626 THE PLACENTA. 



the decidua, they immediately dilate into the placental sinuses 

 (represented, in the diagram, in black), which extend through the 



Fig. 228. 



c 



Vertical section of PLACENTA, showing arrangement of maternal and foetal vessels, a, a. Cho- 

 rion. ft, 6. Decidua. c, c, c, c. Orifices of uterine sinuses. 



whole thickness of the organ, closely embracing all the ramifica- 

 tions of the foetal tufts. It will be seen, therefore, that the placenta, 

 arrived at this stage of completion, is composed essentially of no- 

 thing but bloodvessels. No other tissues enter into its structure ; 

 for all those which it originally contained have disappeared, except- 

 ing the bloodvessels of the foetus, entangled with and adherent to 

 the bloodvessels of the mother. 



There is, however, no direct communication between the foetal 

 and maternal vessels. The blood of the foetus is always separated 

 from the blood of the mother by a membrane which has resulted 

 from the successive union and fusion of four different membranes, 

 viz., first, the membrane of the foetal villus ; secondly, that of the 

 uterine follicle ; thirdly, the wall of the foetal bloodvessel ; and, 

 fourthly, the wall of the uterine sinus. The single membrane, how- 

 ever, into which these four finally coalesce, is extremely thin, as 

 we have seen, and of enormous extent, owing to the extremely 

 abundant branching and subdivision of the foetal tufts. These tufts, 

 accordingly, in which the blood of the foetus circulates, are bathed 

 everywhere, in the placental sinuses, with the blood of the mother ; 

 and the processes of endosmosis and exosmosis, of exhalation and 

 absorption, go on between the two with the greatest possible 

 activity. 



