Nursing Entrance Exam

Page 120 of 175 nasopharynx (the superior portion of the pharynx) through the auditory tube. About 4 cm (1.6 in.) long, the auditory tube consists of two portions. The portion near the middle ear is fairly narrow and elastic cartilage supports it. The portion near the opening into the nasopharynx is relatively broad and funnel-shaped. The auditory tube permits the equalization of pressures on either side of the tympanic membrane. The middle ear contains three miniscule ear bones, collectively known as the auditory ossicles. The ear bones connect the tympanic membrane with one of the receptor complexes of the inner ear. The three auditory ossicles bones are the malleus, the incus, and the stapes. The malleus (hammer) attaches at three points to the interior surface of the tympanic membrane. The incus (anvil), which is the middle ossicle, attaches the malleus to the stapes (stirrup), or the inner ossicle. When the tympanic membrane vibrates, it converts the arriving soundwaves into mechanical movements. The auditory ossicles move like levers conducting the vibrations to the inner ear. The ossicles are connected so that any in-out movement of the tympanic membrane produces a rocking motion of the stapes. Thus, the ossicles function as a lever system, collecting the force applied to the tympanic membrane and focusing it on the oval window. The Inner Ear A layer of dense bone known as the bony labyrinth establishes the superficial contours of the inner ear. The walls of this labyrinth are continuous with the surrounding temporal bone. The inner contours of the bony labyrinth closely follow the contours of the membranous labyrinth, which is a delicate, interconnected network of fluid-filled tubes. The inner ear receptors are found within these tubes. Between the bony and membranous labyrinths flows perilymph, a liquid whose properties closely resemble those of cerebrospinal fluid. Themembranous labyrinth contains endolymph, a fluid with electrolyte concentrations different from those of typical body fluids. The bony labyrinth can be subdivided into the cochlea, vestibule, and three semicircular canals. The vestibule is a pair of membranous sacs: the saccule and the utricle, or sacculus and utriculus, respectively. Receptors in the utricle and saccule deliver sensations of gravity, as well as linear acceleration. The cochlea, which resembles a snail shell, is a spiral-shaped, bony chamber that contains the cochlear duct of the membranous labyrinth. Receptors within the cochlear duct provide the sense of hearing. The duct is sandwiched between a pair of perilymph- filled chambers. The walls of the bony labyrinth are comprised of dense bone everywhere except at two small areas located near the base of the cochlear spiral. The round window is a thin, membranous partition that separates the perilymph of the cochlear chambers from the air in the middle ear. The bony margins of the opening – known as the oval window – are connected to the base of the stapes by collagen fibers.

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