Encased within the dense temporal bone of the skull is an organ called the cochlea, about the size of a pea and shaped like the shell of a snail. Together with the vestibular apparatus (previously described), it composes the inner ear.
Vibrations of the stapes and oval window displace peri-lymph fluid within a part of the bony labyrinth known as the scala vestibuli, which is the upper of three chambers within the cochlea. The lower of the three chambers is also a part of the bony labyrinth and is known as the scala tympani. The middle chamber of the cochlea is a part of the membranous labyrinth called the cochlear duct, or scala media. Like the cochlea as a whole, the cochlear duct coils to form three turns (fig. 10.19), similar to the basal, middle, and apical portions of a snail shell. Since the cochlear duct is a part of the membranous labyrinth, it contains endolymph rather than perilymph.
The perilymph of the scala vestibuli and scala tympani is continuous at the apex of the cochlea because the cochlear duct ends blindly, leaving a small space called the helicotrema between the end of the cochlear duct and the wall of the cochlea. Vibrations of the oval window produced by movements of the stapes cause pressure waves within the scala vestibuli, which pass to the scala tympani. Movements of perilymph within the scala tympani, in turn, travel to the base of the cochlea where they cause displacement of a membrane called the round window into the middle-ear cavity (see fig. 10.18). This occurs because fluid, such as perilymph, cannot be compressed; an inward movement of the oval window is thus compensated for by an outward movement of the round window.
When the sound frequency (pitch) is sufficiently low, there is adequate time for the pressure waves of perilymph within the upper scala vestibuli to travel through the helicotrema to the scala tympani. As the sound frequency increases, however, pressure waves of perilymph within the scala vestibuli do not have time to travel all the way to the apex of the cochlea. Instead, they are transmitted through the vestibular membrane, which separates the scala vestibuli from the cochlear duct, and through the basilar membrane, which separates the cochlear duct from the scala tympani, to the perilymph of the scala tympani (fig. 10.19).
Damage to the tympanic membrane or middle-ear ossicles produces conduction deafness. This impairment can result from a variety of causes, including otitis media and otosclerosis. In otitis media, which sometimes follows allergic reactions or respiratory disease, inflammation produces an excessive accumulation of fluid within the middle ear. This, in turn, can result in the excessive growth of epithelial tissue and damage to the eardrum. In otosclerosis, bone is resorbed and replaced by "sclerotic bone" that grows over the oval window and immobilizes the footplate of the stapes. In conduction deafness, these pathological changes hinder the transmission of sound waves from the air to the cochlea of the inner ear.
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