876 



THE F<ETAL CIRCULATION. 



returned from the placenta by the umbilical vein, although the colour of the blood 

 cannot be distinguished from the venous or impure blood in the umbilical arteries. 

 The umbilical vein (fig. 673, 3, u) returns to the umbilicus, passes upwards under 

 the margin of the liver, gives a branch to the vena porta? (a), and runs as the 



J. | ductus venosus into the inferior vena 



1 I J cava, which carries the blood into the 



right auricle. Directed by the Eusta- 

 chian valve and the tubercle of Lower 

 (fig. 675, 6, tL), the great mass of the 

 blood passes through the foramen ovale 

 into the left auricle, owing to the pre- 

 sence of the valve of the foramen ovale. 

 From the left auricle it passes into the 

 left ventricle, aorta, and hypogastric 

 arteries, to the umbilical arteries. The 

 blood of the superior vena cava of the 

 foetus passes from the right auricle into 

 the right ventricle (fig. 675, 6, Cs). 

 From the right ventricle it passes into 

 the pulmonary artery (fig. 675, 7, p), 

 and through the ductus arteriosus of 

 Botalli (B) into the aorta. There are, 

 therefore, two streams of blood in the 

 right auricle which cross each other, the 

 descending one from the head through 

 the superior vena cava, passing in front 

 of the transverse one from the inferior 

 vena cava to the foramen ovale.] Only 

 a small amount of the blood passes 

 through the as yet small branches of the 

 pulmonary artery to the lungs (fig. 675, 

 7, 1, 2). The course of the blood makes 

 it evident that the head and upper limbs 

 of the foetus are nourished by purer 

 blood than the remainder of the trunk, 



Fig. 670. 

 Course of the fcetal circulation (Cleland). 



which is supplied with blood mixed with the blood of the superior vena cava. 

 After birth, the umbilical arteries are obliterated, and become the lateral ligaments 

 of the bladder, while their lower parts remain as the superior vesical arteries. The 

 umbilical vein is obliterated, and remains as the ligamentum teres, or round 

 ligament of the liver, and so is the ductus venosus Arantii. Lastly, the foramen 

 ovule is closed, and the ductus arteriosus is obliterated, the latter forming the lig. 

 arteriosus. 



The condition of the membranes where there are more fetuses than one : (1) With twins 

 there are two completely separated ova, with two placentae and two deciduse reflexae. (2) Two 

 completely separated ova may have only one reflexa, whereby the placentae grow together, while 

 their blood-vessels remain distinct. The chorion is actually double, but cannot be separated 

 into two lamell at the point of union. (3) One reflexa, one chorion, one placenta, two 

 umbilical cords, and two amuia. The vessels anastomose in the placenta. In this case there 

 is one ovum with a double yelk, or with two germinal vesicles in one yelk. (4) As in (3), but 

 only one amnion, caused by the formation of two embryos in the same blastoderm of the same 

 germinal vesicle. 



Formation of the foetal membranes. The oldest mammals have no placenta or umbilical 

 vessels ; these are the Mammalia implacentalia, including the monetremata and marsupials. 

 The second group includes the Mammalia placentalia. Amongst these (a) the non-deciduata 

 possess only chorionic villi supplied by the umbilical vessels, which project into the depressions 

 of the uterine mucous membrane, and from which they are pulled out at birth (PI. diffusa, e.g., 



