766 



THE VASCULAR SYSTEMS 



(page 764). The middle sacral artery of the adult was formerly regarded as the direct con- 

 tinuation of the adult aorta, but Young and Robinson 1 maintain that it is a secondary branch 

 probably representing fused segmental arteries. The caudal continuations of the aorta- in the 

 adult are the common iliac, internal iliac, and the partially impervious hypogastric arteries. 



The hypogastric arteries are continued into the umbilical cord as the umbilical arteries. After 

 birth they become impervious cords from the umbilicus as far as the origin of the superior veical 

 arteries. 



The primary arterial stem for the lower limb is formed by the sciatic artery, which accom- 

 panies the great sciatic nerve along the posterior aspect of the thigh to the back of the knee, 

 where it is continued as the peroneal artery. The femoral artery arises later as a branch of the 

 common iliac, and, passing down the front and inner side of the thigh to the bend of the knee, 

 joins the sciatic artery. The femoral quickly enlarges, and coincidently with this the part of 

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

 arteries are branches of the main arterial stem. 



Right primitive jugular rein. 

 Right cardinal vein.~.. 



Right duct of Cuvier.-, 



Sinus venosus.--. 

 Right hepatic vein.- -, 



Portal vein... 



Left primitive 

 .'"jugular vein. 



Portal vein 



Right umbilical vein.. 



Umbilical cord. _./.._ 



cardinal vein. 



'Left duct of Cuvier. 



-Left hepatic vein. 

 -~Left umbilical veins. 



Left umbilical vein. 



FIG. 545. Human embryo with heart and anterior body wall removed to show the sinus venosus and 

 its tributaries. (After His, from Kollmann's Entwickelungsgeschichte.) 



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 are the two vitelline or omphalomesenteric veins bringing the blood from 

 the yolk sac, and the two umbilical or allantoic veins returning the blood from the placenta; 

 these four veins open close together into the sinus venosus (Fig. 547). 



The vitelline veins run cephalad 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 cross branches, the first on the dorsal, the second on the ventral aspect of the 

 duodenal portion of the intestine, which is thus encircled by two venous rings (Fig. 546). The 

 portions of the veins above the upper ring become invaded by the developing liver and broken 

 up by it into a plexus of small capillary-like vessels termed sinusoids (Minot). The branches 

 conveying the blood to this plexus are named the venae advehentes, and become the branches of the 



1 Journal of Anatomy and Physiology, vol. xxxii. 



