The  cochlea  and  sensory  areas  in  the
vestibule.  Turning  from  the  semicir-
cular canals  to  the  other -canal  leading
out  of  the  vestibule,  namely  the  coch-
lea, we  find  here  the  organs  which
are  concerned  in  the  reception  of  tonal
stimulations.  It  is  not  clearly  known
whether  noise  stimulations  are  received
in  the  cochlea  or  not.  The  probabili-
ties are  that  noise  stimulations  affect
certain  cells  constituting  sensory  areas
in  the  wall  of  the  vestibule.  At  all
events,  it  is  true  that  there  are  cells
situated  in  the  wall  of  the  vestibule


SENSATIONS


109


which  seem  to  be  suited  to  the  reception  of  simple  stimuli
(see  Fig.  29).  The  vestibule  is  the  direct  descendant  of
the  primitive  vesicle.  This  fact  would  seem  to  argue  in
favor  of  the  view  that  noise  stimulations,  which  are  undif-
ferentiated  and  probably  earlier  than  tonal  stimulations,
affect  these  cells  in  the  vestibule.  Whatever  may  be  true
of  noise,  it  is  certain  that  the  tonal
excitations  are  received  through  the
complicated  structures  which  have
been  developed,  and  appear  in  the
cochlea.  The  cochlea  is  a  highly
developed  organ,  richly  supplied
with  cells  and  fibers  for  the  recep-
tion of  a  great  number  of  different
stimulations.  It  consists  of  a  double
spiral  canal,  which  winds  around
two  and  a  half  times.  The  winding
of  this  canal  is  merely  an  anatomi-
cal device  for  compressing  the
whole  organ  into  as  small  a  space
as  possible.  The  canal,  which  is
cylindrical  in  form,  is  divided  into
three  parts,  —  the  scala  vestibuli,
the  scala  tympani,  and  the  ductus
cochlearis.  This  division  can  best
be  seen  by  making  a  section  across
the  cylindrical  passage.  Fig.  30
shows  such  a  section  with  the  division.  The  scala  tympani
is  partially  separated  from  the  rest  of  the  cochlea  by  a  bony
shelf  which  extends  for  some  distance  into  the  canal.  The
division  is  completed  by  an  important  membrane.  This
membrane,  known  as  the  basilar  membrane,  is  made  up  of
a  series  of  fibers  which  differ  in  length  as  the  membrane
passes  from  the  lower  to  the  upper  extremity  of  the  canal.
