Anatomy of the external and middle ear

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Introduction0:00–0:32

The ear is made up of three parts: the external, middle, and inner ear. Together, these structures help process air vibrations as sound, and this helps us enjoy our favourite music, as well as alert us of potential threats.
Now, while the inner ear also plays a role in balance, the main role of the external and middle ear is to transfer and amplify sound to the inner ear with the help of the three smallest bones in the body: the auditory ossicles.
Let’s start with the external ear, which is by far the most common anatomical spot to hang earrings from. The external ear is actually a complex structure made of the auricle, also called the pinna; and the external acoustic meatus.

External ear - auricle0:32–1:27

Now, let’s take a closer look at the auricle, which is the part of the external ear that surrounds the opening of the external acoustic meatus, and funnels soundwaves towards the external acoustic meatus.
It is made of irregularly shaped cartilage covered with skin. The elevated outer rim of the auricle is the helix.
The smaller elevated rim parallel to the helix is called the antihelix. The depression in the middle of the auricle is called the concha.
Right next to the concha, there’s the opening of the external acoustic meatus. Inferior to the concha, there’s the lobule of the auricle.
And finally, anterior to the opening, there’s an elevation called the tragus. The next part of the external ear is the external acoustic meatus, which is an S-shaped canal that extends from the concha, through the temporal bone, to the tympanic membrane.

External ear - external acoustic meatus1:27–1:47

The entire canal is covered with skin, and is divided into a lateral third, made of cartilage, and a medial two-thirds, made of bone.
Now, at the end of the external acoustic meatus, there’s the thin, oval tympanic membrane, more commonly known as the eardrum, which separates the external ear from the middle ear.

External ear - tympanic membrane1:47–2:59

When sound waves hit the tympanic membrane, it vibrates and transfers the vibration to the three auditory ossicles of the middle ear.
If you take a look through an otoscope, you will notice the tympanic membrane is concave externally, with the center of the concavity called the umbo.
One of the ossicles, called the malleus, attaches to the internal surface of the tympanic membrane. The handle of the malleus extends superiorly from the umbo.
At its end, there’s the lateral process of the malleus which can be seen as a small lump. Two lines extend from the lump: the posterior, and the anterior malleolar folds.
The thin membrane superior to these lines is called pars flaccida as this portion is more flaccid, while the rest of the tympanic membrane is called pars tensa due to this portion of the membrane being thicker and more tense.
During examination with an otoscope, the light from the otoscope reflects off the tympanic membrane and creates a cone of light that extends anteriorly and inferiorly from the umbo.
Now, blood supply for the external ear and the tympanic membrane comes from direct and indirect branches of the external carotid artery.

External ear - arteries, nerves, lymphatics2:59–3:53

The great auricular and auriculotemporal nerves supply the majority of the skin of the auricle. The skin in front of the external acoustic meatus is innervated by the auriculotemporal nerve, while the external acoustic meatus is innervated by branches of the auriculotemporal nerve, the vagus nerve, and the facial nerve.
Most of the external surface of the tympanic membrane is innervated by the auriculotemporal nerve with some contributions from the vagus and facial nerves, while the glossopharyngeal nerve supplies the internal surface of the tympanic membrane.
Finally, lymph of the external ear and the tympanic membrane drains into the superficial parotid lymph nodes, mastoid lymph nodes, deep cervical lymph nodes, and superficial cervical lymph nodes.All right, before we move on, can you recall all of the features of the tympanic membrane on this image?Ok, now, let’s switch gears and look at the middle ear, which is also known as the tympanic cavity.

Quiz3:53–4:04

Middle ear4:04–4:34

The middle ear is a hollow chamber found in the petrous part of the temporal bone, lined with a mucous membrane that houses the auditory ossicles.
It is made of the tympanic cavity proper, which is the space immediately next to the tympanic membrane, and the epitympanic recess, which is an extension of the tympanic cavity superior to the tympanic membrane.The middle ear has six sides, so it is essentially shaped like a narrow box.

Middle ear - walls4:34–5:41

The roof is called the tegmental wall, and it separates the middle ear from the middle cranial fossa. The floor is called the jugular wall, because it separates the middle ear from the internal jugular vein.
The lateral wall is called the membranous wall, and it’s made up of the tympanic membrane and the lateral bony wall of the epitympanic recess.
The medial wall, or the labyrinthine wall, separates the middle ear and the inner ear. Its most important features are the promontory, the oval window, and the round window.
The posterior wall is called the mastoid wall, and it features a pyramidal eminence and an opening in its superior aspect called the aditus to the mastoid antrum, which connects the tympanic cavity to the mastoid air cells.
Finally, the anterior wall is called the carotid wall because it separates the middle ear from the carotid canal. It also features the canal of the tensor tympani muscle superiorly and the opening of the pharyngotympanic tube that creates a connection between the tympanic cavity and the nasopharynx.The pharyngotympanic tube, also called the auditory tube, extends from the opening on the anterior wall to its opening in the nasopharynx, posterior to the inferior meatus of the nasal cavity.

