SECRETION. 



423' 



glands. The nerves are derived from the supraclavicular and the 

 iritercostals. No secretory nerves of the mammae exist. 



Each gland-acinus, or vesicle, consists of a membrana propria, 

 surrounded externally with a network of branched connective-tissue 

 corpuscles. Internally there is a somewhat flattened polyhedral layer 

 of nucleated secretory cells. The size of the lumen of the acini de- 

 pends upon the secretory activity of the glands ; when it is large the 

 vesicle is filled with milk containing numerous refractive, fatty 

 granules. 



In the gland of a woman who is not pregnant or suckling the 

 alevoli are very small and solid. They are filled with a mass of 

 granular, polyhedral cells. During pregnancy the alveoli enlarge, 

 while the cells undergo rapid multiplication. With the beginning of 



Fig. 157. Dog's Mammary Gland in First Stage of Secretion. 

 (HEIDENHAIN.) 



a, 6, Section through the center of two alveoli of the mammary gland, the 

 epithelial cells seen in profile, c, Surface view of the epithelial cells. 



lactation the cells in the center of the alveolus undergo fatty degen- 

 eration and are eliminated in the first milk as colostrum-corpuscles. 

 The lining cells of the alveolus remain to form a single layer of 

 granular, short, columnar cells. Each possesses a spherical nucleus, 

 and is attached to the limiting membrana propria. By means of 

 metabolic processes within the protoplasm of the cells the fats, salts. 

 milk-sugar, etc., are formed. During glandular activity, instead of 

 one, two or more nuclei are seen; the well-formed one is near the 

 base, the other nearer the free end of the cell. Near the border of 

 the cell are seen numerous oil-globules and granules. Some of the 

 larger oil-globules are seen projecting from the surface of the cell 

 as if about to be extruded from it. 



In addition to this, a division of the cell itself takes place: a 

 parting of the cell-substance with a nucleus in it. The daughter- 

 cell thus cast off passes into the alveolus to form a part of the milk. 



