REPRODUCTIVE ORGANS IN THE FEMALE. 333 



twenty separate compound tubulo-alveolar ' glands 

 which open separately on the nipple by an equal 

 number of pores. These glands are arranged radi- 

 ally and enclosed by a variable supply of fat and 

 connective tissue in such a way that it is possible to 

 divide the breast into fifteen to twenty lobes, which 

 may be further divided into lobules. Each pore 

 leads to a narrow 

 vertical tube, the 

 lactiferous duct, 

 which widens just 

 below the base of 

 the nipple to form 

 a receptacle called 

 the milk sinus, be- 

 yond which it 

 again becomes a 

 narrow tube. 

 The latter be- 

 comes branched to 

 form interlobular 

 ducts that open 

 into distal dilata- 

 tions or alveoli, which constitute the secreting por- 

 tions of the gland. 



The lactiferous ducts and sinus are lined with 

 simple columnar epithelium, which becomes strati- 

 fied near the orifices where it is directly continuous 

 with the stratified epithelium of the skin. The finer 

 structure of the alveoli varies according to the func- 

 tional activity of the organ. During lactation the 

 alveoli are distended with milk. The cells of the 



Lactiferous duct. 

 Milk sinus. 



Alveoli. 



Fig. 238. Diagram of one-half of mammary 

 gland, dissected to show gland. 