Middle ear - auditory tube5:41–7:12

Its posterolateral aspect is bony, while the rest is cartilaginous and it is lined with a mucous membrane. The pharyngotympanic tube equalizes the pressure in the middle ear with the atmospheric pressure.
This is an important function, as equal pressure on both sides of the tympanic membrane allows it to move freely. Now, the cartilaginous walls of the tube are normally pressed up against the other, and the tube is closed.
For the tube to work, it has to be opened, and two muscles of the soft palate help with this, namely the levator and the tensor veli palatini.
The levator veli palatini muscle extends along the tube. When it contracts it pushes on the wall of the tube, while the tensor veli palatini muscle attaches to the wall of the tube on the other side and pulls in the opposing direction.
Here’s a real life example to illustrate this. Do you know when your ears get clogged on an airplane?
Well, that’s because the atmospheric pressure changes, and it’s no longer equal to that inside the middle ear. To relieve that pesky feeling, we often swallow or chew gum - this makes the muscles of the soft palate open the pharyngotympanic tube, equalizing the pressure on both sides of the tympanic membrane, and making our ears “pop”!
Finally, let’s go into some detail about the three very small, but very important auditory ossicles, which are housed within the middle ear and named after the shape they resemble.

Middle ear - auditory ossicles7:12–7:55

The malleus, which is Latin for the hammer, is connected to the tympanic membrane on one side and the incus on the other side.
The incus, or the anvil, is found in the middle of the three ossicles, attached to the innermost bone, called the stapes, on its medial aspect.
The stapes resembles a stirrup, and its base attaches to the oval window. These tiny bones form a chain that extends from the tympanic membrane to the oval window on the labyrinthine wall and transfers vibrations from the tympanic membrane to the inner ear.Now, air vibrations made by loud sounds could easily damage the inner ear.

Middle ear - muscles7:55–8:42

That’s why there are two muscles that dampen the movement of the auditory ossicles: the tensor tympani and the stapedius.
The tensor tympani muscle, which is innervated by the mandibular nerve, extends from the pharyngotympanic tube to the malleus.
It pulls on the malleus and tenses the tympanic membrane to reduce the amplitude of its oscillations. The stapedius is a small muscle inside the pyramidal eminence on the posterior wall of the middle ear.
Its tendon inserts on the stapes and pulls it posteriorly also reducing the amplitude of oscillations transmitted to the oval window and movement of the stapes.
The stapedius is innervated by a branch of the facial nerve.Speaking of nerves, there are also some nerves that can be found in, or traveling through the middle ear.

Middle ear - Nerves8:42–9:28

The tympanic nerve, which is a branch of the glossopharyngeal nerve, contributes to the tympanic plexus on the promontory of the medial wall.
This plexus innervates the mucous membrane that lines the middle ear and the pharyngotympanic tube. A branch of the tympanic plexus called the lesser petrosal nerve exits the middle ear to provide parasympathetic innervation to the parotid gland.
Another important nerve is the chorda tympani nerve, a branch of the facial nerve. It enters the middle ear through the posterior wall, passes near the upper part of the tympanic membrane between the malleus and the incus, and exits through the anterior wall to travel towards the infratemporal fossa.All right, as a quick recap.

Review9:28–10:53

The external ear is made of the auricle and the external acoustic meatus. The auricle is made up by the helix, the antihelix, the concha, and the tragus.
The external acoustic meatus is an S-shaped canal that extends from the concha, through the temporal bone, to the tympanic membrane, which separates the external ear from the middle ear.
The middle ear, also known as the tympanic cavity, is a hollow chamber found in the petrous part of the temporal bone. The middle ear has six sides: the tegmental wall or the roof, the jugular wall or the floor, the membranous lateral wall, the labyrinthine medial wall, the mastoid posterior wall, and the carotid anterior wall.
The pharyngotympanic tube, also called the auditory tube, extends from an opening on the anterior wall to its opening in the nasopharynx.
The three auditory ossicles; the malleus, incus and stapes, are also housed within the tympanic cavity. These ossicles create a chain that connects the tympanic membrane to the oval window to transfer and amplify air vibrations into the inner ear to be processed as sound.
The two muscles that dampen the movement of the auditory ossicles are the tensor tympani and the stapedius.