26 MILK SECRETION 



All of the small milk-ducts end in small sac-like bodies, each 

 of which is called a gland-lobule or ultimate follicle. The gland- 

 lobules enclose numerous individual microscopical bodies called 

 alveoli or acini; these are the organs which possess the proper 

 secretory functions. Their outer covering is a membrane called 

 the tunica propria; within this there is an intermediate layer of 

 cell-tissue, and an inside layer 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, swelling and extending into the 

 cavity of the alveoli when the cow is yielding milk abundantly, 

 and when she is not in milk the alveoli are flat. A certain 

 number of alveoli are 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 likened 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 

 secretion have been studied considerably, many things in this 

 connection are not well understood. Previous to the year 

 1840 it was thought that the only function of the milk-gland 

 was to filter the milk as it transuded from the blood. It was 

 supposed that the quality and quantity of milk depended entirely 

 upon the food. The theory has also been advanced that the 

 major portion of the milk constituents is a decomposition of 

 the product of the lymph bodies of the blood. It was believed 

 that the lymph bodies were a source of nourishment to the fetus. 



