pulse  to  the  incus  (c),  thence  to  the  stapes  (d)  in  contact  with  the
membrane  of  the  fenestra  ovalis  (e).  The  contraction  and  tension
of  the  membrana  tympani,  in  regulating  or  responding  to  sound,
may  be  compared  with  the  regulating  power  of  the  iris  of  the
eye.  The  intensity  of  sound  produces  a  reflex  action,  through  the
nervous  system,  to  the  tensor  tympani  muscle  (s),  which  then  pro-


THE   EAR    AND   THE   SENSE    OF    HEARING.  203

duces  more  or  less  tension  of  the  membrana  tympani  (a).  The
reflex  by  which  the  impressions  are  regulated  is  effected  by  a  nerve
from  the  trigeminus,  through  the  otic  ganglion.  When  the  tensor
tympani  muscle  contracts,  the  handle  of  the  malleus  is  drawn
inward,  producing  more  or  less  tension  in  the  membrana  tympani;
but  when  it  relaxes,  the  membrane,  by  its  elasticity,  returns  to  its
position  of  inactivity.  This  variation  of  tension  is  its  accommodat-
ing power  to  receive  and  transmit  modified  sound  impulses,  such  as
those  of  pitch,  loudness,  and  quality.  The  membrana  tympani  is
also  the  gate  guard,  permitting  nothing  to  pass  into  the  middle  ear
that  would  disturb  the  delicate  mechanical  action  of  the  three  little
bones.  The  construction  of  both  the  external  and  middle  ear  is
evidently  designed  for  the  propagation  of  sound,  and  its  multipli-
cation by  resonance.
The  Middle  Ear,  or  Tympanum  (B,  3,  Fig.  59),  is  a  cavity,
or  space,  between  the  membrana  tympani  (a)  and  internal  ear  (5),
filled  with  air  through  the  nasal  cavity  and  Eustachian  tube  (4).
