1194 



EMBRYOLOGY. 



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

 may be best considered under two groups, visceral and parietal. 



The visceral veins are the two vitelline or omphalo-mesenteric veins bringing 

 the blood from the yolk-sac, arid the two umbilical or allantoic veins returning 

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

 venosus (Fig. 747). 



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 

 before they reach the liver, and then encircle the intestinal tube by forming 

 around it two venous rings, the first on its dorsal, the second on its ventral aspect. 

 The portions of the veins above the upper ring become invaded by the developing 



vagus nerve 



common 

 carotid 

 recurrent 

 laryngeal 

 nerve 



right, 

 subclavian 



pulmonary 

 trunk 



descending aorta 



FIG. 752. Showing the destination of the arterial arches in man and mammals. (Modified from Rathker.) 

 {From Quain's Anatomy, 1890, vol. i., pt. 1.) The truncus arteriosus and the five arterial arches springing from 

 it are represented in outline only; the permanent vessels in shade those belonging to the aortic system in 

 heavy shaded line, to the pulmonary system in light shaded line. 



liver and broken up by it into a network of smaller vessels, the central part 

 of the network consisting of a capillary plexus. The branches which convey 

 the blood to this plexus are named the vence advehentes, and become the 

 branches of the portal vein ; while the vessels which drain the plexus into the 

 sinus venosus are termed the vence revehentes, and form the future hepatic veins 

 (Figs. 753 and 754). 



The lower part of the portal vein is formed from the fused vitelline veins 

 which receive the veins from the alimentary canal ; its upper part is derived from 

 the venous rings by the persistence of the left half of the lower and the right 

 half of the upper ring, so that the vessel forms a spiral turn round the duodenum 

 (Fig. 754). 



The two umbilical veins fuse early to form a single trunk in the allantois, but 

 remain double for some time in the embryo and pass forward to the sinus venosus 

 in the side walls of the body. Like the vitelline veins, their direct connection 

 with the sinus venosus becomes interrupted by the invasion of the liver, and thus 

 at this stage the whole of the blood from the yolk-sac and placenta passes through 

 the substance of the liver before it reaches the heart. The right umbilical vein 



