520 H. M. WAHL 



of the ducts. The peripheral vessels also send branches into 

 the integument surrounding the nipple. For each of the larger 

 arterial rami there is usually a vein which takes a corresponding 

 course but does not always run close to the artery. About the 

 free extremities of the ducts as they extend outwards from the 

 nipple region a rich capillary plexus is developed. Near the tips 

 of the ducts this capillary plexus is continuous with that of the 

 surrounding stroma, but toward the nipple it exhibits a greater 

 independence. As this independence is established there become 

 developed in the capillary plexus of the stroma arterioles and 

 venules which supply the capillary plexus of the ducts. First 

 an arteriole is differentiated, then a venule, so that the capillary 

 plexus of the ducts is supplied alternately by arterioles and ven- 

 ules as one passes from the nipple region outwards. At the grow- 

 ing tips of the ducts one usually finds either a developing arte- 

 riole or a developing venule. The branches which supply these 

 arterioles and venules arise directly from the main subcutaneous 

 vessels, and from the overlying cutaneous or from the under- 

 lying muscular vascular plexuses. The chief venous and arterial 

 rami supplying the venules and arterioles to a given part of the 

 duct may parallel one another or the venule may come from a 

 direction different from that of the arteriole. 



In the fullgrown virgin rabbit (fig. 4) the ducts have ramified 

 out extensively from the nipple region. The glands are from 

 2 to 3 cm. in diameter. The ducts branch in a plane parallel 

 with the surface of the body but no branches project perpendicular 

 to this plane. The blood supply of the nipple region corresponds 

 essentially with that described for the rabbit at two months, but 

 the plexus about the necks of the ducts is more fully developed. 

 As a rule, the terminal venules and arterioles enter the plexus 

 on opposite sides of a given segment of the duct. The arteries 

 and veins distributed to the free parts of the ducts are far more 

 extensively developed than at the preceding stage, but other- 

 wise are similar in origin and distribution. As the ducts extend 

 outwards branches from the superficial cutaneous vessels and the 

 superficial muscular plexus are called upon more and more for 

 a vascular supply, although the chief subcutaneous vessels also 



