SECRETION. 337 



After the end of lactation, the mamma gradually returns to its ori- 

 ginal size (involution). The acini, in the early stages of involution, are 

 lined with cells in all degrees of vacuolation. As involution proceeds 

 the acini diminish considersbly in size, and at length, instead of a 

 mosaic of lining epithelial cells (twenty to thirty in each acinus), we 

 have five or six nuclei (some with no surrounding protoplasm) lying in 

 an irregular heap within the acinus. During the later stages of involu- 

 tion, large, yellow granular cells are to be seen. As the acini diminish 

 in size, the connective tissue and fatty matter between them increase, 

 and in some animals, when the gland is completely inactive, it is found 

 to consist of a thin film of glandular tissue overlying a thick cushion of 

 fat. Many of the products of waste are carried off by the lymphatics. 



During pregnancy the mammary glands and mammae undergo 

 changes (evolution) which are readily observable. They enlarge, become 

 harder and more distinctly lobulated: the veins on the surface become 

 more prominent. The areola becomes enlarged and dusky, with pro- 



Fia. 229. Section of mammary gland of bitch, showing acini, lined with epithelial cells of a 

 polyhedral or short columnar form. X 200. (V. D. Harris.) 



jecting papillae; the nipple too becomes more prominent, and milk can 

 be squeezed from the orifices of the ducts. This is a very gradual pro- 

 cess, which commences about the time of conception, and progresses 

 steadily during the whole period of gestation. The acini enlarge, and a 

 series of changes occur, exactly the reverse of those just described under 

 the head of Involution. 



The Mammary Secretion : Milk. 



.The secretion of the mammary glands, or milk, is a bluish- white 

 opaque fluid with a pleasant sweet taste. It is a true emulsion. Under 

 the microscope, it is found to contain a number of globules of various 

 sizes (Fig. 230), the majority about ru-sinr f an i ncn i n diameter. They 

 are composed of oily matter, probably coated by a fine layer of albumi- 

 nous material, and are called milk-globules; while, accompanying these, 

 are numerous minute particles, both oily and albuminous, which exhibit 

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