lake  in  the  moonlight,  we  get  the  perception  of  a  "shiny"  or
brilliant  surface.  The  moonlight  is  reflected  from  the  waves
to  the  eyes  in  a  number  of  bright  points:  these  are  not  ex-
actly the  same  for  both  eyes,  since  the  lines  of  light-reflection
from  the  surface  of  the  water  to  each  are  different.  The
perception  of  brilliancy  seems  largely  to  depend  on  this
slight  non-agreement  of  the  light  and  dark  points  on  the  two
retinas.  A  rapid  change  of  luminous  points,  to  and  fro  be-
tween neighboring  points  on  one  retina,  seems  also  to  pro-
duce it.
CHAPTER  XXXIV.
THE  EAR  AND   HEARING.
The  External  Ear.  The  auditory  organ  in  man  consists
-of  three  portions,  known  respectively  as  the  external  ear,  the
miadle  ear  or  tympanum,  and  the  internal  ear  or  labyrinth;
the  latter  contains  the  end  organs  of  the  auditory  nerve.
The  external  ear  consists  of  the  expansion  seen  on  the  ex-
terior of  the  head,  called  the  concha,  M,  Fig.  162,  and  a  pas-
sage leading  in  from  it,  the  external  auditory  meatus',  G.
Fig.  162.— Semidiagrammatio  section  through  the  right  ear  (Czermak).  M,
concha;  (?,  external  auditory  meatus  ;  T,  tympanic  membrane  ;  P,  tympanic
cavity  ;  o,  oval  foramen  ;  r,  round  foramen  ;  B,  pharyngeal  opening  of  Eusta-
chian tube ;  V,  vestibule  ;  B,  a  semicircular  canal ;  S,  the  cochlea ;  Vt,  scala  ves-
tibuli ;  Pt,  scala  tympani  ;  A,  auditory  nerve.
This  passage  is  closed  at  its  inner  end  hy  the  tympanic  or
drum  membrane,  T.  It  is  lined  by  skin,  through  which
numerous  small  glands,  secreting  the  wax  of  the  ear,  open.
The  Tympanum  (P,  Fig.  162)  is  an  irregular  cavity  in
the  temporal  bone,  closed  externally  by  the  drum  membrane.
557


558  THE  HUMAN  BODY.
