404 



THE ORGANS 



connective tissue varies greatly, some inactive mammae consisting 

 almost wholly of fat tissue. 



The Active Mammary Gland. — Throughout pregnancy the 

 gland undergoes extensive developmental changes and becomes func- 

 tional at about the time of birth of the child. The microscopic 

 appearance of the active gland differs greatly from that of the inactive 

 (Fig. 285). There is a marked reduction in the connective tissue of 

 the gland, its place being taken by newly developed ducts and alveoli. 

 The alveoli are spheroidal, oval, or irregular in shape, and vary con- 



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Fig. 285. — From Section of Human Mammary Gland during Lactation. Xso. 

 (Stohr.) a, Branch of excretory duct; b, interlobular connective tissue; c, alveoli. 



siderably in size. The alveoli are lined by a single layer of low col- 

 umnar or cuboidal epithelial cells which rest upon a homogeneous 

 basement membrane. The appearance of the cells differs according 

 to their secretory conditions. The resting cell is cuboidal and its 

 protoplasm granular. With the onset of secretion the cell elongates, 

 and a number of minute fat droplets appear. These unite to form 

 one or two large globules of fat in the free end of the cell. The fat 

 is next discharged into the lumen of the alveolus, and regeneration of 

 the cell takes place from the unchanged basal portion (Fig. 287). 

 As to the number of times a cell is able to go through this process 

 of secretion and repair before it must be replaced by a new cell. 



