FCETAL CIRCULATION. 705 



Fcdal 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 toward 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 re- 

 ceives 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 toward 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 

 ventricle reaches the aorta by the ductus arteriosus, and only a 

 very small part goes to the lungs, to be returned from them by 

 the pulmonary veins to the left auricle. 



The blood from the ductus arteriosus blends, therefore, with 

 that in the aorta which is passing to the viscera and lower ex- 

 tremities. The main part of this blood travels by two large 

 branches of the aorta (the hypogastric arteries) to the placenta, 

 where it is aerated and purified, etc. 



It is evident, then, that, as the placenta is the great renovating 

 organ of the blood of the foetus, the blood in the umbilical vein 

 is the most arterial in the fcetal circulation. The blood in the 

 ascending vena cava and first part of the aorta is likewise fairly 



