THE MAMMARY GLANDS. 



263 



open. Each lobe is subdivided by connective tissue into several lobules, 

 which in turn are made up of the ultimate divisions of the secreting tissue, 

 the alveoli. The walls of the latter consist of a membrana propria, lined, in 

 the resting condition, by a double layer of cells. Those next the membrana 

 propria are flat and probably muscular in nature, thus emphasizing the 

 resemblance between the mammary and sweat-glands. The inner cells, the 

 secretory elements, are cuboid or low columnar. 



During lactation, the alveoli become greatly enlarged and distended 

 and the intervening connective tissue correspondingly reduced, so that the 

 alveoli are pressed closely together. The cytoplasm of the cells engaged in 

 the production of milk contains minute oil droplets, which, as they increase 

 in size, displace the nucleus towards the membrana propria and project into 

 the lumen of the alveolus, being separated from the latter by only a thin 

 protoplasmic envelope. With the rupture of the cells the oil drops escape 

 into the albuminous fluid, additionally secreted by the cells, that occupies 

 the alveolus. After liberation of the oil droplets, the epithelial cells are 

 much reduced; after a 

 time, however, they 

 again become the seat 

 of renewed secretory ac- 

 tivity, the accumulation 

 of fat and the production 

 of milk. Destruction 

 of the secreting cells, 

 therefore, does not take 

 place. 



The excretory 

 ducts begin as the small 

 canals into which the al- 

 veoli open and, at first, 

 resemble the terminal 

 compartments of the 

 gland, being lined with 

 a delicate stratum of 

 striped muscle, upon 

 which rests a simple 

 cuboidal epithelium. Within the lactiferous ducts, formed by the junction of 

 the smaller canals, the cuboid cells are succeeded by columnar ones. On 

 approaching the base of the nipple, beneath the colored areola, each milk- 

 duct enlarges into a spindle-form ampulla or simis lactiferus, from 10-12 

 mm. long and about half as wide, that serves as a temporary reservoir for 

 the secretion of the gland. Beyond the ampulla the duct narrows (2 mm. ), 

 passes into the nipple, and ends, after ascending the latter parallel with the 

 other ducts, in a minute orifice (.5-. 7 mm.) at the summit of the nipple. 

 Just before terminating, the epithelium lining the duct assumes the stratified 

 squamous character of the adjacent epidermis. 



The skin covering the areola and nipple, delicate but more or less 

 pigmented, contains well marked bundles of unstriped muscle, whose con- 

 tractions cause the nipple to become prominent and erect. Within the 

 areola, this contractile tissue forms a layer, in places almost 2 mm. thick, 

 that encircles the base of the nipple and extends into its substance as a 

 muscular network through which the milk-ducts pass. Deeper longitudinal 

 strands of unstriped muscle occupy the axis of the nipple. 



FIG. 311. Section of mammary eland during lactation, showing dis- 

 tended alveoli lined with fat-bearing cells. X 170. 



