462 NUTRITION OF FARM ANIMALS 



TABLE 121. AVERAGE COMPOSITION OF Cow's MILK 



548. Milk glands. The milk glands, properly speaking, 

 are two in number, one on each side of the median line of the 

 body, although in many animals each gland is subdivided into 

 two or more lobes having separate outlets 

 or teats. Thus in the horse and sheep 

 each gland has two lobes, in the cow 

 two or three, and in the hog from ten to 

 fourteen. The milk gland is classified 

 as a compound tubulo-acinous gland. Its 

 structure may be roughly compared to 

 that of a bunch of grapes. It consists 

 of a great number of acini or alveoli, 

 three of which are shown schematically 

 FIG. 40. Lobule of milk in Fig. 40, corresponding to the single 

 berries of the grape cluster. Each alve- 

 olus consists of an outer layer of con- 

 nective tissue carrying capillary blood vessels, nerves and 

 lymphatics. These alveoli are about ^ of an inch in diameter 

 and are united in groups of 3 to 5 to form lobules having 

 a common outlet as shown in the figure. Internally, the 

 alveoli are lined with a single layer of epithelial cells (Fig. 41), 

 which are the active agents in secreting milk. The ducts or 

 passages leading from the alveoli are also lined with epithelial 

 cells but of a different sort and which do not produce milk. 

 These ducts unite to form larger ones, as shown in Fig. 42, which 

 lead finally to the teat, emptying first into the so-called " milk 

 cistern," a cavity lying near the base of the teat. In compound 



