THE PLACENTA. 611 



b. The Placenta. 



The fully formed placenta, at the close of gestation, is a dis- 

 coidal or cake-shaped body, of spongy consistency, measuring 

 from 16 to 21 cm. in diameter and 3 to 4 cm. in thickness. It 

 is attached to, or rather forms part of, the inner wall of the 

 ■uterus (c/. Pig. 254), and to its inner or free surface, usually a 

 little distance from its centre, the umbilical cord is attached, the 

 opposite end of the cord being connected with the foetus. 



The placenta consists of outer or maternal, and inner or 

 foetal layers, derived respectively from the decidua serotina, and 

 from the chorion. The distinction between the foetal and the 

 maternal elements is easily made in the early stages of develop- 

 ment ; but in the fully formed placenta, owing to the intricacy 

 of the relations between the chorionic villi and the maternal 

 blood-vessels, and the profound histological modifications which 

 almost all parts undergo, it becomes a matter of the greatest 

 difficulty to determine the real nature of the several structures 

 met with ; and there are important points, more especially in 

 regard to the relations of the maternal blood-vessels, on which 

 our knowledge still remains imperfect and unsatisfactory. 



The placenta consists of three chief layers or strata : (i) an 

 inner layer formed by the chorion ; (ii) an outer layer formed 

 by the decidua serotina, or modified mucous membrane of the 

 uterus ; (iii) a middle or intermediate layer, which is much 

 thicker than the other two, forming four-fifths or more of the 

 ■entire thickness of the placenta, and which consists of the intri- 

 cately branched foetal villi, together with the maternal sinuses 

 with which these are in relation. 



Of these three layers, the inner one is distinctly fcetal, and 

 the outer one maternal in origin ; the middle layer is foetal as 

 regards the villi themselves, but the precise relations of the 

 maternal vessels are still undecided. This middle, or villous 

 zone is the characteristic, and the functionally active part of the 

 placenta. At the margin of the placenta it thins out and dis- 

 appears, and the inner and outer layers, chorionic and decidual, 

 •come into contact with each other in the region of the decidua 

 reflexa. 



The inner or chorionic layer of the placenta has already been 

 described (pp. 601 to 605). The amnion is closely united with its 

 inner or free surface over its whole extent. The outer surface, 



R B 2 



