Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sense Organs. Analyzers
Organs of Hearing and Balance (Vestibulocochlear Organ)
Organ of Hearing

The Organ of Hearing perceives sound signals and consists of three parts: the outer, middle, and Inner ear. The middle and inner ears are located within the pyramid of the Temporal bone, while the outer ear lies outside of it (see Fig. 108).

The outer ear comprises the auricle (pinna) and the external acoustic meatus. The auricle captures sounds and directs them into the external acoustic meatus. It is constructed of elastic Cartilage covered with Skin. The external acoustic meatus is a curved tube, cartilaginous on the outside and bony in the deep part. In an adult, the length of the external acoustic meatus is about 35 mm, with a lumen diameter of 6–9 mm. The skin of the meatus is covered with fine, sparse hairs. The ducts of glands that produce a specific secretion—earwax (cerumen)—open into the lumen of the meatus. Both the hairs and earwax perform a protective function, shielding the acoustic meatus from dust, insects, and microorganisms.

Deep within the external acoustic meatus, at its boundary with the Middle ear, lies the thin, elastic tympanic membrane (eardrum), covered externally by thinned skin. Internally, facing the tympanic cavity of the middle ear, the tympanic membrane is lined with a mucous membrane. When acted upon by sound waves, the tympanic membrane vibrates; its vibrational movements are transmitted to the auditory ossicles of the middle ear, and through them to the inner ear, where these vibrations are perceived by the corresponding receptors.

The middle ear is situated inside the petrous part of the temporal bone, within its pyramid. It consists of the tympanic cavity and the auditory (Eustachian) tube, which connects this cavity to the Pharynx.

The tympanic cavity lies between the external acoustic meatus (tympanic membrane) and the inner ear. In shape, the tympanic cavity is a mucous membrane-lined slit, often compared to a drum resting on its edge. Housed within the tympanic cavity are three mobile, miniature auditory ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). The malleus is fused with the tympanic membrane and is movably connected to the incus. The stapes is movably articulated with the incus and the oval window, which separates the tympanic cavity from the Vestibule of the inner ear. The auditory ossicles are joined to one another by movable joints. Vibrations of the tympanic membrane are transmitted via the malleus to the incus, and from the incus to the stapes, which, acting through the oval window, sets the fluid in the cavities of the inner ear into motion (Fig. 109).

The tension of the tympanic membrane and the pressure exerted by the stapes on the oval window in the medial wall of the tympanic cavity are regulated by two small Muscles, one of which attaches to the malleus and the other to the stapes.

The auditory (Eustachian) tube connects the tympanic cavity to the pharynx. Internally, the auditory tube is lined with a mucous membrane. The auditory tube is 35 mm long and 2 mm wide. Its significance is immense: the air entering the tympanic cavity from the pharynx via the tube equalizes the air pressure on the tympanic membrane from the side of the external acoustic meatus. For example, during aircraft takeoff or descent, air pressure on the eardrum changes abruptly, which manifests as a sensation of "ears popping" or fullness. Swallowing movements, which stretch the auditory tube and allow air to enter the middle ear more actively, relieve these unpleasant sensations.

The inner ear is located in the pyramid of the temporal bone between the tympanic cavity and the internal acoustic meatus. It houses the sound-perceiving apparatus and the vestibular apparatus. The inner ear comprises the osseous labyrinth—a system of bony cavities—and the membranous labyrinth, which is situated within the osseous cavities and replicates their shape. The walls of the ducts of the membranous labyrinth are formed of Connective Tissue. The interior of the membranous labyrinth ducts (cavities) contains a fluid called endolymph. The fluid that bathes the membranous labyrinth externally, occupying the narrow space between the walls of the osseous and membranous labyrinths, is called perilymph. Both the osseous labyrinth and the membranous labyrinth enclosed within it are divided into three sections: the cochlea, the semicircular canals, and the vestibule. The cochlea belongs exclusively to the sound-perceiving apparatus (The Organ of hearing). The semicircular canals are part only of the vestibular apparatus. The vestibule, located between the cochlea anteriorly and the semicircular canals posteriorly, relates to both the organ of hearing and the Organ of Balance, with which it is anatomically connected.

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Fig. 109. Propagation of a sound wave in the outer, middle, and inner ear (indicated by arrows):

1 — tympanic membrane; 2 — malleus; 3 — incus; 4 — stapes; 5 — round window; 6 — tympanic scala; 7 — cochlear duct; 8 — vestibular scala

The osseous vestibule, which forms the middle part of the inner ear labyrinth, features two openings, or windows, in its lateral wall: the oval window and the round window. Both windows connect the osseous vestibule with the tympanic cavity of the middle ear. The oval window is closed by the Base of the stapes, while the round window is sealed by a mobile, elastic connective tissue plate—the secondary tympanic membrane.

The cochlea, which houses the sound-perceiving apparatus, resembles a snail shell in shape. It is a spirally curved bony canal forming 2.5 turns around its axis. The base of the cochlea faces the internal acoustic meatus. Running inside the curved bony canal of the cochlea is the membranous cochlear duct, which also forms 2.5 turns and contains endolymph. The cochlear duct has three walls. The outer wall is bony and also serves as the outer wall of the osseous cochlear canal. The other two walls are formed by connective tissue plates known as membranes. These two membranes extend from the middle of the cochlea—from its bony axis (modiolus)—to the outer wall of the osseous canal, dividing it into three narrow, spirally curved channels: the upper, middle, and lower. The middle channel is the cochlear duct, the upper is called the scala vestibuli, and the lower is the scala tympani. Both the scala vestibuli and the scala tympani are filled with perilymph. The scala vestibuli originates near the oval window, then spirals up to the apex of the cochlea, where it communicates with the scala tympani through an opening (the helicotrema). The scala tympani also spirals downward and ends at the round window, which is sealed by the elastic secondary tympanic membrane.

Inside the endolymph-filled cochlear duct, on its basilar membrane bordering the scala tympani, lies the sound-perceiving apparatus—the spiral (Corti's) organ. Corti's organ consists of 3–4 rows of receptor Cells, the total number of which reaches 24,000. Each receptor Cell bears 30 to 120 fine hairs, or microvilli, which end freely in the endolymph. Positioned above the Hair cells along the entire length of the cochlear duct is the movable tectorial membrane, whose free margin faces the interior of the duct, while its other margin is attached to the basilar membrane.