670 MANUAL OF PHYSIOLOGY. 



The nutrient materials from the maternal blood together with 

 oxygen diffuse through the walls of the foetal capillaries, the effete 

 matter, on the other hand, passing from the capillaries to the 

 blood in the veins which surrounds and bathes these vessels. The 

 placenta increases with the growth of the foetus till shortly before 

 birth, when it is said to undergo a certain amount of degenera- 

 tion. It is cast out of the uterus after the expulsion of the foetus 

 with the membranes attached to it. It is, however, only the 

 superficial layer of the maternal placenta (which is intimately 

 connected with the foetal placenta) that is cast off, the deeper 

 layer remaining in the uterus, and undergoing various changes 

 during the reduction of this organ to its normal size. 



After ligature of the umbilical cord, the intimate relationships 

 of the maternal and foetal circulations cease, and it is thought 

 that this causes the inlets of the uterine sinuses to contract, so 

 that when the placenta separates from the uterine walls, the arte- 

 rioles leading to the sinuses are contracted and possibly occluded 

 with clots. The uterine blood current is thus prevented from 

 escaping into the uterine cavity after parturition, and causing 

 profuse haemorrhage. 



The uses of the placenta may be briefly summed up as: 



(1) Alimentary, as it supplies the place of the alimentary canal. 



(2) Respiratory, as it performs the function of the lungs. 



(3) Excretory, as it does duty for the kidneys and some other 

 excretory organs. 



