THE PLACENTA 357 



The fetus is enclosed in the amniotic sac which is formed by the fusion 

 of two membranes (amnion and chorion) derived from layers of the original 

 cell. It is often called "the membranes." 



It contains amniotic fluid a clear saline solution in which the fetus 

 floats. The placenta is developed in the outer layer of the sac (chorion). 



Clinical note. It is sometimes possible to separate the two layers of mem- 

 brane to a partial extent. 



The placenta is a mass of uterine and fetal blood and lymph 

 vessels, held loosely together. The vessels of the decidua serotina 

 are branches of the uterine arteries, which form thinly covered 

 loops called mill. These are received between similar villi of the 

 fetal vessels, forming the placenta. The blood of these two sets of 

 vessels is separated only by the thinnest of membranes, so that 

 the respiration (or exchange of O and COz) of the infant is thus 

 provided for, 1 the impure blood arriving from the fetus by two 

 arteries (right and left hypogastric) and returning purified to the 

 fetus by one vein, the umbilical (Fig. 231). 



Point of interest. The function of these vessels is similar to that in the 

 lungs of extra-uterine life, where the right and left pulmonary arteries carry 

 impure blood to the lungs and pulmonary veins carry pure blood to the heart. 



Note. The placenta is not within the amniotic sac. The fetal surface is 

 smooth, a part of the sac itself; the maternal or uterine surface is irregular, 

 dark red and friable (easily separated into its original masses of vessels and 

 connective tissue called cotyledons). 



Pregnancy or gestation is the condition in which these proc- 

 esses are going on. It begins with conception and ends with the 

 expulsion of the fully developed fetus from the uterus, or 

 parturition. 



The normal duration of pregnancy is 280 days or ten lunar 

 months. During the first five months the growth is very rapid; 

 the length of the fetus increases from one centimeter in the first 

 month to twenty-five in the fifth (about 10 inches). After the 

 fifth month the growth advances steadily but not so rapidly. 

 The average length at birth is about fifty centimeters, and the 

 average weight 2737 grams, or 7 1/3 Ibs. 



The growth of the uterus to accommodate the growing fetus 



has been alluded to. The great blood supply required for all 



these changes is provided by the uterine and ovarian arteries which 



are normally very large in proportion to the size of the organs. 



1 By processes of diffusion and osmosis as in the lungs. 



