THE DEVELOPMENT OF THE VASCULAR SYSTEM. 



219 



peroneal artery. The branch of the original sciatic, which was given off just 

 below the knee, unites with the branch which was given off just above the 

 ankle to form a continuous vessel, the anterior tibial artery. A new branch 

 arises from the proximal portion of the peroneal, extends down the back of 

 the leg, and unites with the distal part of the embryonic saphenous to 

 form the posterior tibial artery. The proximal part of the saphenous then 

 atrophies, leaving but one of the small genu branches of the popliteal (Fig. 

 190) (McMurrich). 



Ant. tibial artery 



| Peroneal artery 



j Post, tibial artery 



"A B 



FIG. 190. Diagrams showing three stages in the development of the arteries 

 of the lower extremity. McMurrich. 



The Veins. The changes which occur during the development of the 

 venous system are so complicated, and in some cases so varied, that the scope 

 of this book permits only a brief outline of the growth of the more important 

 of the venous trunks. 



Corresponding to the arterial system, the first veins to appear are the 

 omphalomesenteric veins. These vessels, which carry blood from the yolk sac 

 to the heart, arise in the area vasculosa, enter the embryonic body at the sides 

 of the yolk stalk, pass cranially along the intestinal tract, and join the caudal 

 end of the heart (Figs. 160, 162, 164 and 193). Next in point of time to ap- 

 pear are the umbilical veins which carry back to the heart the blood which 

 has been carried to the placenta by the umbilical arteries. These also are 

 paired veins within the embryo, although they form a single trunk in the 

 umbilical cord. They extend cranially on each side through the ventro- 

 lateral part of the body wall and join the duct of Cuvier (see below) in the 

 septum transversum (Figs. 163, 164 and 193). Very soon after the appear- 

 ance of the umbilical veins two other longitudinal vessels develop, one on 



