712 



UTEEOGESTATION. 



arrival of the ovum in the uterus, corresponds closely with that which 

 takes place at every successive menstrual period in the uterine 

 membrane. This process consists essentially in a thickening or hyper- 

 trophy of the lining membrane, and is mainly due to an extremely 

 rapid proliferation of the subepithelial cells and fibro-cellular tissue, 

 and an increased development of the blood-vessels and glands. 



Fig. 518. 



Fig. 518. — The DEciDrA opened 



AND VIEWED FKOil BEFORE (after 



W. Hunter). 



This is a representation of the 

 thickened membrane of the uterus 

 thrown off as the product of abor- 

 tion at a very early period of gesta- 

 tion. A and B mark superiorly tue 

 passage of two bristles through the 

 openings from the Fallopian tulies 

 into the cavity of the uterus (cavity 

 of the decidua), and inferiorly the 

 exit of the bristles at the os iiteri. 

 In these three situations the torn 

 edges are seen where the decidua 

 has been separated from the con- 

 tinuous part of the mucous mem- 

 brane. At 0, where an ovum has pro- 

 bably been lodged, the inner part of 

 the decidua is made to bulge towards 

 the cavity of the uterus, and begins 

 to form decidua reflexa. 



The formation of a com- 

 ^ plete decidua within the 



uterus has been observed in several cases in which, although the ovum 

 was not discovered, or had not yet arrived in the uterus, there was 

 reason to believe impregnation had occurred six or eight days previously. 

 (Von Baer, E. H. Weber.) And a similar condition has been observed 

 in several examples of extra-uterine pregnancy (Hunter and others) ; 

 from which ii, appears that the earlier changes connected with the 

 formation of the decidua are independent of the presence of the ovum 

 in the uterus. 



When the ovum has been recently imbedded in the decidua, it forms 

 a swelling or projection of the surface within the uterine cavity, on 

 opening into which the villous chorion is found surrounded by the 

 substance of the decidua or thickened mucous membrane ; but the 

 covering of this substance which passes over the free surface of the. 

 ovum, or that which is towards the uterine cavity, is thinner and 

 simpler in its structure than at the place of attachment of the ovum 

 and in other parts of the uterine surface. 



The most projecting part or summit of the swelling formed by the 

 imbedded ovum more especially is somewhat different from the rest, and 

 indicates, by a sort of cicatricial mark, a place where the substance 

 of the decidua, as it gradually covered in the ovum, may be supposed 

 to have finally closed. 



The decidual thickening of the mucous memln-ane affects nearly 

 equally the whole of the lining of the uterine cavity, but towards the 



