24 THE HUMAN BODY. 



The Intra-Uterine Nutrition of the Embryo. At first 

 the embryo is nourished by absorption of materials from 

 the soft vascular lining of the womb; as it increases in size- 

 this is not sufficient, and a new organ, the placenta, is 

 formed for the purpose. The allantois plants- itself firmly 

 against the decidua serotina (p. 18), 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. Into 

 the allantoic villi total blood-vessels run and form capil- 

 lary networks; the blood flowing through these receives, by 

 dialysis, oxygen and food materials from the maternal 

 'blood, and gives up to it carbon dioxide, 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 becoming closely 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. 



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

 fertilization of tire 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-~lmibilical cord, through which arteries run to, and 

 veins from, the placenta. Shortly afterwards the latter is 

 detached and expelled (both its allantoic and decidual 

 parts), and where it is torn loose from the uterine wall 

 large blood sinuses are left open and exposed. Hence a 

 certain amount of bleeding occurs, but in normal labor this 

 is speedily checked by firm contraction of the uterus. 

 Should this fail to take place profuse haemorrhage occurs 

 (flooding] and the mother may bleed to death in a few min- 

 utes unless prompt measures are adopted. 



For a few days after delivery there is some discharge (the- 

 lochia) from the uterine cayity: the whole decidua being 

 broken down and carried off, to be subsequently replaced by 

 new mucous membrane. The muscular fibres developed in 

 the uterine wall in such large quantities during pregnancy, 



