Chapter 369

### Anatomy

The ear is divided into the external, middle, and inner ear compartments (Fig. 369-1). The external ear consists of the auricle and the external auditory canal. The primary function of the auricle is to channel sound energy toward the middle ear conducting apparatus. The lateral opening of the external auditory canal is the external meatus, which is bordered medially by the tympanic membrane. The lateral one third of the external canal is cartilaginous, and the medial two thirds are bony. The canal is lined by skin that possesses cerumen glands and other adnexal structures (hair follicles, sebaceous glands) in its lateral half.

###### Figure 369-1.

Anatomy of the ear: external, middle, and inner components.

The normal tympanic membrane (Fig. 369-2) seals the opening between the external auditory canal and the middle and inner ear. The portion of the tympanic membrane inferior to the short process of the malleus (pars tensa) is a three-layered structure composed of a medial mucosal epithelium continuous with the middle ear mucosa, a middle fibrous tissue layer, and, finally, a lateral surface of squamous epithelium continuous with the external ear canal skin. The region of the tympanic membrane superior to the short process of the malleus (pars flaccida) does not have a middle fibrous layer, which is clinically significant because it allows the development of retraction pockets and acquired cholesteatomas.

###### Figure 369-2.

Normal tympanic membrane anatomy and landmarks.

(Source: Knoop KJ, Stack LB, Storrow AB, Thurman RJ. The Atlas of Emergency Medicine. 3rd ed. New York: McGraw-Hill; 2010. Photo contributor: Richard A. Chole, MD, PhD.)

The middle ear compartment is an aerated cavity that houses the three ossicles: malleus, incus, and stapes. The function of the ossicles is to efficiently transmit sound energy to the inner ear. The middle ear is connected to the nasopharynx anterosuperiorly via the eustachian tube. Posterior and superiorly, the middle ear cavity is connected to the mastoid air cell system by means of the mastoid antrum. These connections provide a pathway for extension of middle ear infection into the mastoid. This can cause coalescent mastoiditis.

The inner ear is divided into an auditory portion (the cochlea), a vestibular portion (three semicircular canals, the utricle, and the saccule), and the endolymphatic apparatus (the endolymphatic duct and sac). The cochlea is a coiled structure that houses the machinery responsible for transducing sound energy into neural impulses. The actual transducers are hair cells, which are precisely arranged in the organ of Corti. The organ of Corti, in turn, rests on the basilar membrane, which resonates in response to the incoming acoustic stimuli. The cochlea maintains a very specific fluid balance. The endolymph has a composition similar to intracellular fluid (ie, high ...

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