276 



THE DEVELOPMENT OF THE VASCULAR SYSTEM 



plcxiform at first, and later becomes a single stem which forms successively the subclavian, 

 axillary, brachial, and interosseous arteries. Later, in the arm are formed the median, radial 

 and ulnar arteries. For details as to their arrangement students are referred to textbooks of 

 anatomy. 



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

 lateral trunk of the umbilical artery a small branch which forms the chief stem of the vascular 

 plexus in the lower extremity. This, the arteria ischiadica, is superseded in embryos of 15.5 

 mm. by the external iliac and femoral arteries. The arteria ischiadica persists as the inferior 

 gluteal. We have already seen that the secondary lateral root of the umbilical artery becomes 

 the common iliac. 



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 cardinal veins, anterior and posterior, which unite in the common 



Atrium 



Com. cardinal vein 



Sinusoids of liver 



R. vitelline vein 



Ventricle 



L. umbilical vein 



L. vitelline vein 



Fig. 271. — Reconstruction of the veins and arterial arches of a 4.2 mm. embryo in ventral view (His). 



cardinal veins, from the body of the embryo. Thus three veins open into the 

 right and three into the left horn of the sinus venosus (Fig. 263). 



Changes in the Vitelline and Umbilical Veins.— Vena porta. — With the in- 

 crease in size of the liver anlages there is an intercrescence of the hepatic cords 



