668 MANUAL OF PHYSIOLOGY. 



called menstruation. If, however, the ovum be impregnated, the 

 mucous membrane of the uterus not only becomes turgid, but its 

 cells proliferate, and considerable thickening of the tissue takes 

 place. The mucous membrane is then called the decidua. When 

 the ovum reaches the uterus, it ordinarily becomes imbedded in 

 that part of the decidua which occupies the fundus of the uterus. 

 The decidua here grows excessively, and becomes much thickened, 

 and on either side of the ovum a projection is sent from the de- 

 cidua which meets below the ovum, and completely encircles it. 



To the membrane lining the general cavity of the uterus the 

 name decidua vera is given, whilst that part lining the fundus, to 

 which the ovum is attached, is called the decidua serotina, its 

 processes surrounding the ovum receiving the name of the decidua 

 reflexa. 



The placenta is developed from two sources, one arising from 

 the membranes of the foetus, and the other belonging to the 

 mother. 



Relation of the Foetal to Maternal Placenta. The maternal part 

 is formed from the decidua serotina, which becomes much thick- 

 ened and very vascular where the placenta is attached. The 

 fcetal placenta is derived from the chorion, which sends out a 

 number of finger-like processes, which subdivide, and into which 

 the allantois, as it spreads over the chorion, sends prolongations. 

 The mesoblastic layer of the allantois gives rise to the capillaries 

 which are in these processes. The capillaries spring from the 

 branches of the umbilical arteries which pass along the umbilical 

 cord to reach the chorion. The vessels of the decidua serotiua 

 or maternal placenta end in large sinuses, lined by endothelial 

 cells. The blood is carried to these sinuses by the uterine arte- 

 ries, and from them by the uterine veins. The walls of the si- 

 nuses are provided with unstriped muscular tissue, which can close 

 the inlets from the arteries, and thus shut out the blood. The 

 villi of the foetal placenta, dipping into these uterine sinuses, are 

 covered with a single layer of thin scaly cells, so that the fcetal 

 blood is only separated from the maternal by the walls of the 

 capillaries and these thin cells, and thus the interchange of nu- 

 trient materials and gases readily goes on between them ; it is very 



