252 REPRODUCTION 



Here the circulation of the child is brought into intimate relation to 

 that of the mother, but they are nevertheless separated by four layers 

 of cells. Although there is no direct communication, there is an 

 exchange of material between the mother's blood and the fetal 

 blood. The mother's blood furnishes to the fetal blood food and 

 oxygen, and in turn removes the carbon dioxide and excremen- 

 titious material which the fetus must lose. The placental cir- 

 culation supplies the place taken in after life by the alimentary 

 and respiratory tracts. When the placenta is expelled, a part 

 of the maternal tissue is left behind, and there is, of course, a 

 loss of blood contained in the uterine sinuses, but the general 

 balance of the circulation is not disturbed at childbirth. The 

 reason for this is the oblique entrance of the placental vessels. 

 They enter the sinuses at an angle and are therefore compressed 

 by the muscular tissue of the uterus in its contracted state. 

 There are two distinct types of circulation in fetal life the 

 vitelline and the placental circulation. In both types the blood 

 is driven on by the heart, the essential difference being the 

 site where the fetal blood is enriched. The vitelline circulation 

 precedes that of the placenta, and as soon as the latter is formed 

 the former disappears. The vitelline circulation in the human 

 is very short-lived. 



The placental circulation presents two prominent features in 

 which it differs from adult circulation: 



1. Modifications are necessary in the heart and great blood- 

 vessels in order that the blood may not enter the lungs. 



2. In the circulation through the liver the veins are modified 

 so as to allow for the return of placental circulation. 



The course of the fetal circulation is as follows: The fetal 

 blood, purified and enriched in the placenta, passes by the 

 umbilical vein in the umbilical cord to the under surface of the 

 liver; here the vein divides into two parts. One portion of the 

 blood enters the liver substance, and after traversing its cap- 

 illaries is poured out by the hepatic veins into the inferior vena 

 cava. The other portion .of the blood passes directly from the 

 umbilical vein to the inferior vena cava by means of a blood 

 channel, the ductus venosus. The blood of the vena cava infe- 

 rior is carried to the right auricle of the heart, and instead of 

 passing from there into the right ventricle, as in the case of 

 the adult heart, it goes directly into the left auricle by means 



