664 MANUAL OF PHYSIOLOGY. 



THE PLACENTA. 



The placenta is a most important organ to the mammalian 

 embryo. It conveys not only nourishment, but also oxygen from 

 the maternal blood to that of the fostus. It is, of course, neces- 

 sary that the animals whose ova do not contain large stores of 

 food, should in some way provide the substances necessary for the 

 life of their embryo, and it is by means of the placenta that this 

 is brought about. The embryo of oviparous animals does not 

 require a placenta for its nutrition, since inside the egg is a large 

 store of highly nutritious albuminous and fatty materials ; the 

 shell is pervious to air, and the chick's blood can in the allautois 

 be oxidized by the air directly. A bird's egg contains in itself 

 all the necessaries which the placenta supplies, and when impreg- 

 nated only requires the heat of the mother's body to develop a 

 chick. 



While an ovum is descending the Fallopian tube, the mucous 

 membrane of the uterus becomes turgid, and, as before mentioned, 

 if the ovum be unimpregnated it is cast out of the body, part of 

 the substance of the lining membrane of the uterus is desqua- 

 mated and discharged with a fluid largely composed of blood. 

 This takes place approximately every four weeks, and hence is 

 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 

 decidua 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, while 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 





