268 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



vessels of the vitelline series (Fig. 270), and are gradually shifted caudalward until 

 they arise from the dorsal aorta opposite the twenty-third segment (fourth lum- 

 bar). In 5 mm. embryos the umbihcal arteries develop secondary lateral con- 

 nections with the aorta (Fig. 278 A). The new vessels pass lateral to the mes- 

 onephric ducts, and, in 7 mm. embryos, the primitive ventral stem-artery has 

 disappeared. The segment of this new trunk, proximal to the origin of the exter- 

 nal iliac artery which soon arises from it, becomes the common iliac. The re- 

 mainder of the umbilical trunk constitutes the hypogastric artery. When the 

 placental circulation ceases at birth, the distal portion of the hypogastric arteries, 

 from pelvis to umbilicus, atrophy, forming the soHd obliterated hypogastric arteries 

 of adult anatomy. 



The middle sacral artery is the direct caudal continuation of the aorta. Its 

 dorsal position is the result of secondary growth changes. 



Arteries of the Extremities. — It is assumed that in man, as in observed birds and 

 mammals, the first vessels of the limb buds form a capillary plexus. 



Upper Extremity. — The capillary plexus takes origin by several lateral branches from 

 the aorta. In human embryos of 5 mm. but one connecting vessel remains and this takes its 

 origin secondarily from the seventh dorsal intersegmental artery, forming the ventral ramus 

 of this artery and its lateral offset (Fig. 274). The portion of this vessel in what will 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. 



Arteries of the Lower Extremity. — In embryos of 7 mm. there is given ofi from the second- 

 ary lateral trunk of the umbUical artery (i. e. from the future common ihac) a small branch 

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

 arteries. This, the arteria ischiadica, is superseded in embryos of 15.5 mm.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 cardinal veins, anterior and posterior, which unite in the commcn 

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

 right horn, and three into the left horn, of the sinus venosus (Fig. 270). 



Changes in the Vitelline and Umbilical Veins.— Feno portce.~Wi\h the in- 

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

 and the endothelium of the viteUine 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 wlrich 



