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Ear

Ear

The ear is the organ of hearing and balance. It's divided into three parts: the external, middle, and internal ears. The external and middle ears are mainly involved in sound transfer to the internal ear, which contains the organ of hearing and equilibrium. The tympanic membrane (eardrum) separates the external ear from the middle ear.


External Ear

The external ear consists of two parts. The part projecting to the lateral aspect of the head is the auricle or pinna, and the canal leading inward is the external acoustic meatus. 

The auricle is an irregular-shaped plate of elastic cartilage with a series of various elevations and depressions, and a surface covered by skin. Its fleshy lobule consists of fibrous tissue, fat, and blood vessels, and is the only part of the auricle that is not supported by cartilage. The auricle's main function is to assist in sound capture. 

The external acoustic meatus is an S-shaped ear canal that extends from the auricle to the tympanic membrane, in a distance of 2-3 cm in adults. The lateral one-third of this canal is formed by cartilaginous extensions from some of the auricular cartilages. The medial two-thirds is a bony tunnel in the temporal bone that is continuous with the external layer of the tympanic membrane. The external acoustic meatus contains hair and modified sweat glands producing cerumen (earwax). Cerumen is a yellowish waxy substance that assists in cleaning, lubrication and protection from foreign bodies. The tympanic membrane is a thin oval connective tissue membrane that separates the external acoustic meatus from the middle ear. On the outside, it is lined with skin, while internally by a mucous membrane. The tympanic membrane moves in response to air vibrations from the external environment. These movements are transmitted via the auditory ossicles present in the middle ear to the internal ear.

Middle Ear

The middle ear is an air-filled, mucous-lined chamber that lies in the petrous part of the temporal bone, between the tympanic membrane laterally and the lateral wall of the internal ear medially. It consists of two parts: the tympanic cavity and the epitympanic recess. 

The tympanic cavity is the area immediately adjacent to the tympanic membrane, while the epitympanic recess is the area superior to it. The middle ear connects with the mastoid cells (via the mastoid antrum) posteriorly, and with the nasopharynx (via the pharyngotympanic tube) anteriorly. The middle ear contains various structures. The auditory ossicles are a mobile osseous chain that consists of the malleus, incus, and stapes. Their function is to transmit vibrations of the tympanic membrane across the middle ear cavity, to the internal ear. The stapedius and tensor tympani are the two muscles of the middle ear. They are responsible for the dampening of both the auditory ossicles and tympanic membrane vibrations. The other contents of the middle ear include the chorda tympani nerve (branch of facial nerve CN VII), and the tympanic plexus of nerves.

Internal Ear

The internal ear consists of the bony labyrinth, a series of bony cavities, and the membranous labyrinth formed by membranous ducts and sacs. These structures are buried in the petrous part of the temporal bone, between the middle ear laterally and the internal acoustic meatus medially. The bony labyrinth is comprised of the vestibule, three semicircular ducts and the cochlea. These cavities are lined with periosteum and contain a clear fluid called the perilymph. The membranous labyrinth is suspended within the perilymph by delicate filaments and substantial spiral ligament. It consists of the semicircular ducts, cochlear duct, and two sacs: the utricle and saccule. These membranous spaces contain the endolymph. The cochlear duct is the organ of hearing, while the semicircular ducts, utricle and saccule are the organs of equilibrium.

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