PART II. 



MILK AND ITS COMPOSITION. 



I. THE MILK GLANDS AND MILK SECRETION. 



The tissue of the milk glands is shown by macro- 

 scopic examination (Fig. 1) to be composed of small 

 lobnles separated by bands of connective tissue in which 

 lie the larger blood vessels, the nerves and the excretory 

 ducts and in which there is sometimes a considerable 

 quantity of fat tissue. The gland tissue itself is com- 

 posed of complexly branching glandular tubes which, 

 during the period of lactation, are provided with 

 numerous globular distensions, so that the type of the 

 milk gland is intermediate between that of the alveolar 

 and tubular types. The glandular pockets are provided 

 with a membrana propria (Fig. 2) and are lined with 

 epithelial cells. In young and farrow animals, the cells 

 are thin and square or high and narrow, and resemble 

 superficial epithelium. During the period of lactation 

 these cells become large and tense. This epithelium is 

 usually composed of but a single layer, although, in old 

 cows, it is' sometimes found to be composed of several 

 layers. 



Toward the end of gestation, the secretory function 

 of the udder begins with the production of colostrum or 

 the so-called "beast milk." This is a thick, reddish or 

 yellowish fluid with a taste more salty than that of nor- 

 mal milk, and under the microscope (Fig. 3) it is seen 

 that it contains numerous free fat globules and a large 

 number of round or mulberry shaped cells — ^the so-called 

 colostrum bodies — ^that are filled with fat globules. 



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