Middle  Ear The  middle  ear  is  a  cavity  in  the  temporal bone,  lined  with  epitheliums.  It  contains  three auditory ossicles —the malleus (hammer), the in- cus   (anvil),   and   the   stapes   (stirrup)—which transmit  vibrations  from  the  tympanic  membrane to  the  fluid  in  the  inner  ear.  The  malleus  is  at- tached  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   oval   window   (fenestra   ovalis),   the membrane-covered   opening   of   the   inner   ear. These tiny bones link together to span the mid- dle  ear.  They  are  suspended  from  its  bony  wall by ligaments and provide the mechanical means for transmission of sound vibrations to the inner ear. The eustachian tube connects the middle ear with the pharynx. It is lined with a mucous mem- brane  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 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 be  a  pathway  for  infection  of  the  middle  ear. Inner Ear The inner ear is filled with a fluid called en- dolymph. 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. The cells protruding from the organ  of  Corti  are  stimulated  by  the  ripples  to convert  these  mechanical  vibrations  into  nerve  im- pulses, which are relayed through the cochlear (8th cranial) nerve to the auditory area of the cortex in the temporal lobe of the brain. Here they are interpreted as the sounds we hear. Other structures of the inner ear are 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   round   window   (fenestra   rotunda)   is another  membranecovered  opening  of  the  inner ear. It contracts the middle ear and flexes to ac- commodate  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 then discovered that different types of  nerve  ending  receptors  are  widely,  but  un- evenly, distributed in the skin and mucous mem- branes.  For  example,    the   skin   of   the   back possesses  relatively  few  touch  and  pressure  recep- tors while the fingertips have a great many. The skin of the face has relatively few cold receptors, and the mucous membranes have few heat recep- tors.  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. There  are  five  kinds  of  receptors.  The  most important, those for the sense of touch, are bare nerve  endings  next  to  hairs  and  specialized  encap- sulated nerve endings called Meissner’s corpuscles. Cold   receptors   also   have   encapsulated   nerve endings. The   receptors   for   pain   are   naked   nerve filaments  and  are  the  most  numerous;  they  are also the only kind present in the deeper tissues, although stimulation of these usually causes the pain to be referred to a skin area. Three kinds of pain  may  be  experienced:  superficial  or  cutaneous pain;  deep  pain  from  muscles,  tendons,  joints, and  fascia;  and  visceral  pain. OTHER  SENSES Certain nerve receptors, located in muscles and tendons, are stimulated by changes in tension and pressure  and  continually  inform  the  brain  regard- ing the position of parts of the body (body sense). Hunger results from rhythmic contractions of the  stomach  when  it  has  emptied  its  contents. Blood  sugar  levels  also  influence  the  feeling  of hunger. Habit is another factor; for example, per- sons who habitually snack in midmorning will feel hunger contractions at normal snacktime. If the snack is not eaten for several days, and adequate food  intake  continues  at  mealtime,  the  hunger contractions at snacktime will diminish. The ner- vous   system   also   plays   a   part   in   controlling 3-39