SECRETION. 309 



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

 or six nuclei (some with no surrounding protoplasm) lying in an irregu- 

 lar heap within the acinus. During the later stages of involution, 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 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 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. In the gland itself solid columns of cells bud off 

 from the old alveoli to form new alveoli. But these solid columns after 

 a while are converted into tubes by the central cells becoming fatty and 

 being discharged as the colostrum corpuscles above mentioned. 



Milk. 



The mammary secretion, or milk, is a bluish- white, opaque fluid with 

 a pleasant, sweet taste, of specific gravity of 1028-1034. It is a true 



Fig. 233. Globules and molecules of cow's milk, x 400. 



emulsion. Under the microscope, it is found to contain a number of 

 globules of various sizes (fig. 223), the majority about ^^ of an inch 

 (.%5/j.) in diameter. They are composed of oily matter, and are called 

 milk-globules, but the old view that they had an investing membrane of 

 albuminous material is now generally discarded. Accompanying these 

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

 ordinary molecular movements. The milk which is secreted in the first 



