4.  Chemical  Stimulation.     (Purposive  Characters  of  Reflex.)
(a.)  In  a  small  glass  place  some  strong  acetic  acid  and  a  few

pieces  of  filter-paper  3  mm.  square.  Either  when  the  frog  is
lying  on  its  back  or  while  it  is  suspended,  apply  with  a  pair  of
forceps  one  of  the  pieces  of  paper  moistened  with  acid — the  surplus


320  PRACTICAL   PHYSIOLOGY.  [lXVIII.
removed — to  the  skin  on  the  inner  side  of  the  thigh.  At  once
the  leg  on  that  side  is  violently  drawn  up,  perhaps  both  legs  are
drawn  up,  and  the  foot  of  the  leg  first  drawn  up  is  swept  over  the
spot  stimulated,  as  if  to  remove  the  piece  of  paper,  i.e.,  purposive,
co-ordinated  movements  are  executed.  At  once  dip  the  frog  in
water  to  remove  the  acid ;  allow  it  to  rest  for  some  time.  It  is
much  easier  to  obtain  irradiation  of  the  reflex  movements  by
chemical  than  by  mechanical  stimuli.
(6.)  After  five  minutes  repeat  the  experiment,  but  hold  the  leg
to  which  the  acid  is  applied.  Probably  the  other  leg  will  move,
and  the  opposite  foot  will  remove  the  irritating  acid  paper.  Wash
the  frog  and  allow  it  to  rest.
(c.)  Test  further,  by  applying  papers  to  the  flank,  the  skin  over
the  gastrocnemius,  Sac,  and  in  all  cases  characteristic  but  diflTerent
reflex  movements  will  be  elicited, if  sufficient  interval  for  recovery
(five  minutes  at  least)  be  allowed  between  the  successive  experi-
ments.
(c?.)  Destroy  the  spinal  cord,  all  reflex  action  is  abolished.  The
nerves  and  rausclesretaintheirexcitabilityand  the  heart  continues
to  beat.  Expose  the  heart :  it  beats.  Muscle  and  nerve  respond
to  electrical  and  other  stimuli.
5.  Action  of  Strycliniiie.
