270 HUMAN PHYSIOLOGY. 



increases in size until the seventh month, when a retrogressive 

 metamorphosis takes place until its separation during labor, at which 

 time it is of an oval or rounded shape, and measures from seven 

 to nine inches in length, six to eight inches in breadth, and weighs 

 from fifteen to twenty ounces. It is most frequently situated at 

 the upper and posterior part of the inner surface of the uterus. 



The placenta consists of two portions, a fetal and a maternal. 



The fetal portion is formed by the villi of the chorion, which, 

 by developing, rapidly increase in size and number. They become 

 branched and penetrate the \uterine tubules, which enlarge and re- 

 ceive their many ramifications. The capillary blood-vessels in the 

 anterior of the villi also enlarge and freely anastomose with one 

 another. 



The maternal portion is formed from that part of the hypertrophied 

 and vascular decidual membrane between the ovum and the uterus, 

 the decidua serotina. As the placenta increases in size, the maternal 

 blood-vessels around the tubules become more and more numerous, 

 and gradually fuse together, forming great lakes, which constitute 

 sinuses in the walls of the uterus. 



As the terminal period of gestation approaches, the villi extend 

 deeper into the decidua, while the sinuses in the maternal portion 

 become larger and extend further into the chorion. Finally, from ex*- 

 cessive development of the blood-vessels, the structures between 

 them disappear, and as their walls come in contact they fuse to- 

 gether, so that, ultimately, the maternal and fetal blood are separated 

 only by a thin layer of a homogeneous substance. When fully 

 formed, the placenta consists principally of blood-vessels interlacing 

 in every direction. The blood of the mother passes from the uterine 

 vessels into the lake surrounding the villi ; the blood from the fetus 

 flows from the umbilical arteries into the interior of the villi ; but 

 there is not at any time an intermingling of blood, the two being 

 separated by a delicate membrane formed by a fusion of the walls 

 of the blood-vessels and the walls of the villi and uterine sinuses. 



The function of the placenta, besides nutrition, is that of a 

 respiratory organ, permitting the oxygen of the maternal blood to 

 pass by osmosis through the delicate placental membrane into the 

 blood of the fetus ; at the same time permitting^ the carbonic acid 

 and other waste products, the result of nutritive changes in the 

 fetus, to pass into the maternal blood, and so to be carried to the 

 various eliminating organs. 



