64  Comparative  Composition  of  Milks.      { A February^  1908?' 
in  ass's  milk  I  to  2-3.    Human  milk  is  intermediate,  being  nearly 
1  to  1.  It  is  important  to  note,  also,  that  the  maximum  ratio  of 
casein  to  albumin  in  cow's  milk  is  about  7  to  1,  and  the  minimum 
4-5  to  I  ;  the  maximum  of  goat's  milk  is  3  to  I,  and  the  minimum 
2  to  I  ;  and  the  maximum  of  sheep's  milk  is  4  to  i,and  the  mini- 
mum 3  to  1. 
These  variations  in  the  ratios  of  casein  to  albumin  in  the  milks 
of  different  animals  are  not  accidents  of  nature.  They  are  in  direct 
obedience  to  the  law  of  supply  and  demand.  They  mean  simply 
that  the  proportion  of  food-material  that  is  digested  with  diffi- 
culty (casein)  to  the  food-material  that  is  digested  with  ease  (al- 
bumin), is  adjusted  by  nature  to  meet  the  needs  of  the  individual 
animal  for  the  proper  development  of  the  motor  and  chemical  func- 
tions of  its  stomach  and  intestines.  Naturally,  with  the  larger  ani- 
mals, more  casein  and  less  albumin  is  required,  than  with  the 
smaller  animals.  So  far  as  the  food-value  of  the  two  proteids  to  the 
body  is  concerned,  they  are  probably  of  equal  value.  Not  only 
does  the  ratio  vary  with  different  animals,  but  in  the  same  animals 
of  the  same  age,  in  obedience  to  individual  needs,  though  the  latter 
variations  are  within  much  narrower  limits  than  the  former.  A 
sixteen  pound  infant,  for  example,  requires  more  casein  than  a  twelve 
pound  one,  though  both  may  be  six  months  of  age.  Hence,  it  fol- 
lows that  human  milk  is  a  food  of  constantly  changing  composition. 
At  birth  much  albumin,  and  little  casein  is  needed,  and  then  as  the 
child  grows  and  develops,  more  and  more  casein  is  required,  and 
less  and  less  albumin. 
As  to  the  chemical  reactions  of  milks  with  gastric  juice,  the  for- 
mation of  curds,  the  kinds  of  curds,  and  the  functions  of  curds,  and 
the  mechanical  and  chemical  modifications  of  curds — these  are 
questions  of  the  deepest  scientific  interest  to  the  pediatrist,  but 
wholly  outside  the  province  of  this  paper. 
The  composition  of  cream  varies  greatly  according  to  the  method 
used  in  obtaining  it.  It  is  obtained  (1)  by  setting  milk  in  shallow 
pans  and  removing  the  cream  by  hand-skimming,  or  (2)  by  placing 
it  in  deep  vessels  surrounded  by  cold  water,  the  skimmed  milk 
being  drawn  off  from  below  (both  of  these  are  gravity  creams),  or 
(3)  by  means  of  the  centrifugal  separator.  The  United  States  stand- 
ard for  cream  (U.  S.  Dept.  of  Agri.,  Office  of  Sec,  Cir.  10)  is  13  per 
cent,  milk  fat. 
