50 



HUMAN ANATOMY. 



Its convex external or uterine surface is rough, owing to the separation from the 

 deeper part of the lining of the uterus which has taken place at the termination of 

 labor. This surface, moreover, presents a number of divisions, the cotyledons^ de- 

 fined by deep fissures. The inner or foetal surface is smooth, being covered by the 

 amnion, and slightly concave. The weight of the fully developed placenta averages 

 about 500 grammes. 



The position of the placenta is determined, evidently, by the point at which the 

 ovum forms its attachment with the maternal tissues ; in the majority of cases this 

 location is at the fundus of the uterus in the vicinity of the oviduct, right or left, 

 the orifice of which becomes occluded by the expansion of the placental structures. 

 Less frequently the placenta occupies the more dependent portions of the uterine 

 wall and, in exceptional cases, its position is in the immediate vicinity of the internal 

 mouth of the uterus ; in these latter cases the placenta may partially, or even com- 

 pletely, grow over the latter opening, thus constituting the grave condition known as 

 placenta praevia. The general constitution of the placenta (Fig. 59), as consisting 



Uterine blood-vessels 



Fig. 59. 



Maternal blood -spaces 



Foetal villi 



Umbilical vesicle' 



Decidua placentalis 



Allantois 



Decidua capsularis 



Chorion 



Decidua vera 



Interdecidual space 



Amniotic sac 



Diagram illustrating the relations of the fcetus, the membranes, and the uterus during the early months of 



pregnancy. 



of the foetal and the maternal portions, has already been sketched ; it now remains to 

 consider briefly the arrangement of these structures. 



The foetal portio7i of the placenta, the contribution of the chorion frondosum, 

 soon becomes a mass of richly branching villi, the more robust main stalks of which 

 are attached to the maternal tissue, while the smaller secondary ramifications are 

 free, completely surrounded by the contents of the maternal blood-sinuses in which 

 they float. , In all cases the villous processes support the terminal loops of the foetal 

 blood-vessels, the blood being conveyed to and from the placenta, along the umbil- 

 ical cord, by the umbilical arteries and vein. Although coming into close relation, 

 the syncytium and the meagre connective tissue surrounding the foetal capillaries 

 alone intervening, the blood-streams of the mother and of the child never actually 

 mingle ; the delicate septum, however, allows the free interchange of gases necessary 

 for the respiratory function as well as the passage of nutritive substances into the 

 foetal circulation. 



