THE PLACENTA. 759 



at c, c, c, c, are the orifices of uterine vessels which penetrate it from 

 below. These vessels enter the placenta in an extremely oblique direc- 

 tion, though they are represented in the diagram, for the sake of dis- 

 tinctness, as nearly perpendicular. When they have once penetrated 



Fig. 274 



c 



Vertical section of the PLACENTA, showing the arrangement of the maternal and festal 

 bloodvessels. a. a. Chorion. 6,6. Decidua. c, c, c, c. Orifices of uterine sinuses. 



the lower portion of the decidua, they immediately dilate into the pla- 

 cental sinuses (represented, in the diagram, in black), which extend 

 through the whole thickness of the organ, closely embracing all the 

 ramifications of the foetal tufts. It will be seen, therefore, that the pla- 

 centa, arrived at this stage of completion, is composed essentially of 

 nothing but bloodvessels. The other tissues which originally entered 

 into its structure have disappeared, leaving 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 suc- 

 cessive union and fusion of four different membranes, namely : 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 membrane, however, thus produced, is of great 

 extent, owing to the abundant branching and subdivision of the foetal 

 tufts. These tufts, 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 absorption and exhalation go on between the two with 

 a corresponding activity. 



It is easy to demonstrate the arrangement of the foetal tufts in the 

 human placenta. They can be readily seen by the naked eye, and may 



