when you think about your own ear.

Superior

Meninges of middle cranial fossa
Temporal lobe of brain

Inferior

Sternomastoid muscle
Jugular vein

Medial

Cochlea
Semicircular canals
Facial nerve

Posterior

Mastoid
Meninges of posterior cranial fossa
Cerebellum
Sigmoid sinus

There are anterior relations as well but are seldom affected by infective disease.

Physiology

So why do we have a middle ear? The easy answer is that it is there to amplify sound. And this is true, it does amplify it about twenty times.

However, there is another reason. To understand the reason consider what happens when you are in a swimming pool. Let us imagine that your friend is calling to you from the side of the pool but that you are under water. Do you hear him clearly? The answer is no, but the question is, why not? It is because most of the sound is reflected from the surface of the water and only a small portion causes vibration in the water. Thus he sounds very quiet.

Now think about the ear again. We are trying to hear sound waves that are in air but our organ of hearing (cochlea) is a fluid filled organ. Without a middle ear mechanism most of the sound vibrations would be reflected off the fluid surface of the inner ear. However, with a middle ear mechanism we are able to focus all of the sound energy, amplify it and 'funnel' it into the inner ear. In technical jargon it is a mechanism that minimises the impedance mismatch for sound in air and fluid.

Without a middle ear mechanism the world would be a lot quieter.