708 MANUAL OF PHYSIOLOGY. 



left innominate vein. The right duct of Cuvier and the right 

 jugular below the entrance of this cross branch, forms the supe- 

 rior vena cava ; whilst the part of the right primitive jugular 

 immediately above the entry of the left innominate vein, forms 

 the right innominate vein. 



The posterior vertebral vein of the right side forms the vena 

 azygos major ; the corresponding branch of the opposite side, 

 together with the part of the left primitive jugular below the 

 cross branch, forms the left superior intercostal vein and the supe- 

 rior vena azygos minor. The lower part of the left posterior 

 vertebral vein, together with the connecting branch to the right 

 vein, remain as the inferior vena azygos minor. 



Foetal Circulation. The course taken by the blood through 

 the heart and vessels of the embryo differs essentially from that 

 which persists in adult life. 



Tracing the blood from the placenta, it passes along the um- 

 bilical vein towards the liver, here it may take either of two courses 

 to reach the vena cava, one which follows the ductus venosus and 

 avoids the liver, the other which passes by the venae advehentes 

 (portal veins) to the liver, and proceeds by the venae revehentes 

 (hepatic veins) to the inferior vena cava, which receives all the blood 

 passing by both of these channels. From this the blood is emptied 

 into the right auricle, and hence is guided by the Eustachian 

 valve through the septum by the patent foramen ovale to the left 

 auricle. From the left auricle it passes to the left ventricle, 

 which contracts and sends the blood into the aortic arch, where it 

 is split up into two streams, one of which passes into the vessels 

 of the head and neck, the other by the descending aorta to the 

 trunk and lower extremities. 



The blood from the head and neck is returned to the right 

 auricle by the superior vena cava. The blood from this vein 

 passes through the auricle to the right ventricle, which sends it 

 through the pulmonary artery towards the lungs. 



The pulmonary artery, however, in the embryo, has one very 

 large branch, called the ductus arteriosus, which joins the aorta 

 at a point just below the origin of the vessels of the head and 

 neck ; hence the main part of the blood passing from the right 



