The  closure  of  the  joint  is  completed,  in  front  and  behind,  b}  a
thin  capsular  membrane  attached  to  the  bones  near  their  respec-
tive articular  surfaces,  and  sufficiently  loose  to  permit  the  necessary
range  of  motion.
We  have  said  that,  besides  flexion  and  extension,  the  construc-
tion of  the  ankle-joint  admits  of  a  very  slight  rotatory  movement
in  a  horizontal  direction,  the  centre  being  on  the  fibular  side.  In
adaptation  to  this  movement  the  internal  malleolus  is  made  much
shorter  than  the  outer;  it  is  not  so  tightly  confined  by  its  liga-
ments, and  its  articular  surface  is  part  of  a  cylinder.
By  laying  open  the  joint,  we  have  an  opportunity  of  observing
that  the  breadth  of  the  corresponding  articular  surfaces  of  the
bones  is  greater  in  front  than  behind :  the  object  of  this  is  to
render  the  tibia  less  liable  to  be  dislocated  forwards  over  the  astra-
galus. Whenever  this  happens,  the  astragalus  must  of  necessity
become  firmly  locked  between  the  malleoli.
106.    LZOAMENTS   CONNECTING    THE  BONES   CF  THE  FOOT.
The  astragalus  is  the  key-stone  of  the  arch  of  the  foot,  and  sup-
ports the  whole  weight  of  the  body.  It  articulates  with  the  os
calcis  and  the  os  scaphoides  in  such  a  manner  as  to  permit  the  ab-
duction and  adduction  of  the  foot,  so  useful  in  the  direction  of  our
steps.  We  must  carefully  examine  this  for  ourselves,  since  no
mere  verbal  description  can  do  the  subject  justice.
The  astragalus  articulates  with  the  os  calcis  by  two  distinct  sur-
faces,   respecting  which    it  is   necessary  to  remark,    that    the


478       LIGAKXHTS  OONKECTINO  TSB  BONBS  OF  THE  FOOT.
