X  In  many  of  the  nocturnal  carnivorous  quadrupeds,  the  inner  surface  of
the  choroid  at  the  bottom  of  the  eye  presents  a  brilliant  colour  and  metallic
lustre.  It  is  calird  the  tapetum.  By  reflecting  the  rays  of  light  a  second
time  through  the  retina,  it  probably  causes  the  animal  to  see  better  in  the
dusk.  It  is  the  cause  of  the  well-known  glare  of  the  eyes  of  cats  and  other
animals ;  and  the  great  breadth  of  the  luminous  appearance  arises  from  the
dilatation  of  the  pupil.
526  DISSECTION  OP  THE  EYE.
eighth  of  an  inch  broad,  situated  just  beneath  the  union  of  the
sclerotica  and  the  cornea.  It  sen'es  as  a  connecting  uiedium
between  several  structures — ^namely,  the  choroid,  iris,  ciliar)'  pro-
cesses, and  sclerotica.*

Iris, — This  is  a  moveable  curtain  suspended  freely  in  a  clear
fluid,  which  is  situated  in  the  space  between  the  cornea  and  crystal-
line lens.  The  iris  divides  this  space  into  two  unequal  parts,
called  the  anterior  and  posterior  chambers ;  and  these  commimi-
cate  with  each  other  through  a  circular  aperture  in  the  centre,
called  the  pupil. f  Its  use  is  to  regulate  the  amount  of  light
which  shall  be  admitted  into  the  eye  :  for  this  purpose  its  inner
circumference  is  capable  of  enlarging  and  contracting  according  to
circumstances,  while  its  outer  circumference  is  immoveably  con-
nected to  the  ciliary  ligament.
The  colour  of  the  iris  varies  in  different  individuals,  and  gives
the  peculiar  tint  and  brilliancy  to  the  eye.  The  colouring  matter
or  pigment  itself  is  contained  in  minute  cells.  The  posterior
surface  of  the  iris,  called  the  uvea,X  is  in  all  cases  covered  by  a
layer  of  black  j)igment.
