rated  from  each  other  by  the  vomer.  Through  these  apertures  the
atmospheric  air  is  constantly  streaming  in  and  out  of  the  pharynx
in  its  passage  to  and  from  the  lungs.
161.  Isthmus  faudum, — This  name  is  given  to  the  o|>eningby
wliich  the  mouth  communicates  with  the  pharynx.  If  we  look  into
the  throat  of  a  living  person,  we  may  observe  that  this  aperture  is
bounded  by  the  .soft  palate  and  uvula  above,  below  by  the  root  of
the  tongue,  and  on  either  side  by  the  arches  of  the  palute,  enclosing
the  tonsils  between  them.  The  size  and  shape  of  this  aperture
admits  of  great  variation,  and  we  shall  presently  find  that  it  may
be  completely  closed  by  the  contraction  of  its  lateral  boundaries.
152.  Soft  palate — This  is  a  moveable  prolongation  of  the  roof  of
the  mouth,  attached  to  the  posterior  border  of  the  hard  palate.  It
constitutes  an  imperfect  diaphragm  or  septum,  interposed  between
the  mouth  and  the  posterior  openings  of  the  nose.  Its  upper  or
nasal  surface  is  convex,  and  continuous  with  the  floor  of  the  nose ;
its  lower  surface  is  concave,  in  adaptation  to  the  back  of  the  tongue,
and  is  marked  in  the  mesial  line  by  a  prominent  white  ridge,  indi-
cating its  original  formation  by  two  lateral  halves.  The  general
shape  and  direction  of  the  soft  palate  may  be  easily  seen  by  looking
into   the  mouth  :    we  may  then  observe  that,  anteriorly,  it  haai


288  ARCHX8  Of  THE  PALATE.
nearly  the  same  hoiisontal  direction  as  the  hard  palate  to  which  it
18  attached ;  hut  it  gradually  hecomes  more  curved  as  it  passes
hackwards,  decreasing  at  the  same  time  in  thickness,  so  that  the
free  posterior  border  hangs  loosely  into  the  pharynx.  Hence  the
name  vtlvm  pendulum  palati.
