286 HAND-BOOK OF PHYSIOLOGY. 



CIRCULATION OF BLOOD IK THE F(ETUS. 



The circulation of blood in the foetus differs considerably from that 

 of the adult. It will be well, perhaps, to begin its description by tracing 

 the course of the blood, which, after being carried out to the placenta by 

 the two umbilical arteries, has returned, cleansed and replenished, to the 

 foetus by the umbilical vein. 



It is at first conveyed to the under surface of the liver, and there the 

 stream is divided, a part of the blood passing straight on to the inferior 

 vena cava, through a venous canal called the ductus venosus, while the 

 remainder passes into the portal vein, and reaches the inferior vena cava 

 only after circulating through the liver. Whether, however, by the direct 

 ronte through the ductus venosus or by the roundabout way through the 

 liver, all the blood which is returned from the placenta by the umbilical 

 vein reaches the inferior vena cava at last, and is carried by it to the right 

 auricle of the heart, into which cavity is also pouring the blood that has 

 circulated in the head and neck and arms, and has been brought to the 

 auricle by the superior vena cava. It might be naturally expected that 

 the two streams of blood would be mingled in the right auricle, but such is 

 not the case, or only to a slight extent. The blood from the superior 

 vena cava the less pure fluid of the two passes almost exclusively into 

 the right ventricle, thrpugh the auriculo- ventricular opening, just as it 

 does in the adult; while the blood of the inferior vena cava is directed by 

 a fold of the lining membrane of the heart, called the Eustacliian valve , 

 through the foramen ovale into the left auricle, whence it passes into the 

 left ventricle, and out of this into the aorta, and thence to all the body. 

 The blood of the superior vena cava, which, as before said, passes into 

 the right ventricle, is sent out thence in small amount through the pul- 

 monary artery to the lungs, and thence to the left auricle, as in the adult. 

 The greater part, however, by far, does not go to the lungs, but instead, 

 passes through a canal, the ductus arteriosus, leading from the pulmo- 

 nary artery into the aorta just below the origin of the three great vessels 

 which supply the upper parts of the body; and there meeting that part 

 of the blood of the inferior vena cava which has not gone into these large 

 vessels, it is distributed with it to the trunk and lower parts, a portion 

 passing out by way of the two umbilical arteries to the placenta. From 

 the placenta it is returned by the umbilical vein to the under surface of 

 the liver, from which the description started. 



Changes after Birth. After birth the foramen ovale closes, and so 

 do the ductus arteriosus and ductus venosus, as well as the umbilical 

 vessels; so that the two streams of blood which arrive at the right 

 auricle by the superior and inferior vena cava respectively, thenceforth 



