Secretion of Milk. 7 
smaller ducts become much _ retracted, and the 
follicles shrink in size and finally become rudimen- 
tary, or even entirely disappear, until under the 
stimulus of a succeeding pregnancy, the whole 
gland renews its activity, and the ducts and _ fol- 
licles regain their former size and appearance. 
New ducts and follicles may also be formed up to 
about the fifth or sixth year, and the power of 
the animal to secrete milk be thereby increased. 
Surrounding the follicles, and intimately attached 
to them, are capillary blood vessels, both veins and 
arteries, and through the cells of the membranes 
making up the walls of all these vessels the fiuids 
of the blood freely pass into the cavity of the 
follicles by means of osmosis, or transudation. 
The cavity of the follicle is lined with epithelial 
cells, that during lactation are filled with proto- 
plasm, and are capable of rapid multiplication, 
growth, and degeneration, at the same time that 
the cell contents are undergoing rapid and exten- 
sive changes. 
The secretion of milk.—The milk is formed from 
the blood, partly by the transudation of the blood 
serum directly into the cavity of the milk follicle, 
and partly by a transformation of the contents of 
the epithelial cells lining the cavity of the follicle, 
which at this time are especially active. The 
water passes directly from the capillaries into the 
milk follicles and ducts, carrying with it the min- 
eral constituents in solution and a part,of the al- 
bumin of the blood serum; but by far the larger 
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