ioo8 



A MANUAL OF ANATOMY 



transverse anastomotic vessels, two of which, lower and upper, lie across 

 the ventral aspect of the duodenal gut, and one — middle — dorsal to it. A 

 double venous ring {sinus annularis) is now formed, the rings being known 

 as lower or caudal and upper or cephalic, and these embrace the duodenal 

 gut. The portions of the tw^ vitelline veins above the upper venous ring 

 cease to be connected directly with the sinus venosus, owing to the develop- 

 mental changes which are now taking place in the hepatic diverticulum or 

 liver-bud. They are invaded l5y the hepatic cylinders, and are freely sub- 

 divided into sinusoids. The veins which carry the blood from the upper 

 venous ring to these sinusoids are called the vencs hepatica; advehentes, and 

 the veins which carry the blood from the sinusoids to the sinus venosus are 



Fig. 417.— The Venous Trunks of the Human Embryo (His). 



P.J.V. Primitive Jugular Vein X.X. Upper separated portions of Umbilical Veins 



C.V. Cardinal Vein R.U.V. Right Umbilical Vein 



R.S.V.C. Right Superior Vena Cava L.U.V. Left Umbilical Vein 



L.S.V.C. Left Superior Vena Cava V.P. Vena Portse 



V.V. Vitelline Veins 



called the vencB hepatices revehentes, the latter giving rise to the hepatic veins 

 of adult life. 



A summary of the portal vein has just been given. The initial portions of 

 the vitelline veins atrophy. 



The vitelline veins thus give rise to (i) the portal vein and its ramifications, 

 and (2) the hepatic veins. 



Umbilical or Allantoic Veins. — The two umbilical veins return the blood 

 from the placenta to the sinus venosus. They are of small size during the 

 period of the vitelline circulation, but become enlarged as the yolk-sac atrophies 

 and as the allantois undergoes increase in size. The two veins unite and 

 form a single trunk within the umbilical cord. At the umbilicus this trunk 

 enters the body of the embryo, and immediately divides into two umbilical 

 veins, right and left, which traverse the septum transversum and open into 

 the sinus venosus. As they traverse the septum transversum they are 

 intimately related to the developing liver. 



The two umbilical veins soon lose their connection with the sinus venosus, 

 and they are now subdivided, like the vitelline veins, into hepatic sinusoids 



