522 H. M. WAHL 



these plexuses becomes much more active, the differentiation of 

 the veins of the ducts continuing as before to exceed that of the 

 arteries. 



The subcutaneous vessels in the region of the gland and all 

 of the vessels supplied to the gland meanwhile increase rapidly 

 in size. Not only do the vessels enlarge in diameter, but they 

 also elongate with the outward growth of the ducts and send 

 rami to the branches which arise from the ducts. The new ducts 

 are supplied in part by branches thus derived from the vessels 

 of the parent ducts and in part by new vessels developed in the 

 capillary plexus of the surrounding stroma. As mentioned 

 above, there is a greater tendency for the new veins than the 

 arteries to be derived from the preexisting duct vessels. 



In the latter part of pregnancy the chief interest lies in the 

 development of the vascular supply of the glandular lobules. 

 At first the anlage of the lobule is surrounded by a network of 

 capillaries continuous with the capillaries of the duct. As the 

 lobular anlage enlarges the surrounding capillary plexus becomes 

 more richly developed and one or more arterioles and venules 

 are developed between it and the arteries and veins of the ducts. 

 As the alveolar ducts develop at the periphery of the lobular 

 ducts the same process is repeated, venules and arterioles for the 

 alveoli being developed between the capillary plexus about the 

 alveoli and the vessels of the lobule (fig. 6). Frequently, how- 

 ever, the arterioles, and less frequently the venules, of the capil- 

 lary plexus join vessels which approach the lobule from the side 

 opposite the duct. Both the veins and the arteries may extend 

 into a lobule from the extremity opposite the duct. The venules 

 and arterioles supplied to the alveoli both however, almost 

 invariably extend from the neck of the alveolus outwards into 

 the capillary plexus surrounding it. 



During lactation the gland still further increases in size owing 

 to the expansion of the existing alveoli and the formation of new 

 ones. Between the expanded secreting alveoli the connective 

 tissue becomes very slight in amount. The ducts beiome 

 greatly distended with milk. There is a great elaboration and 



