BLOOD VESSELS OF THE MAMMARY GLAND 521 



furnish new branches to them. The alternation of venules and 

 arterioles continues as at the preceding stage. In the capillary 

 plexus about the ducts, however, venules and arterioles are 

 beginning to appear. The venules are better developed than 

 the arterioles and exhibit a greater tendency to form longitudinal 

 trunks accompanying the main ducts and joining the main 

 vessels which pass to the gland directly from the chief subcutane- 

 ous vessels. 



During the early stages of pregnancy the ducts expand rapidly 

 and their ramification increases (fig. 5). If the territory sup- 

 plied by a single main duct and its various branches be called 

 a lobe it soon becomes impossible on mere gross inspection to 

 distinguish one lobe clearly from another owing to the inter- 

 digitation of branches from neighboring lobes. The territories 

 of the chief branches of the main ducts similarly become difficult 

 to distinguish clearly as sub-lobes. As pregnancy advances the 

 ducts of the smaller compound units of structure, the lobules, 

 begin to appear on the ends and sides of the main ducts and 

 their branches. The first lobular ducts to appear may arise 

 from a plane perpendicular to the skin on both the superficial 

 and deep surfaces of the ducts. The lobular ducts may be simple 

 or compound. The simple ducts arise as solid bulbular proc- 

 esses which elongate and become hollowed out. The sides 

 of these ducts become studded with knob-like processes which 

 later expand into alveoli. The compound lobular ducts give 

 rise to branches of one or more orders and these, in turn, become 

 studded by alveoli. 



Meanwhile, in the nipple region the necks of the ducts become 

 much enlarged to form the lacteal sinuses, although in sections 

 the walls appear irregular and collapsed. The mouths of the 

 ducts, hitherto expanded and funnel-shaped, become greatly 

 constricted. At the base of the nipple the ducts are likewise 

 constricted but beyond here they again dilate. 



At first the vascular development proceeds essentially along 

 the lines described in the growing animal. The capillary plexuses 

 about the ducts, however, become much more richly developed 

 than before and the development of arterioles and venules in 



