DEVELOPMENT OF THE VEINS 



271 



originate. Later there are differentiated first four, then six, such opposed trunks within the 

 liver, and the six primary lobules supplied and drained by these trunks may be recognized in 

 the adult liver. 



Of the umbilical veins the right disappears early; the left persists during 

 fetal Ufe, shifts to the median line, and courses in the free edge of the falciform 

 hgament. After birth its lumen is closed and from the umbilicus to the liver it 

 forms the ligamentum teres. In early stages veins from the body wall drain into 

 the umbiHcal veins. 



The Anterior Cardinal Veins and the Origin of the Superior Vena Cava. — 

 The anterior cardinal veins consist each of two parts (Fig. 271): (1) The true 

 anterior cardinals, located laterad in the segmented portion of the head and neck 



Efferent Common Ductus Efferent ' 



' : vein hepatic win venosus Pancreas hepatic vein 



R. umbilical vein- 



Stomach-xf^i 

 |i 

 Afferent hepatic vein 



Portal vein 



L. umbilical vein 



\~- Liver 

 i .' 



'^^&i-=i~S it~ Afferent hepatic vein 



Obliterated L. vitelline vein 



R. umbilical vein 



Vitelline i umbilical vein 

 vein 



Fig. 281. — A diagram showing the development of the portal vein as illustrated in a human embryo of 



about 7 mm. (modified after His). 



and draining into the common cardinal veins; (2) the vena capitis medialis, ex- 

 tending into the unsegmented head proper and running ventro-lateral to the 

 brain wall. In embryos of 20 mm. there has formed by anastomosis a large con- 

 nection between the right and left anterior cardinals, which carries the blood 

 from the left side of the head into the right vein (Fig. 282 C). Soon the left 

 anterior cardinal loses its connection with the common cardinal on the left side 

 (Fig. 282 D). The proximal portion of the left common cardinal, with the trans- 

 verse portion of the sinus venosus, persists as the coronary sinus. The right com- 

 mon cardinal and the right anterior cardinal vein as far as its anastomosis with 

 the left anterior cardinal become the superior vena cava. The anastomosis itself 

 forms- the left vena anonyma, while that portion of the right anterior cardinal be- 



