518 



ANGIOLOGY 





arterial stem for the upper arm, the direct continuation of this stem in the forearm 

 is the volar interosseous artery. A branch which accompanies the median nerve 

 soon increases in size and forms the main vessel (median artery) of the forearm, 

 while the volar interosseous diminishes. Later the radial and ulnar arteries are 

 developed as branches of the brachial part of the stem and coincidently with their 

 enlargement the median artery recedes; occasionally it persists as a vessel of some 

 considerable size and then accompanies the median nerve into the palm of the hand. 

 The primary arterial stem for the lower limb is formed by the inferior gluteal 

 (sciatic) artery, which accompanies the sciatic nerve along the posterior aspect of 

 the thigh to the back of the knee, whence it is continued as the peroneal artery. 

 This arrangement exists in reptiles and amphibians. The femoral artery arises 

 later as a branch of the common iliac, and, passing down the front and medial 

 side of the thigh to the bend of the knee, joins the inferior gluteal artery. The 

 femoral quickly enlarges, and, coincidently with this, the part of the inferior gluteal 

 immediately above the knee undergoes atrophy. The anterior and posterior tibial 

 arteries are branches of the main arterial stem. 



Anterior detacfod portions 

 of umbilical veins 



Stomach 

 Venae advehentes 

 Pancreas 

 Bile-diict 



Obliterated portions 

 of venous rings 



Right umbilical vein 



Ductus venosus 



Liver 



Left umbilical vein 



Duodenum/ 



FIG. 475. The liver and the veins in connection with it, of a human embryo, twenty-four or twenty-five days old, 



as seen from the ventral surface. (After His.) 



Further Development of the Veins. The formation of the great veins of the 

 embryo may be best considered by dividing them into two groups, visceral and 

 parietal. 



The Visceral Veins. The visceral veins are the two vitelline or omphalomesenteric 

 veins bringing the blood from the yolk-sac, and the two umbilical veins returning 

 the blood from the placenta; these four veins open close together into the sinus 

 venosus. 



The Vitelline Veins run upward at first in front, and subsequently on either 

 side of the intestinal canal. They unite on the ventral aspect of the canal, and 

 beyond this are connected to one another by two anastomotic branches, one on the 

 dorsal, and the other on the ventral aspect of the duodenal portion of the intestine, 

 which is thus encircled by two venous rings (Fig. 475); into the middle or dorsal 

 anastomosis the superior mesenteric vein opens. The portions of the veins above 

 the upper ring become interrupted by the developing liver and broken up by it into 



