FCETAL MEMBRANES. 



595 



cut in all directions, many sections of villi being entirely free from their basal 

 connections. The villi thus present the appearance of projections, peninsulas, 

 or islands lying in spaces filled with blood (Fig. 501). 



Branches from the arteries of the uterine muscularis enter the decidua basa- 

 lis. They take very tortuous courses through the latter and in it lose their con- 

 nective tissue and muscular coats, and, while of considerably larger diameter 

 than most capillaries, become reduced to endothelial tubes. These follow the 

 intervillous (placental) septa in which they branch and from which they finally 

 open directly into the intervillous spaces along the edges of the cotyledons. 

 The maternal blood is thus poured into the intervillous spaces at their peri- 

 phery. After flowing through them it passes into veins which leave the 

 intervillous spaces near the center of the cotyledons (Fig. 500). 



Chorion laeve + 

 Decidua parietalis 



Decidua basalis 



Cotyledon 

 (lobe) 



Cotyledon 

 (lobe) 



FIG. 502 . Placenta at birth, seen from the uterine side. Bonnet. 



The relation of these spaces to the maternal blood vessels is not easy to make 

 out in ordinary sections, but many observations have established the fact that 

 both arteries and veins open directly into the spaces. The entire system of 

 intervillous spaces may thus be considered as a part of, or an appendage to, the 

 maternal vascular system, the maternal blood flowing from the arteries into 

 these spaces and returning from these spaces to the mother through the veins. 

 The foetal blood, on the other hand, circulates in the capillaries of the connective 

 tissue of the villi separated from the maternal blood of the intervillous spaces by 

 the epithelial villous covering already described (p. 589) . It is between the 

 maternal blood of the intervillous spaces and the foetal blood in the villous 

 capillaries that the interchange of material takes place. Both the maternal 

 and foetal vascular systems are closed systems so that no blood can pass directly 



