Notwithstanding  these  objections,  there  can  be  no  doubt  that  many
of  the  functions  of  an  organ  are  retained  much  longer  than  they  would
be  if  the  organ  were  not  perfused ;  for  example,  the  contractility  of  the
muscle  or  the  power  of  forming  urea  in  the  liver.  Perfusion  experiments
are  of  value  therefore  when  they  yield  positive  results.  Negative  re-
sults may  indicate  either  that  the  organ  does  not  perform  the  particular
function  that  is  being  investigated  or  that  it  has  lost  this  function  as  a
result  of  partial  death.  That  a  perfused  muscle  retains  its  power  of
contraction  does  not  necessarily  indicate  that  it  maintains  all  of  its
metabolic  functions;  neither  does  the  fact  that  the  liver  forms  urea
prove  that  it  is  capable  of  performing  its  other  functions.  It  is  easy  to
show  that  the  liver  dies  piecemeal;  some  functions,  such  as  glycogen-
formation,  die  early,  while  others,  such  as  urea-formation,  remain  for  a
long  time  intact.  The  use  of  perfusion  experiments  for  the  settling  of
questions  of  metabolism  should  therefore  always  be  very  carefully  con-
trolled and  never  used  as  the  sole  line  of  evidence  on  which  to  base  impor-
tant conclusions.
