666 MANUAL OF PHYSIOLOGY. 



the membranes of the foetus, and the other belonging to the 

 mother. 



Relation of the Foetal to Maternal Placenta. The maternal part 

 is formed from the decidua serotina, which becomes much thick- 

 ened and very vascular where the placenta is attached . The 

 foetal placenta is derived from the chorion, which sends out a 

 number of finger-like processes, which subdivide, and into which 

 the allantois, as it spreads over the chorion, sends prolongations. 

 The mesoblastic layer of the allantois gives rise to the capillaries 

 which are in these processes. The capillaries spring from the 

 branches of the umbilical arteries which pass along the umbilical 

 cord to reach the chorion. The vessels of the decidua serotina 

 or maternal placenta end in large sinuses, lined by endothelial 

 cells. The blood is carried to these sinuses by the uterine arte- 

 ries, and from them by the uterine veins. The walls of the 

 sinuses are provided with unstriped muscular tissue, which can 

 close the inlets from the arteries, and thus shut out the blood. The 

 villi of the foetal placenta, dipping into these uterine sinuses, are 

 covered with a single layer of thin, scaly cells, so that the foetal 

 blood is only separated from the maternal by the walls of the 

 capillaries and these thin cells, and thus the interchange of nu- 

 trient materials and gases readily goes on between them ; it is very 

 similar to the conditions of the lung alveoli, where the blood is 

 separated from the air with which it interchanges gases by the 

 cells of the capillary wall and of the lung alveolus. 



Though the capillaries of the foetus are in such close relation 

 to the blood of the mother, it must be distinctly understood that 

 there is no direct communication between the vessels of the foetus 

 and those of the mother, and therefore it is not possible to inject 

 the vessels of the mother through those of the foetus, or vice versa. 



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 de- 

 generation. It is cast out of the uterus after the expulsion of 



