62  Animal  Digestive  Ferments. 
methods  for  ascertaining  the  composition  and  food  value  of  the  pro- 
ducts of  enzyme  action,  viz.,  the  various  albumoses  and  peptones  of 
the  market. 
From  the  very  first,  we  observe  speculations  upon  the  nature  of 
the  changes  produced  by  digestion.  Von  Helmont  believed  it  to 
be  a  fermentation ;  Eberle,  chemical  solution  ;  Schwann,  contact 
action  differing  from  true  fermentation  ;  Frerich  again  speaks  of  it 
as  fermentation ;  then  Pasteur  proved  that  fermentation  was  due  to 
micro-organisms,  and  a  distinction  was  made  between  organized 
and  unorganized  ferments,  Kuehne  suggesting  the  name  enzyme  for 
the  latter. 
Since  Buchner's  undoubted  discovery  in  1897  that  alcoholic 
fermentation  of  sugar  is  produced  by  an  enzyme — Zymase — which 
can  be  isolated  from  the  yeast  cell,  the  old  theory  that  fermenta- 
tions could  only  be  produced  by  living  cells,  being  inseparably 
associated  with  the  life  of  these  cells,  has  been  shattered ;  and  the 
thought  that  all  true  fermentations  are  caused  by  enzymes,  and  that 
the  digestive  processes,  among  others,  should  be  classed  as  such  is 
rapidly  gaining  credence. 
The  Zymase  has  not  received  complete  chemical  analysis,  not 
having  been  prepared  in  sufficiently  pure  form.  It  appears  to  be 
very  closely  related  to  proteids. 
Loew  has  described  the  enzymes  as  being  very  labile  proteid  sub- 
stances, containing  both  aldehyde  and  amide  groups.  Oppenheimer 
denies  that  the.  enzymes  themselves  are  labile,  but  capable  of  pro- 
ducing cleavage  in  other  labile  molecules,  they  themselves  remaining 
unaltered. 
The  products  of  the  digestion  of  food  have  naturally  been  the 
subject  of  scientific  investigation  related  to  the  study  of  the  ferments 
themselves,  and  to  our  subject  ;  for  the  reason  that  foods  increas- 
ingly enter  into  therapeutics  both  in  the  prevention  and  in  the  cure 
of  disease.  The  main  point  of  view  from  which  it  must  be  regarded 
in  medicine  is  the  physiological — that  the  prime  function  of  diges- 
tion is  the  conversion  of  food  of  all  classes  into  a  soluble,  assimilable 
and  nutritive  form. 
The  obstacles  which  we  have  encountered  in  the  chemical  inves- 
tigation of  the  enzymes  in  a  measure  exist  in  relation  to  various 
proteids,  native  and  derived,  owing  to  their  colloidal,  non-volatile 
and  readily  decomposable  nature.    However,  it  is  sufficient  to  state 
