163S A MANUAL OF ANATOMY 



surface of the chorion covered by syncitium beneath which is the 

 cellular layer of Langhans. 



The blood-lacunae are situated entirely within the maternal 

 placenta, and are filled with maternal blood, by which the cotyledons 

 are freely bathed. They are lined with syncitium, and are by some 

 regarded as dilated capillaries, their syncitial lining being con- 

 tinuous with the endothelium of the maternal arterioles in the basal 

 layer of the decidua basalis. More probably, however, the syncitial 

 lining is derived from the syncitium which forms the superficial 

 layer of epithelium of the chorion frondosum. 



The arteries of the maternal part of the placenta are derived 

 from the uterine and ovarian arteries. 



Placental Circulation. — Both foetal and maternal blcod circulate 

 through the placenta. The impure blood of the foetus is conveyed 

 to the placenta by the two hypogastric and umbilical arteries, which 

 are represented, at an earlier period, by the two allantoic arteries. 

 After being purified in the placenta, the blood is conveyed from 

 that organ into the body of the foetus by the umbilical vein, which 

 is represented, at an earlier period, by the two allantoic veins. The 

 arteries of the maternal part of the placenta are derived, as just 

 stated, from the uterine and ovarian arteries. The maternal arterial 

 blood eventually leaves the capillaries and enters the maternal 

 blood-lacunae, which are formed in connection with the stratum 

 compactum of the decidua basalis or serotina, and into which the 

 capillaries open. The blood is conveyed away from the intervillous 

 spaces by minute venous radicles which open into veins in the deep 

 part of the decidua basalis. It thus freely bathes the chorionic 

 villi, each of which contains, as stated, an arteriole, a venous radicle, 

 and a rich network of capillary bloodvessels. There is no direct 

 intermingling of the foetal and the maternal blood. The two are 

 separated by (i) the ectodermic (syncitial) envelope of each villus, 

 and (2) the core of mesoderm within. This mesoderm, however, 

 really represents the endothelial walls of the capillary ramifications 

 of the arteriole within the villus. These tissues, however, are so- 

 thin that interchanges readily take place through them, by osmosis, 

 between the foetal and the maternal blood. These interchanges are 

 of a threefold nature — namely, respiratory, nutritive, and excretory, 

 (i) The foetal blood is here supplied with oxygen and deprived of 

 carbon dioxide; (2) it is furnished with nutritive materials; and 

 (3) excretory products of the foetus are removed from it. Being 

 now purified, it is conveyed into the body of the foetus by the 

 umbilical vein. 



It is to be noted, in connection with the placental circulation, 

 that the foetal blood never leaves closed vessels, whilst the maternal 

 blood does do so, and enters blood-lacunae. It is right, however, to- 

 state that these blood-sinuses are regarded by some authorities as 

 greatly enlarged capillaries. 



The veins of the maternal fart of the placenta commence as venous 

 radicles wjiich lie partly in the basal layer, and partly in the syncitial 



