THE,PREGNANT UTERUS. 



343 



definitions of the terms decidua reflexa, serotina, and vera are given. -As the 

 ovum increases in size the decidua reflexa also increases, and gradually becomes 

 thinner and thinner, until it ultimately disappears. The exact date of its disap- 

 pearance is not known ; it falls somewhere within the fifth month. Accordingly, 

 we may divide the period of pregnancy into two phases or stages, each com- 

 prising about half of the whole period. During the first stage the decidual 

 reflexa is present (Fig. 227); during the second stage it is absent, so that the 

 chorion l.aeve comes into direct contact with the decidua vera. In the following 

 sections a typical uterus of the first and second stages' each is described. 



FIG. 227. HUMAN UTERUS ABOUT ONE MONTH PREGNANT. NATURAL SIZE. (THE UTERUS HAS BEEN OPENED 



BY AN INCISION ALONG ITS MEDIAN LlNE, SO AS TO DISPLAY THE SMALL OVAL BAG FORMED BY THE DECIDUA 



REFLEXA.) 



Human Uterus Three Months Pregnant. 



The uterus measures about 3^ inches in transverse diameter, and shows 

 well-marked venous sinuses on its external surface. It should be opened by a 

 crucial incision on the anterior side; its walls will -be found about an inch or more 

 in thickness; it contains a grayish red bag (decidua reflexa), which nearly fills 

 the cavity of the uterus and encloses the embryo, so that upon opening the womb 

 we do not encounter the fetus directly. The inner bag has a smooth surface, 

 but shows a few small pores; it is without blood-vessels and is attached to the 

 dorsal wall of the uterus. The inner surface of the uterus shows a rich network 



