578 THE HUMAN BODY 



family physician ought perhaps to warn the husband more fre- 

 quently than he does of the risk to a delicate wife's health, or even 

 life, of frequent pregnancies : and the husband should control him- 

 self accordingly. 



The Intra-Uterine Nutrition of the Embryo. At first the em- 

 bryo is nourished by absorption of materials from the soft vas- 

 cular lining of the womb; as it increases in size this is not suffi- 

 cient, and a new organ, the placenta, is formed for the purpose. 

 A fetal outgrowth, the allantois, plants itself firmly against the 

 decidua serotina, and villi developed on it burrow from its surface 

 into the uterine mucous membrane. In the deeper layer of this 

 latter are large sinuses through which the maternal blood flows, 

 and into which the allantoic villi project. Blood is brought from 

 the fetus to the allantois by arteries and carried back by veins 

 after traversing the capillaries of the villi, and while flowing 

 through these receives, by dialysis, oxygen and food materials 

 from the maternal blood, and gives up to it carbon dioxid, urea, 

 and other wastes. There is thus no direct intermixture of the two 

 bloods; the embryo is from the first an essentially separate and 

 independent organism. The allantois and decidua serotina be- 

 coming inseparably united together form the placenta, which in 

 the human species is, when fully developed, a round thick mass 

 about the size of a large saucer, connected to the embryo by a 

 narrow stalk, the umbilical cord, in which blood-vessels run to and 

 from the placenta. 



Parturition. At the end of from 275 to 280 days from fertiliza- 

 tion of the ovum (conception) pregnancy terminates, and the child 

 is expelled by powerful contractions of the uterus, assisted by 

 those of the muscles in the abdominal walls. When the child is 

 born, it has attached to its navel the umbilical cord, which is 

 then usually ligatured and cut across: some good authorities, 

 however, maintain that this should not be done until after the 

 contractions which expel the placenta, as. otherwise a quantity 

 of the infant's blood remains in that organ; the loss of which 

 might be serious to a feeble infant. Shortly after the birth of the 

 child renewed uterine contractions detach and expel the placenta, 

 both its fetal or allantoic and maternal or decidual part, as the 

 afterbirth. Where it is torn loose from the uterine wall large blood 

 sinuses are left open; hence a certain amount of bleeding occurs, 



