278 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



passes out by the pulmonary artery. A small amount of this blood is 

 conveyed to the lungs by the pulmonary arteries, but, as the fetal lungs 

 do not function, most of it enters the dorsal aorta by way of the ductus 

 arteriosus. Since the ductus is caudal to the origin of the subclavian and 

 carotid arteries, its less pure blood is distributed to the trunk, viscera, and 

 lower extremities. The placental circuit is completed by the hypogastric, 

 or umbilical arteries by way of the umbilical cord. 



Pohlman (Anat. Rec., vol. 2, 1908) interprets his experiments to indicate that, con- 

 trary to the generally accepted view, there is a mingling of the blood which enters the right 

 atrium through the two caval veins. If this occurs, there would be no difference in the 

 quality of blood distributed to the various parts of the body. 



Changes at Birth. At birth the placental circulation ceases and the 

 lungs become functional. The umbilical' arteries and veins, no longer used, 

 contract and their lumina are obliterated by the thickening of the inner 

 coat (tunica intima). The lumen of the umbilical artery is said to be 

 occluded after four days, that of the umbilical vein within a week. The 

 cord-like vein is persistent as the ligamentum teres of the liver ; the arteries 

 become the obliterated hypogastrics. 



The ductus venosus atrophies because after birth only the blood from 

 the portal vein enters the liver, and this is all drained into the liver sinu- 

 soids, forming the portal circulation. The ductus venosus is obliterated 

 within two months and becomes the fibrous ligamentum venosum, em- 

 bedded in the wall of the liver. 



The ductus arteriosus ceases to function after birth, as all the blood 

 from the pulmonary arterial trunk is conveyed to the expanded lungs. 

 In most cases the ductus becomes impervious within four months after 

 birth and persists as a solid, fibrous cord, the ligamentum arteriosum. 



After birth, the large amount of blood now returned to the left 

 atrium from the functional lungs equalizes the pressure in the two atria 

 (p. 256). As a result, both during diastole and systole, the septum 

 primum, or valve of the foramen ovale, is pressed against the septum 

 secundum, closing the foramen ovale. Eventually, the two septa fuse- 

 in one-third of all cases within three months, in two-thirds by the tenth 

 year (p. 259). 



THE LYMPHATIC SYSTEM 



The lymphatic system originates in a plexus of lymphatic capillaries 

 distributed along the primitive main venous trunks. By the dilation 

 and coalescence of this network at definite regions five lymph sacs appear 

 (Fig. 287). Paired jugular sacs arise in 10 to n mm. embryos, lateral to 

 the internal jugular veins. In embryos of 23 mm. the unpaired retro- 

 peritoneal sac develops at the root of the mesentery, adjacent to the supra- 



