102 



ON CLASSIFICATION. 



mucous membrane, in the region of the placenta, is traversed 

 by numerous arterial and venous trunks, which carry the blood 

 to and from the Dlacenta ; and the layer of decidua into which 

 the chorionic villi do not penetrate acquires a cavernous, or 

 cellular, structure from becoming burrowed, as it were, by the 

 innumerable sinuses into which these arterial and venous trunks 



Fig. 41 



A 



Fig. 41. — Section of the Human Uterus and Placenta at the thirtieth week of pregnancy. 

 (After Ecker.) — A, umbilical cord ; B, chorion ; C, the foetal villi separated by pro- 

 cesses of; D, cavernous decidua; E, F, G, wall of the uterus. 



open. In the process of parturition, the decidua serotina splits 

 through this cellular layer, and the superficial part of it comes 

 away with the umbilical cord, together with the fcetal mem- 

 branes and the rest of the decidua ; while the deeper layer, 

 undergoing fatty degeneration and resolution, is more or less 

 completely brought away with the locliia, and gives place to 

 a new mucous membrane, which is developed throughout the 

 rest of the uterus, during pregnancy; but, possibly, arises only 

 after delivery over the placental area. 



In the Pig the placenta is an infinitely simpler structure. 



