PHYSIOLOGY OF MILK SECRETION 5 
tions seems to be for the purpose of preventing stag- 
nation of milk in the alveoli; they take up the fat glo- 
bules in the alveoli and carry them off to the lymph 
stream (Czerny). Leucocytes and exfoliated epithelial 
cells are also present in the milk in large numbers in in- 
flammation of the udder. 
Under natural conditions, the secretion of milk con- 
tinues only until the offspring is able to masticate and 
digest solid food. But in tthe highly specialized dairy 
cow, in which the function of the udder has been greatly 
developed, lactation may continue for as long as one 
to two years if reimpregnation does not take place, pro- 
vided the milk is regularly withdrawn. When the cow 
is reimpregnated, lactation usually ceases within a few 
weeks of parturition, but in some individuals it continues 
without interruption from one parturition to the next. 
In these latter animals, the secretion changes to colos- 
trum a few days before parturition. Cows in which lac- 
tation is about to cease are called “strippers.” Near the 
end of lactation the milk changes very much in composi- 
tion and the cellular content again increases. Quite fre- 
quently it has a salty or bitter taste, or an animal-like 
taste and odor which are unpleasant. It is considered 
good practice to give the udder and the organs of diges- 
tion an opportunity to rest and recuperate before a new 
lactation period begins, and “ persistent milkers ” are 
frequently “dried off” about a month before the suc- 
ceeding parturition is due. This can usually be accom- 
plished by reducing the feed and gradually stopping 
milking. Periodical emptying of the udder is necessary 
for the continuance of milk secretion, and the opposite 
effect is produced when milking is incomplete or is 
omitted. 
