The ear has the receptors for both hearing and equilibrium. For study purposes, it may be divided into three parts: the outer, middle, and inner ear (Fig. 18-1).
The outer ear consists of the projecting pinna (auricle) and the external auditory canal (meatus). This canal ends at the tympanic membrane or eardrum, which transmits sound waves to the middle ear. Glands in the external canal produce a waxy material, cerumen, which protects the ear and helps to prevent infection.
Spanning the middle ear cavity are three ossicles (small bones), each named for its shape: the malleus (hammer), incus (anvil), and stapes (stirrup). Sound waves traveling over the ossicles are transmitted from the footplate of the stapes to the inner ear. The eustachian tube connects the middle ear with the nasopharynx and serves to equalize pressure between the outer and middle ear.
The inner ear, because of its complex shape, is described as a labyrinth (Fig. 18-2). It consists of an outer bony framework containing a similarly shaped membranous channel. The entire labyrinth is filled with fluid.
The cochlea, shaped like the shell of a snail, has the specialized organ of Corti concerned with hearing. Cells in this receptor organ respond to sound waves traveling through the fluid-filled ducts of the cochlea. Sound waves enter the cochlea from the base of the stapes through an opening called the oval window and leave through another opening called the round window.
The sense of equilibrium is localized in the vestibular apparatus. This structure consists of the chamberlike vestibule and three projecting semicircular canals. Special cells within the vestibular apparatus respond to movement. (The senses of vision and proprioception are also important in maintaining balance.)
Nerve impulses are transmitted from the ear to the brain by way of the vestibulocochlear nerve, the eighth cranial nerve, also called the acoustic or auditory nerve. The cochlear branch of this nerve transmits impulses for hearing from the cochlea; the vestibular branch transmits impulses concerned with equilibrium from the vestibular apparatus.
Oval window Round window
FIGURE 18-2. The inner ear. (Reprinted with permission from Smeltzer SC, Bare BG. Brunner & Suddarth's Textbook of Medical-Surgical Nursing. 9th Ed. Philadelphia: Lippincott Williams & Wilkins, 2000.)
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