THE PLACENTA. 659 



But when the placenta is detached, and its maternal vessels torn off at 

 their necks (Fig. 211, c, c, c, c), the sinuses, emptied of blood by the 

 compression to which they are subjected, are apparently obliterated ; 

 and the foetal tufts, lying in contact with each other, appear to con- 

 stitute the whole of the placental mass. The existence of the sinuses, 

 however, and their extent, may be demonstrated in the following 

 manner : 



If the uterus of a woman who has died undelivered at the full term 

 or thereabout, be opened without wounding the placenta, this organ 

 will be seen attached to the uterine surface, with its vascular con- 

 nections complete. Let the foetus be removed by dividing the umbili- 

 cal cord, and the uterus, with the placenta attached, placed under water, 

 with its internal surface uppermost. If a blowpipe be now inserted into 

 one of the divided vessels of the uterine walls, and air forced through 

 it under gentle and steady pressure, it will inflate, first, the vascular 

 sinuses of the uterus ; next, the deeper portions of the placenta ; and 

 lastly, the air-bubbles insinuate themselves everywhere between the 

 foetal tufts, and appear in the most superficial portions of the placenta, 

 immediately beneath the chorion (a, , Fig. 211). This shows that the 

 placental sinuses, which communicate with the uterine vessels, occupy 

 the entire thickness of the placenta, and are equally extensive with the 

 tufts of the chorion. 



If the placenta be detached and separately examined, it will be found 

 to present on its uterine surface a number of openings, extremely 

 oblique in position, and bounded on one side by a thin crescentic edge. 

 These are the orifices of the uterine blood-vessels, passing into the pla- 

 centa and torn off at their necks, as above described ; and, by careful 

 dissection, they are found to lead into extensive collapsed cavities (the 

 placental sinuses), between the foetal tufts. These cavities are filled 

 during life with the maternal blood ; and there is every reason to 

 believe that before delivery, while the circulation is going on, the 

 placenta is at least twice as large as after it has been expelled from 

 the uterus. 



The part taken by the placenta in the development of the foetus is 

 of primary importance. From the date of its formation, about the 

 beginning of the fourth month, it is the only channel for the convey- 

 ance of nourishment from the mother to the foetus. The nutritious 

 materials, circulating in the maternal sinuses, pass through the inter- 

 vening membrane, and enter the blood of the foetus. The healthy or 

 injurious regimen to which the mother is subjected will, accordingly, 

 exert an influence upon the child. Even medicinal substances taken 

 by the mother, and absorbed into her circulation, may transude through 

 the placental vessels, and thus produce their specific effect on the foetal 

 organization. 



The placenta is an organ of exhalation as well as of absorption. The 

 excrementitious matters in the blood of the foetus are undoubtedly dis- 

 posed of in great measure by transudation through the placental ves- 



