^s]  Appendix,  See  I  ion   ///.,§  lo  299


TI1C  eye  is  adapted  by  three  sorts  of  muscular  adjustment,  for
reaction  to  objects  at  ditTercnt  distances:  (1)  Convergence  and
divergence  are  movements  of  the  eyeballs,  by  the  eye-muscles,
which  facilitate  vision  of  near  and  far  objects.  When  the  eyeballs
are  parallel,  clear  images  of  indetuiitely  distant  objects,  for  ex-
ami)le  of  the  stars,  are  formed.  As  the  eyeballs  converge  more
and  more,  that  is,  as  the  fronts  of  the  eyeballs  roll  together  and
the  backs  roll  apart,  rays  of  light  from  every  point  of  a  nearer  object
are  brought  together  at  corresponding  points  on  the  retina^  of  lioth
eyes,  so  that  the  two  eyes  act  as  one.*  (2)  Accommodation  is
bodily  process  which  changes  the  refractiveness  of  the  lenses  them-
selves. Accommodation  is  due  to  the  contraction  of  the  ciliary
muscle  (C.P).,  "a  muscle  lying  in  the  forward  part  of  the  choroid
coat,  outside  the  iris,  composed  in  small  part  of  circular  fibres
l)arallel  to  the  circumference  of  the  iris  and  in  large  part  of  fibres
radiating  from  this  edge  of  the  iris."  This  muscle,  contracting
somewhat  after  the  fashion  in  which  a  purse-string  is  pulled  up,
"draws  the  forward  half  of  the  choroid  coat  forward  and  inward,
thus  lessening  the  tension  of  the  elastic  capsule  in  which  the  cr}stal-
line  lens  is  swung,  and  allowing  the  lens  to  bulge  from  front  to
back."  t  (3)  The  third  of  these  muscular  adjustments  is  the
jHirely  unconscious  relle.x  movement  by  whk4rthtrpTTpTr  an  opening
in  the  iris  of  the  eye,  is  enlarged  or  narrowed  according  to  the
distance  of  the  object  and  the  intensity  of  the  light.  There  are
great  differences  in  these  reflexes.  The  pupils  of  night-seeing
animals  —  owls,  for  example  —  dilate  far  more  widely  in  the
night  than  the  pu])ils  of  human  eyes,  and  contract,  in  daylight,  to
a  mere  slit.
