The  inferior  rectus  being  paralyzed,  the  eyeball  fails  to  move  down-
ward and  to  some  extent  outward;  of  the  double  vision,  the  secondary
image  is  below.  The  'superior  rectus  paralyzed,  the  eyeball  does  not  move
upward,  nor  perfectly  outward;  the  secondary  image  ia  above.  To  look
w.th  this  eye  the  head  is  thrown  back.  The  inferior  oblique  is  opposite,  in
action  and  in  the  effects  of  paralysis,  to  the  inTirior  rectus.  The  superiw
oblique,  supphed  by  a  separate  nerve,  the  fourth  or  trochlear,  is  often
paralyzed;  the  effects  are  opposite  to  those  of  paralysis  of  the  auperitv
rectus.  The  fourth  nucleus,  under  the  posterior  geminum,  is  a  continua-
tion of  the  third.  The  sixth  nucleus,  in  the  lower  part  of  the  pons,  is  another
link  in  the  chain  of  gray  matter  for  the  ocular  muscles.  The  externij
rectus,  supplied  by  the  sixth  or  abducent  nerve,  is  more  often  affecied
alone  than  any  other  ocular  muscle.  When  the  entire  third  nerve  is  para-
lyzed the  eyelid  droops  (ptosis),  the  eye  turns  outward  by  the  action  of
the  sixth  nerve,  and  slightly  downward  by  the  action  of  the  fourth,  the


DISEASES  OF  THE  CRANIAL  NERVES.
