1164 



EMBRYOLOGY. 



eating blood-channels or sinuses, which are filled with maternal blood, and in 

 which are suspended the now greatly ramified tufts of the chorionic villi. These 

 uterine sinuses anastomose freely with one another, and form, at the edge of the 

 placenta, a venous channel with an irregular calibre, which runs round the whole 

 circumference of the placenta, and is termed the marginal sinus. Some of the 

 chorionic villi are attached by fibrous bands to the basal layer of the decidua and 

 to the imperfect septa between the sinuses, but the majority of them hang free. 

 Circulation through the Placenta. The maternal blood is brought to the 



Placental villi of chorion frondosiim. 



Decidua nerotiiia. 



Allantois.- - 



Umbilical cord, 

 with its con-"' 

 tained vessels. 



Fallopian tube. 



"-Cavity of uterus, 

 --Yulk-sac. 



^Decidua vera. 



Decidua reflexa.'' 



of mucus in the 

 neck of uterus. 



FIG. 708. Sectional plan of the gravid uterus in the third and fourth montns. (Modified from Wagner.) 



uterine sinuses by the "curling arteries" of the uterus and drained away by the 

 uterine veins, while, as already stated, within the chorionic villi are found the 

 ramifications of the foetal vessels derived from the allantoic or umbilical arteries. 

 Since the villi are suspended in the sinuses, they are necessarily bathed in the 

 maternal blood, and hence it follows that the maternal and foetal bloo'd-currents 

 are brought into close relationship. There is, however, no intermingling of the 

 two currents, or, in other words, no direct communication between the vascular 

 system of the mother and that of the foetus, the interchange of materials neces- 

 sary for the growth of the foetus and for the purification of the foetal blood taking 

 place through the walls of the villi. The purified blood is carried back to the 

 foetus by the umbilical vein. From what has been said, it will be understood that 

 the placenta is the organ by which the connection between the foetus and the 

 mother is established, and which subserves the purposes of nutrition, respiration, 

 and excretion. 



Placenta. At the end of the gestation period the placenta presents the form 

 of a disk which weighs about a pound and has a diameter of from six to eight 

 inches. Its average thickness is about an inch and a quarter, but diminishes rap- 

 idly near the circumference of the disk. Its outer or decidual surface blends with 

 the uterine wall, but if examined after the separation of the placenta, it presents 

 a comparatively smooth surface, which on inspection is seen to be incompletely 

 divided into a number of masses named cotyledons. Its inner or chorionic surface 

 is smooth, being closely invested by the amnion. The umbilical cord is attached 

 near the centre of this surface, and from this attachment the larger branches of the 

 umbilical vessels are seen radiating under the amnion. On section the placenta 

 presents a soft, spongy appearance, caused by its freely communicating blood- 



