376 THE ORGANS 



congestion, by decrease in thickness of the mucosa and in the size of 

 the glands, and by restoration of the surface epithelium. 



3. The Mucosa of the Pregnant Uterus 



This is known as the decidua graviditatis, and presents changes in 

 structure somewhat similar to those which occur during menstrua- 

 tion, but more extensive. It is divided into three parts: 



{a) The decidua serotina or decidua basalis — that part of the 

 mucosa to which the ovum is attached. 



{b) The decidua reflexa or decidua capsularis — that part of the 

 mucosa which surrounds the ovum. 



{c) The decidua vera^ — which consist of all the remaining mucosa. 



The development of the decidua vera resembles the changes which 

 take place in the mucosa during menstruation. There is the same 

 thickening of the mucosa, and this thickening is due to the same 

 factor, i.e., distention of the blood-vessels and proliferation of the 

 tissue elements. These changes are, however, much more extensive 

 than during menstruation. The superficial part of the stroma 

 between the mouths of the glands becomes quite dense and firm, 

 forming the compact layer. The deeper part of the stroma contains 

 numerous cavities, which are the lumina of the now widely distended 

 and tortuous glands. This is known as the spongy layer. 



Within the stroma, especially of the compact layer, develop the 

 so-called decidual cells. These are peculiar typical cells derived from 

 connective tissue. They are of large size (30 to ioo«), vary greatly 

 in shape, and in the later months of pregnancy have a rather charac- 

 teristic brown color, which they impart to the superficial layers of 

 the decidua vera. They are mostly mononuclear, although poly- 

 nuclear forms occur. 



During the latter half of pregnancy there is a gradual thinning of 

 the decidua vera, due apparently to pressure. The necks of the 

 glands in the compact layer disappear, and the gland lumina in the 

 spongy layer are changed into elongated spaces, which lie parallel to 

 the muscular layer. 



The decidua reflexa and decidua serotina have at first the same 

 structure as the decidua vera. The decidua reflexa undergoes hyaline 

 degeneration during the early part of pregnancy, and by the end of 

 gestation has either completely disappeared (Minot) or has fused with 

 the decidua vera (Leopold). 



