6o  Animal  Digestive  Ferments.        { AFebJr°uarV!i™* 
Krassilinikow,  a  student  of  Brucke,  first  made  use  of  dialysis  in 
purifying  pepsin  in  1 864. 
Hoppe-Seyler,  in  1864,  published  a  comprehensive  table,  classify- 
ing the  various  forms  of  proteids,  characterized  by  their  solubility 
and  by  their  precipitability  with  various  neutral  alkaline  salts,  as 
sodium  chloride,  magnesium  sulphate,  etc. 
One  of  the  earlier  contributions  on  the  pancreas,  its  ferments,  the 
nature  of  its  action  on  proteids,  the  cleavage  products  formed,  etc., 
a  subject  that  was  made  the  special  study  of  Kuehne  and  his  pupils, 
appeared  in  1867.  Since  then,  up  to  1880,  numberless  papers  on 
the  subject  have  appeared,  and  our  present  knowledge -of  this  sub- 
ject we  owe  largely  to  this  investigator.  Kuehne  also  gave  the 
name  trypsin  to  the  proteolytic  ferment,  and  introduced  the  term 
,( enzymes "  to  designate  this  class  of  active  principles,  viz.,  the 
digestive  ferments. 
Von  Wittich,  in  1869,  suggested  the  use  of  glycerin  in  extracting 
the  pepsin,  and  this  has  since  been  employed  extensively  both  in 
scientific  research  as  well  as  in  the  manufacture  of  the  digestive  fer- 
ments and  their  preparations. 
Scheffer,  in  1872,  published  a  method  for  preparing  commercial 
pepsin  by  precipitation  from  an  acid  infusion  of  the  stomach  with 
common  salt.  He  also  stated  that  other  neutral  alkali  salts,  such 
as  sodium  sulphate  and  magnesium  sulphate,  could  be  used  instead 
of  common  salt  with  good  results.  The  action  of  saturated  solu- 
tions of  some  of  the  neutral  salts  of  the  alkalies  on  different  protein 
substances  induced  him  to  try  their  effect  on  pepsin. 
In  1873  we  find  Ebstein  and  Grutzner  demonstrating  that  pepsin 
does  not  exist  as  such  in  the  stomach,  but  is  rapidly  formed  from  its 
progenitor — termed  pepsinogen  by  these  authors — by  the  gastric 
acid.  Schwann  and  Wasmann  seemed  aware  of  this  fact,  but  laid 
no  stress  on  it.  Langley,  more  recently,  and  Glaessner,  during  the 
past  year,  have  given  this  subject  careful  study.  Heidenhain  dis- 
covered a  zymogen,  now  called  trypsinogen,  in  the  pancreas  in 
1875. 
Enabled  by  the  great  improvement  in  technic  and  by  the  intro- 
duction of  antiseptic  methods,  Heidenhain,  Klememsiwicz  and 
Thiry  during  the  seventies,  Pawlow  and  his  pupils  during  the 
nineties,  observed  the  functions  of  the  digestive  glands  under  vari- 
ous conditions  and  influences  in  the  living  animal.    Interesting  and 
