268 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



become the free arm is plexiform at first, but later becomes a single stem which forms 

 successively the subclavian, axillary, brachial, and interosseous arteries. Subsequently 

 the median, radial, and ulnar arteries of the arm are formed. 



Lower Extremity. In embryos of 7 mm. there is given off from the secondary lateral 

 trunk of the umbilical artery (i. e., from the future common iliac) a small branch which 

 forms the chief arterial stem of the lower extremity, the future popliteal and peroneal arter- 

 ies. This, the arteria ischiadica, is superseded in embryos of 15.5 mm. by the external 

 iliac and femoral arteries, of which the latter annexes the branches of the ischiadic distal to 

 the middle 'of the thigh. The arteria ischiadica persists proximally as the inferior gluteal 

 artery. 



DEVELOPMENT OF THE VEINS 



We have seen that in embryos of 23 somites three systems of paired 

 veins are present: the umbilical veins from the chorion, the vitelline veins 

 from the yolk sac, and the anterior and posterior cardinal veins, which 

 unite in the common cardinal veins, from the body of the embryo. Thus, 

 three veins open into the right horn of the sinus venosus, and three into 

 the left (Fig. 270). 



Changes in the Vitelline and Umbilical Veins. Vena portoe. With 

 the increase in size of the liver anlages there is an intercrescence of the 

 hepatic cords and the endothelium of the vitelline veins. As a result, these 

 veins form in the liver a network of sinusoids (Fig. 279), and each vein is 

 divided into a distal portion which passes from the yolk sac to the liver, 

 and into a proximal portion which carries blood from the liver sinusoids to 

 the sinus venosus. Soon the proximal portion of the left vitelline vein is 

 largely absorbed into the sinusoids of the liver and shifts its blood flow into 

 the right horn of the sinus venosus. In the meantime the liver tissue 

 grows laterally, comes into contact with the umbilical veins, and taps 

 them so that their blood flows more directly to the heart through the 

 sinusoids of the liver (Fig. 280). As the channel of the right proximal 

 vitelline is larger, the blood from the left umbilical vein flows diagonally 

 to the right horn of the sinus venosus. When all the umbilical blood 

 enters the liver, as in embryos of 5 to 6 mm., the proximal portions of the 

 umbilical veins atrophy and disappear (Fig. 281). In 5 mm. embryos the 

 vitelline veins have formed three cross anastomoses with each other (Figs. 

 280 and 281) : (i) a cranial transverse connection in the liver, ventral to 

 the duodenum; (2) a middle one, dorsal to the duodenum; and (3) a 

 caudal one, ventral to it. There are thus formed about the gut a cranial 

 and a caudal venous ring. In embryos of 7 mm. the left umbilical vein 

 has enlarged, while the corresponding right vein has degenerated. Of the 

 two venous loops, only the right limb of the cranial ring and the left limb of 

 the caudal ring, together with the median dorsal anastomosis, persist. * A 

 new vein the superior mesenteric, develops in the mesentery of the intes- 

 tinal loop and joins the left vitelline vein just caudal to its dorsal middle 



