286 THE DEVELOPMENT OF THE VASCULAR- SYSTEM 



bilical arteries, which pass from the common iliac arteries by way of the 

 umbilical cord to the placenta. 



Changes at Birth. — At birth the umbilical vessels are ruptured 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 ul- 

 tima). The lumen of the umbilical arteries is occluded after four days, that of 

 the umbilical vein within a week. The wall of the vein is persistent as the liga- 

 mentum teres of the liver. 



The ductus venosus atrophies because after birth only the blood from the por- 

 tal vein enters the liver, and this is all drained into the liver sinusoids, forming the 

 portal circulation. The ductus venosus is persistent as the fibrous ligamentum ve- 

 nosum, embedded 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. The ductus be- 

 comes impervious from ten to twenty days after birth and persists as a solid 

 fibrous cord. 



The foramen ovale does not function after birth, as the large amount of blood 

 returned to the left atrium from the now functional lungs equalizes the pressure 

 in the two atria. As a result, both during diastole and systole, the septum pri- 

 mum, or valve of the foramen ovale, is pressed against the septum secundum, clos- 

 ing the foramen ovale. Eventually the two septa fuse, though they may be 

 incompletely united during the first year after birth, or even longer. 



The Lymphatic System 



The development of the lymphatics is, according to Sabin (Keibel and Mall, 

 vol. 2, p. 709), divided into two stages: (1) the development from the veins of 

 isolated lymph-sacs, which become united with each other by the thoracic duct 

 and acquire a secondary opening into the veins at the jugular valves; (2) the peri- 

 pheral outgrowth of lymphatic vessels as endothelial sprouts from the lymph-sacs. 



In 10 to n mm. embryos appear the jugular sacs lateral to the internal jugu- 

 lar veins and derived from them, first as a plexus of capillaries which becomes iso- 

 lated, but later rejoins the vein, forming a valve at the opening. In 23 mm. em- 

 bryos the retro-peritoneal sac (F. T. Lewis, 1901-2) appears at the root of the mes- 

 entery adjacent to the suprarenal bodies and caudal to the superior mesenteric 

 artery. It is developed from a capillary plexus arising from the neighboring 

 veins. Posterior lymph-sacs are developed from the v. ischiadica. 



