THE FOETAL CIRCULATION. 771 



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, through 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 Eustachian 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 pulmonary 

 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 pulmonary 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 dis- 

 tributed 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 um- 

 bilical vein to the under surface of the liver, from which 

 the description started. 



After birth the foramen ovale closes, and so do the 

 ductus arteriosus and ductus venosus, as well as the um- 

 bilical vessels; so that the two streams of blood which 

 arrive at the right auricle by the superior and inferior 

 vena cava respectively, thenceforth mingle in this cavity of 

 the heart, and passing into the right ventricle, go by way 

 of the pulmonary artery to the lungs, and through these, 

 after purification, to the left auricle and ventricle, to be 

 distributed over the body. (See chapter on Circulation.) 



