524 H, M. WAHL 



enrichment of the vascular supply of the gland, but the essential 

 features are as above described (fig. 6) . 



Within two weeks after weaning the gland appears much 

 thinner than during lactation owing chiefly to disappearance of 

 milk from the ducts and to the retrograde metamorphosis which 

 becomes well marked first in the alveoli and later in the ducts. 

 The alveoli first shrink in size, then the alveolar cells degenerate 

 and are absorbed, but usually a duct stem remains as a small 

 group of epithelial cells with merely a small lumen or no lumen. 

 The stroma appears relatively greatly increased as the alveoli 

 disappear. Different parts of the same gland undergo quite 

 unequal retrograde metamorphosis. The alveoli of one duct may 

 disappear, while along another duct they may appear to be still 

 in the secreting stage. Alveoli of this kind may persist for at 

 least several months after lactation. The walls of the main ducts 

 and their branches first collapse as the contained milk secretion 

 is absorbed and then gradually shrink in size. 



As the alveoli are absorbed the surrounding capillaries dis- 

 appear so that in the lobule the relatively thick-walled venules 

 and arterioles seem disproportionately large compared with the 

 capillary field which they supply. The capillaries about the 

 ducts likewise in part disappear. The various arteries and veins 

 of the gland appear for a time tortuous and shrunken, but gradu- 

 ally they come to resemble more and more the vessels of the gland 

 of the virgin adult. Several characteristic differences, however, 

 remain. The ducts are much longer and more ramified than 

 in the virgin animal and the number of veins and arteries supply- 

 ing them is greater. The larger arteries are more regularly 

 accompanied by veins and the large veins by arteries. 



To sum up briefly: The blood supply of the gland during 

 development and rest appears in the main to be secondary to 

 the blood supply of the skin and the subcutaneous muscles, but 

 during functional activity it becomes more independent, the 

 blood supply of the alveoli being connected with the vessels of 

 the ducts and to a large extent, at least, independent of that of 

 the stroma. The irregularity in the retrograde metamorphosis 

 of the gland and the changes in the blood supply are suggestive 

 from the standpoint of cancer formation. 



