THE UTERUS DURING MENSTRUATION AND PREGNANCY 241 



The maternal arteries and veins pass through the basal plate, taking a sinuous 

 course and opening into the intervillous spaces (Fig. 247). Near their entrance 

 they course obliquely and lose all but their endothelial layers. The original 

 openings of the vessels into the intervillous spaces were formed during the im- 

 plantation of the ovum, when their walls were eroded by the invading trophoderm 

 of the villi. As the placenta increases in size the vessels grow larger. The ends 

 of the villi are frequently sucked into the veins and interfere with the placental 

 circulation. At the periphery of the placenta is an enlarged intervillous space, 

 which varies in extent and never more than partly surrounds the placenta. This 

 space is the marginal sinus through which blood is carried away from the placenta 

 by the maternal veins. The blood of the mother and fetus does not mix, although 

 the epithelial cells of the vilK are instrumental in transferring nutritive sub- 

 stances to the blood of the fetus and in eliminating excreta from the fetal circula- 

 tion into the maternal blood stream of the intervillous spaces. 



The Relation of the Fetus to the Placenta and the Separation of the De- 

 cidual Membranes at Birth.^The relation of the embryo to the fetal membranes 

 has been described on p. 71. During the first months of pregnancy the embryo 

 floats in the cavity of the amnion, attached to the placenta by the umbilical cord 

 (Fig. 241). Later, as we have seen, the amnion fuses more or less completely to 

 the chorion frondosum and laeve. The decidua capsularis largely disappears or 

 is fused to the decidua vera. Before birth, the placenta is concave on its amniotic 

 surface, its curvature corresponding to that of the uterus (Fig. 249). At term, 

 the duration of which is taken as ten lunar months, the muscular contractions of 

 the uterus, termed "pains," bring about a dilation of the cervix uteri, the rupture 

 of the amnion and chorion Iseve, and cause the extrusion of the child. With the 

 rupture of the membranes the amniotic liquor is expelled, the fetal membranes 

 remaining attached to the decidual membranes. The pains of labor begin the 

 detachment of the decidual membranes, the plane of their separation lying in 

 the spongy layer of the decidua basalis and decidua vera, where there are only thin- 

 walled partitions between the enlarged glands. Following the birth of the child, 

 the tension of the umbilical cord and the "after pains" which diminish the size 

 of the uterus, norrnally complete the separation of the decidual membranes from 

 the wall of the uterus. The uterine contractions serve also to diminish the size 

 of the ruptured placental vessels and prevent extensive hemorrhage. From the 

 persisting portions of the spongy layer and from the epithelium of the glands the 

 tunica propria, glands, and epithehum of the uterine mucosa are regenerated. 



The decidual membranes and the structures attached to them when expelled 

 i6 



