(fig. 1-52). The malleus is attached to the inner surfaceof the eardrum and connects with the incus, which inturn connects with the stapes. The base of the stapes isattached to the fenestra ovalis (oval window), themembrane-covered opening of the inner ear. Thesetiny bones, which span the middle ear, are suspendedfrom bony walls by ligaments. This arrangementprovides the mechanical means for transmitting soundvibrations to the inner ear.The eustachian tube, or auditory tube, connectsthe middle ear with the pharynx. It is lined with amucous membrane and is about 36 mm long. Itsfunction is to equalize internal and external airpressure. For example, while riding an elevator in a tallbuilding, you may experience a feeling of pressure inthe ear. This condition is usually relieved byswallowing, which opens the eustachian tube andallows the pressurized air to escape and equalize withthe area of lower pressure. Divers who ascend too fastto allow pressure to adjust may experience rupture oftheir eardrums. The eustachian tube can also provide apathway for infection of the middle ear.Inner EarThe inner ear is filled with a fluid calledendolymph. Sound vibrations that cause the stapes tomove against the oval window create internal ripplesthat run through the endolymph. These pressurizedripples move to the cochlea, a small snail-shapedstructure housing the organ of Corti, the hearingorgan (fig. 1-52). The cells protruding from the organof Corti are stimulated by the ripples to convert thesemechanical vibrations into nerve impulses, and theseimpulses are relayed through the vestibulocochlear(8th cranial) nerve to the auditory area of the cortex inthe temporal lobe of the brain. There they areinterpreted as the sounds we hear.Another structure located in the inner ear iscomposed of the three semicircular canals, situatedperpendicular to each other. Movement of theendolymph within the canals, caused by general bodymovements, stimulates nerve endings, which reportthese changes in body position to the brain, which inturn uses the information to maintain equilibrium.The fenestra rotunda (round window) is anothermembrane-covered opening of the inner ear. Itcontracts the middle ear and flexes to accommodatethe inner ear ripples caused by the stapes.TOUCHUntil the beginning of the last century, touch(feeling) was treated as a single sense. Thus, warmth orcoldness, pressure, and pain, were thought to be part ofa single sense of touch or feeling. It was discoveredthat different types of nerve ending receptors arewidely and unevenly distributed in the skin andmucous membranes. For example, the skin of the backpossesses relatively few touch and pressure receptorswhile the fingertips have many. The skin of the face hasrelatively few cold receptors, and mucous membraneshave few heat receptors. The cornea of the eye issensitive to pain, and when pain sensation is abolishedby a local anesthetic, a sensation of touch can beexperienced.1-48HM3F0152MALLEUSINCUSSTAPESSEMICIRCULARCANALSINNEREARVESTIBULOCOCHLEARNERVECOCHLEAOVALWINDOWROUNDWINDOWTYMPANICCAVITYMIDDLEEARTYMPANICMEMBRANEEXTERNALAUDITORYCANALOUTEREARAURICLEPHARYNXFigure 1-52.—Major parts of the ear.
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