OF THE FOETUS. 323 



thefe the courfe of the circulation is lengthened ; 

 but the vein is larger than the arteries. At the 

 extremity of the cord, each of ihefe veffels di- 

 vide into an infinite number of ramifications, 

 and extend themfelves between two membranes. 

 They fet off from the common trunk in fuch a 

 manner, that the whole ramifications afTume a 

 round form, and are diflinguifhed by the name 

 of placenta, becaufe they refemble a cake. The 

 central part of the placenta is thicker than its 

 edges : Its mean thicknefs is about an inch, and 

 its diameter is eight or nine inches, and fome- 

 times more. Its external furface, which is ap- 

 plied to the uterus, is convex, and the internal 

 furface is concave. The blood of the foetus cir- 

 culates in the cord and in the placenta. The 

 arteries of the cord proceed from two large ar- 

 teries in the foetus, and carry the blood through 

 all the arterial ramifications of the placenta ; 

 and the blood is collecled and returned to the 

 foetus by the venous branches of the placenta 

 and the umbilical vein. 



The concave furface of the placenta is cover- 

 ed with the chorion : Its convex furface is alfo 

 covered with a foft membrane, which feems to 

 be a continuation of the chorion, and is eafily 

 torn ; and the foetus is inclofed in the double 

 covering of the chorion and amnios. The figure 

 of the whole is globular, becaufe the intervals 

 between the foetus and membranes are filled 

 with a tranfparent fluid. This liquor is imme- 



X 2 diatelv 



