856 



DEVELOPMENT 



[CH. LIX. 



together; thus two loops are formed through which the duodenum 

 passes. The veins from the intestine open into the fused trunks, and 

 the splenic vein enters the left vein at the level of the lower transverse 

 anastomosis. Subsequently the left side of the upper and the right 

 side of the lower loop disappear, and the portal vein is produced from 

 the remains. In the meantime the right umbilical vein has disappeared, 

 and the left has united with the left omphalo-mesenteric vein at the 

 point where the latter ends in the left vena advehens. From this 



21 



ater stage iu the development of the veins than that shown in tig. 



FIG. 654.-D: .. 



053. 1, Primitive jugular vein ; 2, duct of Cuvier ; 3, upper part of cardinal vein, now vena azygos 

 major; 5, remains' of light lower limb of loop formed by fusion of ornphalo-mesenteric veins;" 6, 

 common umbilical vein ; 7, sinus venosus ; 8, liver ; 9, left branch of umbilical vein ; 10, right 

 hepatic vein ; 11, left hepatic vein ; 12, right vena advehens ; 13, left vena advehens ; 14, upper 

 part of inferior vena cava ; 15, right renal vein ; 16, lower part of inferior vena cava (cardinal vein) ; 

 17, fused part of omphalo-mesenteric vein ; 18, vein from alimentary canal ; 19, splenic vein ; 20, 

 remains of left upper limb of loop formed by fusion of omphalo-mesenteric veins ; 21, ductus 

 venosus. 



point a direct channel opens up beneath the liver to the upper part of 

 the inferior vena cava ; this is the ductus venosus, and it conducts the 

 greater part of the oxygenated blood from the umbilical vein directly 

 to the inferior vena cava, and so to the right auricle ; but part of the 

 umbilical blood passes into the liver with the omphalo-mesenteric 

 blood. 



A pulmonary vein forms and carries blood from the lungs to the 

 left auricle. It is subsequently replaced first by two veins, one from 

 each lung, and afterwards four veins, two from each lung. 



