THE VASCULAR SYSTEM 



379 



into the foot and ends in the five digital arteries of the toes. From the 

 femoral artery, a saphenous artery grows diagonally across the leg and by 

 making connexion with the digital vessels supplants the ischiadic vessel, 

 which now becomes the peroneal artery of the calf. As a consequence of 

 the union of the saphenous artery with the ischiadic, the latter partly 

 atrophies. In the knee region, the saphenous becomes the popliteal 



duct of Cuviep 



Fig. 319. — Diagrams showing the development of the hepatic portal circulation from 

 the omphalomesenteric veins, and the relations of the umbilical veins to the liver. 

 A, is based on conditions in pig embryos of 3-4 mm.; B, on embryos of about 6 mm.; 

 C, on embryos of 8-9 mm.; D, on embryos of 20 mm. and older. (From Patten's 

 "Embryology of the Pig," adapted from several sources.) 



artery, and in the leg, the tibialis posterior artery. By an outgrowth of 

 the popHteal artery, the tibialis anterior artery is formed. 



Development of the Veins. At the time of its first appearance in a 

 2 mm. embryo, the sinus venosus receives blood from two pairs of veins, 

 the vitelline veins from the yolk-sac and the umbilical veins from the 

 chorionic villi of the placenta. A third pair of veins connected with the 

 sinus appear in a 2.6 mm. embryo. These veins are the common cardinals, 



