330 HAND-BOOK OF PHYSIOLOGY. 



of epithelial cells, and in the state of full activity this transformation is 

 so complete that it may be called a deliquescence" (Creighton). 



In the earlier days of lactation, epithelial cells partially transformed 

 are discharged in the secretion: these are termed "colostrum corpuscles," 

 but later on the cells are completely transformed before the secretion is 

 discharged. 



After the end of lactation, the mamma gradually returns to its original 

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

 with cells in all degrees of vacuolation (Fig. 223). As involution proceeds 

 the acini diminish considerably 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 



FIG. 223. Section of mammary gland of ewe shortly after the end of lactation, showing parts of 

 four acini, which contain numerous epithelial cells undergoing vacuolation in situ; they very closely 

 resemble young fat-cells, and are in fact just like " Colostrum corpuscles. 11 X 300. (Creighton.) 



within the acinus. During the later stages of involution, large yellow 

 granular cells are to be seen. As the acini diminish in size, the con- 

 nective tissue and fatty matter between them increase, and in some ani- 

 mals, 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 undergo changes (evolution) 

 which are readily observable. They enlarge, become harder and more 

 distinctly tabulated: the veins on the surface become more prominent. 

 The areola becomes enlarged and dusky, with projecting papillae; the 

 nipple too becomes more prominent, and milk can be squeezed from the 

 orifices of the ducts. This is a very gradual process, 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 Involu- 

 tion. 



