FUNCTIONS OF THE MAMMARY GLANDS. 



287 



ginning of pregnancy, and the process occupies the entire 

 period of gestation. During the intervals between its periods 

 of functional activity the breast remains in a quiescent func- 

 tionless state: the resting stage. In this condition the gland 

 is shrunken and surrounded by a considerable quantity of 

 fibre-fatty tissue. The acini are shriveled up. On micro- 

 scopical examination of sections of the gland in this stage 

 (Fig. 2) each acinus appears as an alveolar space bounded by 

 a thin layer of fibrous tissue, denuded of epithelium. Its con- 

 tents are irregularly-arranged, polymorphic, epithelial cells, 

 with large nuclei and scanty surrounding protoplasm. . . . 

 "During the rising function the size of the acini grad- 



FlG. 2. 



FIG. 3. 



THE MAMMARY LOBULE NEAR THE RESTING STAGE (UPPER) AND 

 DURING FUNCTIONAL ACTIVITY. (Creighton.) 



ually increases from that of the resting stage. The cells in- 

 crease in number and size and acquire more protoplasm. They 

 gradually arrange themselves so as to form a lining membrane 

 for the wall of the acinus (Fig. 3), which, as lactation ap- 

 proaches, is converted into a regular mosaic. The cells become 

 granular, irregularly shaped, excavated, and vacuolated, secret- 

 ing granular and mucous fluids. The milk of the first few days 

 is always somewhat crude, containing colostrum-cells, which 

 are the last of the long series of secretory products thrown off 

 during the period of rising function. 



"The fully-expanded acinus (Fig. 4) in a state of active 

 secretion is at least four times as large as that of the resting 



