Gelatinoid  or  Albuminoid  Alimentary  Principles.  These
also  contain  nitrogen,  but  cannot  replace  the  proteids  entirely
as  foods;  though  a  man  can  get  on  with  less  proteids  when  he
has  some  albuminoids  in  addition.  The  most  important  is
gelatin,  which  is  yielded  by  the  white  fibrous  tissue  of  animals
when  cooked.  On  the  whole  the  gelatinoids  are  not  foods  of
high  value,  and  the  calf's-foot  Jelly  and  such  compounds,
■often  given  to  invalids,  have  not  nearly  the  nutritive  value
they  are  commonly  supposed  to  possess.
Hydrocarbons  {Fats  and  Oils).  The  most  important  are
.stearin,  palmatin,  and  olein,  which  exist  in  various  propor-
tions in  animal  fats  and  vegetable  oils;  the  more  fluid  contain-
ing more  olein.  Butter  contains  also  a  little  of  a  fat  named
butyrin.  Fats  are  compounds  of  glycerine  and  fatty  acids,
and  any  such  substance  which  is  fusible  at  the  temperature
•of  the  Body  will  serve  as  a  food.  The  stearin  of  beef  and
mutton  fats  is  not  by  itself  fusible  at  the  body  temperature,
but  is  mixed  in  those  foods  with  so  much  olein  as  to  be  melted
in  the  alimentary  canal.  Beeswax,  on  the  other  hand,  is  a
fatty  body  which  will  not  melt  in  the  intestines  and  so  passes
■on  unabsorbed;  although  from  its  composition  it  would  be
useful  as  a  food  could  it  be  digested.  A  distinction  is  some-
times made  between  fats  proper  (the  adipose  tissue  of  ani-
mals consisting  of  fatty  compounds  inclosed  in  albuminous
cell-walls)  and  oils,  or  fatty  bodies  which  are  not  so  organized.
Carbohydrates.  These  are  mainly  of  vegetable  origin.
