Variations in Quality. 27 



Variations in quality of milk. — We have already 

 seen that the amount of milk secreted may vary 

 greatly under the influence of a large number of 

 varying conditions. So, too, we find that there 

 are 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 

 X)eriod 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 



