4 PRINCIPLES AND PRACTICE OF MILK HYGIENE 



pregnant cow contains certain substances called "milk- 

 forming substances." During pregnancy the greater 

 portion of these substances is required for the nourish- 

 ment and development of the foetus, but sufficient is 

 left over to stimulate the development of the glandular 

 tissue in the udder. When the foetus is developed, all 

 are available for action upon the udder and secretion is 

 stimulated. 



At the time of parturition the udder does not secrete 

 milk, but a substance called colostrum. The alveoli at 

 this period contain many cells, entire and disintegrated, 

 and leucocytes are also numerous in the interalveolar 

 connective tissue. The secretion is therefore rich in cells. 

 It also contains comparatively large, round bodies which 

 have the appearance of masses of fat grobules. These 

 are the so-called colostrum corpuscles, which are re- 

 garded by some as leucocytes which have taken up a large 

 number of fat globules, and by others as exfoliated epi- 

 thelial cells containing masses of fat globules. A cow 

 in this stage of lactation is said to be "fresh." 



During the first week the secretion gradually changes 

 to milk. The alveoli of the udder are not all active at 

 this time, but those which are inactive and which have 

 not undergone permanent involution resume their func- 

 tion within the succeeding two or three weeks, when the 

 secretion of milk reaches its highest point. Usually by 

 the end of the first week the leucocytes have disappeared 

 from the alveoli and interalveolar tissue and very few 

 cells are present in the milk. But if milking is delayed 

 or is incomplete at any time, or if stasis of milk occurs 

 from any cause, leucocytes again invade the alveoli and 

 interalveolar tissue in large numbers and become nume- 

 rous in the milk. Their appearance under these condi- 



