The  Movements. — Flexion  and  extension  are  the  only
movements  possible  at  these  joints.  Flexion  is  much  more
free  than  extension,  and  can  be  continued  till  the  one  bone
is  at  right  angles  to  the  other.  The  second  phalanx  can  be
flexed  through  from  110°  to  115°  upon  the  first,  when  the  first
is  not  flexed  upon  the  metacarpal  bone.  Extension  is  checked
when  the  finger  is  straight,  i,e,,  it  cannot  go  beyond  a  right  line.
The  great  freedom  of  flexion  is  due  to  the  forward  slope
and  the  greater  anterior  extent  of  the  articular  facet  of  the
inferior  ends  of  the  bones,  and  to  the  direction  of  the  fibres  of
the  lateral  ligaments  which  pass  a  little  forwards  to  their  inser-
tion into  the  distal  bone.
It  is  interesting  to  remark  that  at  the  metacarpo-phalangeal
articulation,  and  at  each  of  the  inter-phalangeal,  the  extreme  of
flexion  is  reached  when  the  distal  bone  has  been  brought  to  a
right  angle  with  the  one  next  immediately  above  it ;  and  as  we
have  four  bones  and  three  joints  for  each  finger,  when  the  hand
is  closed,  the  several  bones  of  each  finger  form  a  four-sided
figure,  whose  opposite  sides  are  parallel,  and  at  right  angles
with  the  other  two ;  thus  the  metacarpal  bone  and  the  second
phalanx  are  parallel  with  each  other,  and  at  right  angles
to  both  the  first  and  third  phalanges  :  similarly  the  first  and
third  phalanges  are  parallel  with  one  another.
PART   IV.
THE     LOWER     LIMB.
CHAPTER  XVL

THE  HIP-JOINT.
CUuSf  Diarthrosia.  Suhdmium,  Eiuuihrodim.
The  bones  which  enter  into  the  formation  of  this  joint  are  the
innominate  and  the  femur ;  the  former  at  its  acetabular  part,
the  latter  by  its  head  and  neck.
