THE FCETAL CIRCULATION. 601 



trance of the inferior cava. Subsequently, however, the septum, 

 growing from above downwards, becomes directed more and 

 more to the left of the vena cava inferior. During the entire 

 period of foetal life, there remains an opening in the septum, 

 which the valve of the foramen ovale, developed in the third 

 month, imperfectly closes. 



Circulation of Blood in the Fcetus. 



The circulation of blood in the foetus is peculiar, and 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 um- 

 bilical 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 cir- 

 culating through the liver. Whether, however, by the direct 

 route 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 ex- 

 pected that the two streams of blood would be mingled in the 

 right auricle, but such is not the case, or only to a slight ex- 

 tent. The blood from the superior vena cava the less pure 

 fluid of the two passes almost exclusively into the right ven- 

 tricle, through the auriculo-ventricular opening, just as it does 

 in the adult ; while the blood of the inferior vena cava is di- 

 rected 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 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 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 



