THE FCETAL CIRCULATION. 839 



axis, and the point presents to the left. At this time the auricular portion 

 is larger than the ventricles ; but the auricles diminish in their relative capac- 

 ity during the latter half of intrauterine life. The pericardium makes its 

 appearance during the ninth week. 



Early in intrauterine life the relative size of the heart is very great. At 

 the second month its weight, in proportion to the weight of the body, is as 1 

 to 50. This proportion, however, gradually diminishes, until at birth the 

 ratio is as 1 to 120. The weight in the adult is about as 1 to 160. During 

 about the first half of intrauterine life the thickness of the two ventricles is 

 nearly the same ; but after that time the relative thickness of the left ven- 

 tricle gradually increases. 



Peculiarities of the Foetal Circulation. Beginning at the abdominal 

 aorta, the blood passes into the two primitive iliacs, and thence into the in- 

 ternal iliacs. From the two internal iliacs the two hypogastric arteries arise, 

 which ascend along the sides of the bladder, to its fundus, pass to the umbili- 

 cus and go to the placenta, forming the two umbilical arteries. In this way 

 the blood of the foetus goes to the placenta. 



The umbilical vein enters the body of the foetus at the umbilicus ; it 

 passes along the margin of the suspensory ligament, to the under surface of 

 the liver ; it gives off one branch of large size, and one or two smaller 

 branches to the left lobe ; it sends a branch each to the lobus quadratus and 

 the lobus Spigelii; and the vessel reaches the transverse fissure. At the 

 transverse fissure it divides into two branches, the larger of which joins the 

 portal vein and enters the liver ; and the smaller, which is the ductus venosus, 

 passes to the vena cava ascendens, at the point where it receives the left 

 hepatic vein. Thus the greater part of the blood returned to the foetus from 

 the placenta passes through the liver, a relatively small quantity being 

 emptied into the vena cava, by the ductus venosus. 



The vena cava ascendens, containing the placental blood which has passed 

 through the liver, the blood conveyed directly from the umbilical vein by the 

 ductus venosus and the blood from the lower extremities, passes to the right 

 auricle. As the blood enters the right auricle it is directed by the Eustachian 

 valve, passing behind the valve, through the foramen ovale, into the left 

 auricle. At the same time the blood from the head and the superior ex- 

 tremities passes down, by the vena cava descendens, in front of the Eustachian 

 valve, through the right auricle, into the right ventricle. The arrangement 

 of the Eustachian valve is such that the right auricle simply affords a pas- 

 sage for the two currents of blood ; the one, from the vena cava ascendens, 

 through the foramen ovale, passes into the left auricle and the left ventricle ; 

 and the other, from the vena cava descendens, passes through the right 

 auriculo-ventricular opening, into the right ventricle. It is probable, indeed, 

 that there is very little admixture of these two currents of blood in the natu- 

 ral course of the foetal circulation. 



The blood poured into the left auricle, from the vena cava ascendens, 

 through the foramen ovale, passes from the left auricle into the left ventricle. 

 The left auricle and the left ventricle also receive a small quantity of blood 



