The  gastrocnemius  has  very  little  power  of  flexing  the  knee,  but  the  extension  of  the  knee
increases  the  effect  of  the  muscle  on  the  ankle  joint,  especially  in  walking,  just  as  we
have  seen  that  the  extension  of  the  hip  augments  the  force  with  which  the  contracting
rectus  extends  the  knee.  For  direct  extension  of  the  ankle,  the  peroneus  concurs  and
opposes  the  inversion.  The  only  difference  between  the  gastrocnemius  and  soleus  is  that
the  latter,  having  no  attachment  to  the  femur,  can  extend  the  ankle  when  the  knee  is
flexed  as  well  as  when  it  is  extended.  In  paralysis  of  these  muscles  extension  of  the
ankle  (by  the  peroneus  longus  and  flexor  longus  digitorum)  is  extremely  feeble,  and  the
foot  can  scarcely  be  carried  beyond  a  right  angle.     Walking  is  greatly  interfered  with ;

*  These  muscles  have  lately  been  termed  "  plantar  flexors,"  because  they  are  homolo-
gous with  the  flexors  of  the  wrist.  The  term  is  a  bad  one,  since  it  involves  a  use  of  the
word  "  flexor  "  in  absolute  contradiction  to  its  proper  signification.
MUSCLES   OF   LOWER   LIMBS.  ,


57


standing  ontiptoe  is  impossible.  The  unopposed  peroneus  longus  causes  eversion  of  the
foot,  lowers  the  head  of  the  first  metatarsal  bone,  and  deepens  the  plantar  arch.  In  time  the
ankle  joint  becomes  overflexed ;  the  heel  considerably  lowered  and  the  plantar  muscles  and
fascia  become  shortened.  The  resulting  deformity  is  termed  talipes  calcaneus  (Fig.  23).
Peroneus  longus  (musculocutaneous  nerve,  from  external  popliteal  of  sciatic)  everts
the  foot,  lowering  the  inner  border,  narrowing  the  foot,  and  increasing  the  plantar  arch.
It  also  turns  the  whole  foot  out  on  the  axis  of  the  leg.     It  has  a  very  feeble  power  of

Fig.  23.
