Variations in Quality 27 
Variations in quality of milk.— We have already 
seen that the amount of milk secreted may vary 
greatly under the infiuence of a large number of 
varying conditions. So, too, we find that there 
ate a large number of conditions that affect the 
quality of the milk, meaning thereby the relative 
proportion of the various constituents, and particu- 
larly the proportion of fat to other constituents. 
.Some of these changes are regular and progressive 
during the period of lactation. Others are due to 
definite causes, and still others occur from time to 
time to which we have as yet been unable to 
ascribe any definite cause. After about the third 
or fourth week of lactation the percentage of fat 
in the milk remains nearly constant until the seventh 
or eighth month, or until the quantity of milk begins 
to rapidly diminish; but while the percentage of fat 
does not markedly change, the character of the fat 
undergoes several marked and characteristic changes. 
The butter globules are largest in size early in the 
period of lactation, and constantly diminish as lac- 
tation progresses, at the same time that they in- 
crease in number, so that the total amount of fat 
is not greatly changed. Early in the period of lac- 
tation there is a larger proportion of olein. In 
some cases it may amount to 50 per cent of the 
total fat. As the lactation progresses the propor- 
tion of olein decreases and stearin and palmitin in- 
creases, until the proportion of olein may fall as low 
as 20 per cent. This change is more marked when 
the animal changes from fresh to dry food, as the 
