++
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.
++
++
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.
++
++
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 potassium
and ...