MILK SECRETION. 25 



are able to hold up this milk also, but the majority of cows 

 cannot perfectly control the muscle which guards the en- 

 trance to the teat. The milk which is present in the milk- 

 ducts and which has to pass through these junctions referred 

 to above, can be held up by most cows at will. 



All of these small milk-ducts end in small sack-like bodies. 

 Each of these dilated portions is called the gland-lobule 

 or ultimate follicle. These gland-lobules enclose numei-ous 

 individual microscopical bodies called alveoli or acini. These 

 alveoli constitute the organs which possess the proper secre- 

 tory functions. These alveoli are lined on the outside with a 

 membrane called the tunica propria. Next to this membrane 

 is a layer of cell-tissue. The inside layer is composed of cells, 

 which are named the epithelial cells. These epithelial cells 

 within the alveoli are supplied with blood from the cow's 

 system. During lactation they assume a different form. 

 When the cow is yielding milk abundantly, these cells swell 

 and extend into the cavity of the alveoli. When the cow is 

 not in milk these alveolian cells become flat. A certain number 

 of alveoli is tributary to one particular duct leading from the 

 gland-lobule into still larger milk-ducts. 



Each aggregation of gland-lobules, tributary to one milk- 

 cistern, constitutes a lobe, and may be hkened to a side branch 

 of a bunch of grapes. Each separate grape may represent 

 a gland-lobule. The seeds within the grape, if we imagine each 

 seed to be hollowed out and lined with small column-like 

 bodies, may be likened to the alveoli. These column-like 

 bodies would then represent the epithelial cells. The stem 

 leading from each individual grape may represent the small 

 duct which carries the milk on to the larger ducts. The main 

 stems of the bunch may represent the larger ducts that enter 

 into the milk-reservoir. The air which everywhere fills the 

 openings or interstices of the various parts of the bunch of 

 grapes may be likened to the fibrous fatty tissue between the 

 alveoli and the lobules of the gland. 



Theories of Milk Secretion.— Although the theories of milk 



