Secretion of Milk. 7 



smaller ducts become much retracted, aud 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 j^ear, 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 fluids 

 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, carrjdng with it the min- 

 eral constituents in solution and a part of the al- 

 bumin of the blood serum; but by far the larger 



