322 HUMAN UTERUS AND FCETAL APPENDAGES. 



is the amnion already mentioned, and the outer is the chorion united with the 

 decidua. The amnion and chorion are appendages of the embryo; the decidua 

 is the modified mucous membrane of the uterus.. Let us return to the embryo. 

 From its abdomen there springs a long, whitish cord, known as the umbilical cord; 

 it is usually from about one-third to one-half an inch in diameter and 40 cm. 

 long, but its dimensions are extremely variable; it always shows a spiral twist, 

 and contains three large blood-vessels, two arteries and one vein, all of which 

 can be distinguished through the translucent tissue. The distal end of the cord 

 is attached to the wall of the uterus, usually near the middle of the dorsal side of 

 the organ. It is easily seen that the blood-vessels of the umbilical cord radiate 

 out from its end over the surface of the uterus underneath the amnion, branching 

 as they go; they spread, however, only over a circumscribed area, the placental, 

 where the wall of the uterus is very much thickened. A vertical section through 

 the placental area shows that the amnion and chorion are widely separated from 

 the decidua and muscularis by a spongy mass soaked with maternal blood. This 

 mass consists of numerous trees of tissue, which spring with comparatively thick 

 stems from the chorion and branch again and again. In these stems and 

 branches are to be found the final ramifications of the vessels of the umbilical 

 cord; the trees are known as chorionic or placental villi. Some of their end- 

 twigs are very closely attached to the surface of the decidua. In the center of 

 the placental area the villi form a mass about three-fourths of an inch thick, but 

 toward the edge of the area the mass gradually thins out until at the very edge 

 the chorion and decidua come into immediate contact. The mass of villi, 

 together with the overlying portions of the chorionic and amniotic membranes 

 and the underlying portion of the decidua, constitutes what is known as the 

 placenta. The decidua of the placental area is called the decidua serotina ; the 

 chorion of the placenta is known as the chorion frondosum. When birth takes 

 place, the whole placenta is expelled after the delivery of the child ; the placenta 

 of the obstetrician is therefore partly of foetal, partly of maternal, origin. 



Decidua Vera of the First Stage in Section. 



Specimens may be preserved in Zenker's or Tellyesnicky's fluid, or they 

 may be preserved with less good results in Miiller's or Parker's fluid or in picro- 

 sulphuric acid. Sections may be made of the entire wall in celloidin, or, if it is 

 desired to get thinner sections, in paraffin, in which case it is advantageous to 

 remove as much as possible of the muscular coat so as to cut only the decidual 

 membrane. 



The following description is based upon a uterus one month pregnant. 

 Figure 184 was obtained from a vertical section of the decidua, by drawing the 



