The  sensory  organs  are  five  in  number,  viz.,  that
of  sight,  hearing,  touch,  taste  and  smell.  They  are
merely  the  peculiarly  shaped  termination  of  a  par-
ticular nerve.  Impressions  acting  upon  this  termin-
ation, or  anywhere  upon  the  nerve,  whether  coming
from  within  the  body  or  from  without,  affect  it  in  a
way  that  is  peculiar  to  it,  and  concerning  which
nothing  positive  is  known.  Thus,  the  eye-ball
is  so  constructed  as  to  collect  a  great  number  of
rays  of  light  which  affect  the  optic  nerve,  and
thereby  produce  the  sensation  of  sight.  What  be-
comes of  this  sensation — that  is,  in  what  manner  it
produces  the  consciousness  of  sight,  and  in  what
manner  it  ultimately  serves  intellectual  functions,
we  do  not  know,  and  probably  never  shall.  The
ear  is  utterly  blind  to  the  minute  waves  of  light,
but  very  sensitive  to  the  aerial  waves  of  sound.  So
each  organ  has  a  distinct  structure,  in  virtue  of
10


146  FIBST   LESSONS   IN   PHYSIOLOGY.
which  it  has  its  particular  manifestations  ;  and  the
different  senses  may  be  compared  to  the  various
departments  of  government  in  a  country,  all  of
which  together  make  up  the  government  itself.
THE    SENSE   OF   TOUCH.
