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PHYSIOLOGY OF THE DOMESTIC ANIMALS. 

 QUANTITATIVE COMPOSITION OP MILK. 



7. THE SECRETION OF MILK. The mammary glands belong to the 

 t} T pe of compound acinous glands, and are constructed on a similar plan 

 to the salivary glands. The alveoli are lateral expansions at the termi- 

 nations of the excretory ducts and are formed of a closed membrane 

 covered, as is also the case with the ducts, with a single layer of cells, 

 whose appearances vary according to the stage of activity of the glands. 

 The secretory cells are composed of polyhedral cellular structures 

 containing a round nucleus and usually a varying number of oily 

 globules. During the stage of greatest activity the secretory cells 

 increase in size, the nucleus often is apparently reduplicated, and the 

 number of oil-globules greatly increased. In active secretion, during 

 which the oil-globules and cell-contents appear to be extruded, the 

 remaining cells are much smaller and only contain a single nucleus. The 

 excretory ducts, which are short, are likewise supplied with flat epithelial 

 cells and terminate in a canal which in each part of the gland becomes 

 enlarged, especially at the base of the nipple, where it becomes dilated 

 to form the so-called milk-cistern, which is connected at the exterior by 

 several canals which open at the end of the nipple (Fig. 260). This 

 cistern is lined with a mucous membrane composed of cylindrical epi- 

 thelial cells (Fig. 261). In the canals and in the nipple the epithelial 

 coat becomes converted into pavement epithelium. The excretory 

 canals are abundantly supplied with unstriped muscular fibres, which at 

 the opening into the nipple become developed into strong circular layers. 



It is evident from the composition of milk that its most important 

 constituents must result from a special cell activity, since neither casein 

 nor milk-sugar are found in the blood, and, although fat is a constant 

 constituent of the blood, the amount in comparison with that found in 

 the milk is almost infinitesimal. It follows, therefore, that the milk, like 

 the other secretions, cannot be regarded as a transudation, but is a result 

 of the protoplasmic activity of the epithelial cells of the mammary glands. 

 But, further, good cows may yield as much as twenty-five kilos of milk 

 daily, containing as much as 2.5 kilos of albuminoids, fats, and sugar. 



