THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



231 



portal vein is thus a derivative of the omphalomesenterics. After birth, 

 when the placental blood is cut off, blood is distributed in the liver by 

 branches of the portal vein, which represent the advehent hepatic veins; 

 it is collected again by branches which unite to form the revehent hepatic 

 veins, or hepatic veins proper, and the latter open into the inferior vena 

 cava. The advehent and revehent hepatic veins are formed by the 

 enlargement of some of the original sinusoids (Figs. 199 and 201). 



Observations on the development of the veins in the extremities of human 



Ant. cardinal 

 (int. jugular) 



Post, cardinal 



Sinus venosus and 

 orifice of ductus venosus 



Revehent hepatic 



Advehent hepatic 



Right umbilical 



Omphalomesenteric 

 (portal) 



Umbilical vein 



Ant. cardinal 

 (int. jugular) 



Post, cardinal 

 Bronchus 



Revehent hepatic 

 Advehent hepatic 



Left umbilical 

 Umbilical cord 



FIG, 201. Veins in the liver region of a human embryo of 10 mm. Kollmann's Atlas. 



embryos are so fragmentary that it seems advisable to make use of the work 

 that has been done on the rabbit. In the upper extremity the first vein to 

 develop is the primary ulnar vein which begins in the radial (cranial) side of 

 the extremity near its proximal end, extends distally along the radial border, 

 thence proximally along the ulnar (caudal) border, and opens into the 

 anterior cardinal vein (internal jugular) near the duct of Cuvier (Fig. 203). 

 This condition is present in rabbit embryos of thirteen days. A little later a 

 second vessel, the cephalic vein, appears as a branch of the external jugular, 



