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 the 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 vided the milk is regularly withdrawn.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. 



