i-Pearce,  R.  G.,  and  Carter,  E.  P.:     Am.  Jour.  Physiol.,  1915,  xxxviii,  350.
PART  VII
METABOLISM


CHAPTER  LX
METABOLISM

Introductory. — The  object  of  digestion,  as  we  have  seen,  is  to  render
the  food  capable  of  absorption  into  the  circulatory  fluids — -the  blood  and
lymph.  The  absorbed  food  products  are  then  transported  to  the  various
organs  and  tissues  of  the  body,  where  they  may  be  either  used  at  once
or  stored  away  against  future  requirements.  After  being  used,  certain
substances  are  produced  from  the  foods  as  waste  products,  and  these  pass
back  into  the  blood  to  be  carried  to  the  organs  of  excretion,  by  which  they
are  expelled  from  the  body.  By  comparison  of  the  amount  of  these  ex-
cretory products  with  that  of  the  constituents  of  food,  we  can  tell  how
much  of  the  latter  has  been  retained  in  the  body,  or  lost  from  it.  This
constitutes  the  subject  of  general  metabolism.  On  the  other  hand,  we  may
direct  our  attention,  not  to  the  balance  between  intake  and  output,  but  to
the  chemical  changes  through  which  each  of  the  foodstuffs  must  pass  be-
tween absorption  and  excretion.  This  is  the  subject  of  special  metabolism.
In  the. one  case  we  content  ourselves  with  a  comparison  of  the  raw. ma-
terial acquired  and  the  finished  product  produced  by  the  animal  factory;
in  the  other  we  seek  to  learn  something  of  the  particular  changes  to  which
each  crude  product  is  subjected  before  it  can  be  used  for  the  purpose  of
driving  the  machinery  of  life  or  of  repairing  the  worn-out  parts  of  the
body.
