PART IV. DAIRYING 



CHAPTER XVIII 



COMPOSITION AND GENERAL PROPERTIES OF MILK 



Secretion. The external appearance of the cow's udder, or 

 mammary gland, with the four teats from which milk is 

 drawn, is familiar to the most casual observer. Internally, the 

 structure of the gland is extremely complex. 

 The relation of the more important func- 

 tional parts is shown in the diagram 1 

 (Fig. 21). The teats are tubular, and the 

 ducts are irregularly expanded above, form- 

 ing cisterns, or reservoirs, in which the milk 

 accumulates. At a further point the ducts 

 branch, and the innumerable smaller ducts 

 which lead off from them terminate in small, 

 pouch-like structures called alveoli. It is 

 in these that milk is actually formed. 



The cavity of each alveolus is lined with 

 a single layer of secreting cells. When the 

 gland is active, i.e. when the cow is " in 

 milk," these cells become enlarged, and the 

 pressure of the contiguous cells causes them 

 to protrude into the cavity. In this condition, 

 each cell contains two or more nuclei. The 

 innermost nucleus is generally normal in size and appearance. 

 Those which are nearer to the free end of the cell are irregular 

 in outline, and exhibit other signs of degeneration. The cells 

 also contain water, salts, sugar, fat, and protein bodies, all, it 



1 After Percival, " Agricultural Bacteriology." 

 286 



a Alveoli. 



d Lactiferous ducr. 



TTear. 



FIG. 21. 



