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Anatomy of the external and middle ear

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Anatomy of the external and middle ear

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Anatomy of the External and Middle Ear

Figure 1. A. Anterior view of coronal section of the subdivisions of the ear. B. Lateral view of the auricle (external ear). 

Figure 2. A. Anterior view of coronal section highlighting features of external ear. B. Otoscopic view of right tympanic membrane.

Figure 3. A. Coronal section of the middle ear. B. Sagittal section showing pharyngotympanic tube opening in nasopharynx. C. Lateral view of the medial wall of the middle ear showing the tympanic plexus.

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A 39-year-old man is diagnosed with Bell palsy and has paralysis of the stapedius muscle. Which of the following is true regarding the stapedius muscle in the middle ear?  

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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.

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. 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. 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. 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. 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. 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.