156 COURSE OF THE BLOOD IN THE FCETUS. 



The ductus arteriosns establishes a communication between the main 

 pulmonary artery and the aorta, by which the blood from the right ventricle is 

 carried mainly into the dorsal aorta. 



The two large hypogastric or umbilical arteries, prolonged from the iliac 

 arteries, passing out of the body of the foetus, proceed along the umbilical cord, 

 to be distributed in the foetal portion of the placenta. From the placenta the blood 

 is returned by the umbilical vein, which, after entering the abdomen, communi- 

 cates by one branch with the portal vein, and is continued by another, named 

 ductus venosus, into one of the hepatic veins, through which it joins the main stem 

 of the vena cava inferior. 



Course of the blood in the foetus. The right auricle of the foetal heart 

 receives blood from the two vena cavae and the coronary sinus. The blood brought 

 by the superior cava is simply the venous blood returned from the head and upper 

 half of the body ; whilst the inferior cava, which is considerably larger than the 

 superior, conveys not only the blood from the lower half of the body, but also that 

 which is returned from the placenta and the liver. This latter stream of blood 

 reaches the vena cava inferior, partly by a direct passage the ductus venosus and 

 partly by the hepatic veins, which bring to the vena cava inferior all the blood 

 circulating through the liver, whether derived from the supply of placental blood 

 entering that organ by the umbilical vein, or proceeding from the vena portae or 

 hepatic artery. 



The blood of the superior vena cava is believed to pass through the right auricle 

 into the right ventricle, whence it is propelled into the trunk of the pulmonary 

 artery. A small part is distributed through the branches of that vessel to the lungs, 

 and returns by the pulmonary veins to the left auricle ; but, as these vessels remain 

 small up to the time of birth, by far the larger part passes through the ductus 

 arteriosus into the descending aorta, and is thence distributed in part to the lower 

 half of the body and the viscera, and in part along the umbilical arteries to the 

 placenta. From these several organs it is returned by the vena cava inferior, the 

 vena portse, and the umbilical vein ; and, as already noticed, reaches the right 

 auricle through the trunk of the inferior cava. 



Of the blood entering the heart by the inferior vena cava, it is supposed that 

 only a small part is mingled with that of the superior cava, so as to pass into the 

 right ventricle ; by far the larger portion is thought to be directed by the 

 Eustachian valve through the foramen ovale into the left auricle, and thence, 

 together with the small quantity of blood returned from the lungs by the 

 pulmonary veins, to pass into the left ventricle, whence it is sent into the arch 

 of the aorta, to be distributed almost entirely to the head and upper limbs. 



Sabatier was the first to call attention particularly to the action of the Eusta- 

 chian valve in separating the currents of blood entering the right auricle by the 

 superior and inferior venae cavae. This separation, as well as that occurring 

 between the currents passing through the aortic arch and the ductus arteriosus 

 into the descending aorta, was illustrated experimentally by John Eeid. A striking 

 confirmation of the extent to which the last mentioned division of the two currents 

 of the foetal blood may take place, without disturbance of the circulation up to the 

 time of birth, is afforded by the examples of malformation in which a complete ob- 

 literation has existed in the aortic trunk immediately before the place of the union 

 of the ductus arteriosus with the posterior part of the aortic arch. 



In earlier stages of development than those above described, it is certain that 

 there is little or no separation of the two kinds of blood, for both the umbilical 

 vems from the placenta and the veins from the yolk sac and body generally, 

 pour their blood together into the sinus venosus, and the mixed blood is then 

 forced through a single somewhat narrowed orifice (porta vestibuli of His) into the 

 auricle. 



