846 EMBRYOLOGY 



The division of the heart into two ventricles appears before the two 

 auricles are separated. This is effected by a septum, which gradually 

 extends from the apex of the heart upward toward the auricular portion. 

 At the seventh week there is a large opening between the two ventricles. 

 This gradually closes from below upward, the heart becomes more 

 pointed and the separation of the two ventricles is complete at about the 

 end of the second month. 



At about the end of the second month, a septum begins to be formed 

 between the auricles. This extends from the base of the heart toward 

 the ventricles ; but it leaves an opening between the two sides the 

 foramen ovale, or the foramen of Botal which persists during foetal 

 life. At the anterior edge of the opening of the vena cava ascendens 

 into the right auricle, there is a membranous fold which projects into 

 the auricle. This is the valve of Eustachius, and it divides the right 

 auricle incompletely into two portions. 



During the sixth week the heart is vertical and is situated in the 

 median line, with the aorta arising from the centre of its base. At the 

 end of the second month it is raised up by the development of the liver, 

 and its point presents forward. During the fourth month, it is twisted 

 slightly on its 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 capacity during the latter half of intra-uterine 

 life. The pericardium makes its appearance during the ninth week. 



Early in intra-uterine 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 I to 50. This proportion, however, gradually diminishes, until at 

 birth the ratio is as I to 120. The weight in the adult is about as I to 

 1 60. 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. Beginning at the abdominal 

 aorta, the blood passes 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, 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 



