THE PEIMITIVE VASCULAE SYSTEM. 



69 



of the umbilicus, where they enter the septum transversum, in which each vitelline 

 vein joins the corresponding umbilical vein, forming a common vitello-intestinal 

 trunk, which enters the sinus venosus (Fig. 81). 



This trunk also receives the primitive head vein, or anterior cardinal vein, which 

 returns the blood from the cranial part of the embryo (Fig. 82). 



Internal jugular veiir 

 External jugular vein- 



Vertebral artery 

 Left innominate vein - - 

 Subclavian artery 

 Subclavian vein 



Right pulmonary artery--*^. 

 Superior vena cava 



Right atrium - 1 



Vena azygos 

 Right ventricle 



Inferior vena cava, 

 vitelline vein portion 



Inferior vena cava, down- 

 growth from vitelline vein' 

 Right and left branches, 

 of portal vein 



Portal vein 



Remains of vitelline vein s 

 Inferior vena cava 

 (subcardinal party 



Right renal vein 

 Right lumbar vein 



1st aortic arch 

 Internal carotid 



2nd aortic arch 



External carotid 

 Internal carotid 



-f- --Arch of aorta 

 ^^Left subclavian artery 



.,! Left subclavian vein 



- -Ductus arteriosus 



^~ * "Pulmonary artery 



"^"Left superior intercostal vein 

 Left atrium 



Left ventricle 



Accessory hemiazygos vein 







Hemiazygos vein 

 Aorta 



Coeliac artery 



Spleen 

 Splenic vein 



Superior mesenteric vein 

 .-Superior mesenteric artery 



-Kidney 



--Left renal vein 

 Umbilical vein 



->Left lumbar vein 



Placenta 



Umbilical arteries 



Inferior mesenteric artery ' 



Common iliac artery* 

 External iliac artery 



Umbilical artery - 



FIG. 88. DIAGRAM OF THE FCETAL CIRCULATION. 



A little later two veins are formed, one on each side, which return blood from 

 the body wall and the primitive limbs. They are the posterior cardinal veins, and 

 as soon as they are established they join the caudal ends of the anterior cardinal 

 veins to form the ducts of Cuvier, which then open directly into the posterior part 

 of the heart which is called the sinus venosus (Fig. 83). Shortly afterwards the 

 common stems of the vitello-umbilical veins are absorbed into the sinus venosus, 

 forming its right and left horns. When this has happened six veins open into 



