STEUCTURE OF THE PLACENTA. 



719 



closely united with, and so nearly assimilated in its structure to 

 the villi, that it has been found difficult to follow it with certainty, 

 and its existence has even by many been entirely denied. The 

 relations of these two parts of the placenta, as ascertained by the 

 observation of their gradual development in the growth of the 

 placenta, and their comparative anatomy in animals, will be referred to 

 hereafter. 



The whole of the placental mass, together with the layer of decidua 

 on its external or uterine surface, and the united decidua vera and reflexa 

 are separated at birth along with the foetus and its membranes. 



Fig. 522. — Vertical Section thkough the jiiddle part of the Placenta and the 

 XJterixe Wall (from Farre after Wagner). 



Tlie preparation was from a woman who died in the thirtieth week of gestation : the 

 fines V, u, run through the wall of the uterus to the outer surface of the placenta ; d, the 

 decidua serotina ; p, the tufts of foetal vascular villi, of which two larger divisions are 

 separated by decidual septa, as at dp ; /, the placental end of the umhilical cord ; 

 am, the amnion ; cli, the chorion ; vf, divided fcetal blood-vessels ; r, stems of vascular 

 villi ; us, uterine sinuses or veins ; a, a, coiled arteries passing into the placenta. 



Circulation of blood in the placenta. — The existence of a distinct 

 circulation of blood in the fcetal and in the maternal vessels of the 

 placenta, discovered by the Hunters, has long been placed beyond 

 doubt by the experimental investigations of all those who have injected 

 the two sets of vessels with sufficient care and success. The nature of 

 the distribution of the vessels is very different in the two parts of the 

 placenta. In the tufts of foetal villi, the umbilical arteries and veins, 

 possessed of distinct coats, undergo gradual subdivision by ramification 

 into smaller and smaller tubes, until they at last reach capillary minute- 

 ness, and the terminal capillaries run in long and tortuous loops which 



