DEVELOPMENT OF THE CIRCULATORY SYSTEM. 935 



weight in the adult is about 1 to 160. During about the first half of intra-uterine life, 

 the thickness of the two ventricles is nearly the same ; but, after that time, the relative 

 thickness of the left ventricle gradually increases. 



Peculiarities of the Foetal Circulation. In studying the complete course of the blood 

 in the foetus, which constitutes the second, or the placental circulation, we note peculiari- 

 ties in two portions of the circulatory system. In the one, a peculiar arrangement is 

 necessitated by the passage of blood to and from the placenta ; and in the other, the char- 

 acter of the blood coming from the placenta necessitates a peculiar arrangement of the 

 heart and the great vessels. 



The branches from the internal iliac arteries, which pass to the foetal tufts of the pla- 

 centa, do not exist in the adult. The ductus venosus, which conveys a portion of the 

 blood of the umbilical vein to the vena cava ascendens, and the umbilical vein itself do 

 not exist in the adult. 



The Eustachian valve, situated at the inner margin of the vena cava ascendens as it 

 opens into the right auricle, does not exist in the adult. The foramen ovale, or the open- 

 ing between the right and the left auricle, through which the blood from the vena cava 

 ascendens is directed into the left auricle, does not exist in the adult. The ductus arte- 

 riosus, which conveys the blood from the left pulmonary artery to the arch of the aorta, 

 does not exist in the adult. In the adult, the pulmonary arteries receive all the blood 

 from the right ventricle. In the foetus, the pulmonary arteries receive a small quantity 

 of blood, as compared with that which passes to the aorta through the ductus arteriosus. 

 Keeping in view these peculiarities of the circulatory apparatus, the entire course of the 

 blood, during foetal life, is as follows : 



Beginning with the abdominal aorta, we follow the course of blood into the two 

 primitive iliacs, and thence into the internal iliacs. From the two internal iliacs, the two 

 hypogastric arteries arise, which ascend along the sides of the bladder to its fundus, thence 

 pass to the umbilicus 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 extremities passes down, by the vena cava descendens, in front of the Eusta- 

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

 Eustachian valve is such, that the right auricle simply affords a passage for the two cur- 

 rents 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 

 natural course of the foetal circulation. Reid injected the vena cava ascendens with red, 

 dhd the vena cava descendens with yellow, in a foetus of seven months, and he found very 



