(fig. 1-52). The malleus is attached to the inner surface of the eardrum and connects with the  incus, which in turn connects with the stapes. The base of the stapes is attached  to  the   fenestra  ovalis   (oval  window),  the membrane-covered  opening  of  the  inner  ear.  These tiny bones, which span the middle ear, are suspended from   bony   walls   by   ligaments.   This   arrangement provides the mechanical means for transmitting sound vibrations to the inner ear. The  eustachian tube, or auditory tube, connects the  middle  ear  with  the  pharynx.  It  is  lined  with  a mucous   membrane   and   is   about   36   mm   long.   Its function   is   to   equalize   internal   and   external   air pressure. For example, while riding an elevator in a tall building, you may experience a feeling of pressure in the   ear.   This   condition   is   usually   relieved   by swallowing,   which   opens   the   eustachian   tube   and allows the pressurized air to escape and equalize with the area of lower pressure. Divers who ascend too fast to allow pressure to adjust may experience rupture of their eardrums. The eustachian tube can also provide a pathway for infection of the middle ear. Inner Ear The   inner   ear   is   filled   with   a   fluid   called endolymph. Sound vibrations that cause the stapes to move against the oval window create internal ripples that  run  through  the  endolymph.  These  pressurized ripples   move   to   the   cochlea,   a   small   snail-shaped structure   housing   the   organ  of  Corti,   the   hearing organ (fig. 1-52). The cells protruding from the organ of Corti are stimulated by the ripples to convert these mechanical vibrations into nerve impulses, and these impulses  are  relayed  through  the  vestibulocochlear (8th cranial) nerve to the auditory area of the cortex in the   temporal   lobe   of   the   brain.   There   they   are interpreted as the sounds we hear. Another   structure   located   in   the   inner   ear   is composed of the  three semicircular canals, situated perpendicular   to   each   other.   Movement   of   the endolymph within the canals, caused by general body movements,  stimulates  nerve  endings,  which  report these changes in body position to the brain, which in turn uses the information to maintain equilibrium. The  fenestra rotunda  (round window) is another membrane-covered   opening   of   the   inner   ear.   It contracts  the  middle  ear  and  flexes  to  accommodate the inner ear ripples caused by the stapes. TOUCH Until   the   beginning   of   the   last   century,   touch (feeling) was treated as a single sense. Thus, warmth or coldness, pressure, and pain, were thought to be part of a single sense of touch or feeling. It was discovered that   different   types   of   nerve   ending   receptors   are widely   and   unevenly   distributed   in   the   skin   and mucous membranes. For example, the skin of the back possesses relatively few touch and pressure receptors while the fingertips have many. The skin of the face has relatively few cold receptors, and mucous membranes have  few  heat  receptors.  The  cornea  of  the  eye  is sensitive to pain, and when pain sensation is abolished by   a   local   anesthetic,   a   sensation   of   touch   can   be experienced. 1-48 HM3F0152 MALLEUS INCUS STAPES SEMICIRCULAR CANALS INNER EAR VESTIBULOCOCHLEAR NERVE COCHLEA OVAL WINDOW ROUND WINDOW TYMPANIC CAVITY MIDDLE EAR TYMPANIC MEMBRANE EXTERNAL AUDITORY CANAL OUTER EAR AURICLE PHARYNX Figure 1-52.—Major parts of the ear.