786 



HANDBOOK OF PHYSIOLOGY. 



cous membrane of the uterus, at and near the region where the placenta 

 is placed, becomes hollowed out by sinuses, or cavernous spaces, which 

 communicate on the one hand with arteries and on the other with veins 

 of the uterus. Into these sinuses the villi of the chorion protrude, 

 pushing the thin wall of the sinus before them, and so come into inti- 

 mate relation with the blood contained in them. There is no direct 

 communication between the blood-vessels of the mother and those of the 

 foetus; but the layer or layers of membrane intervening between the 



Fig. 483. Diagrammatic view of a vertical transverse section of the uterus at the seventh 

 or eighth week of pregnancy, c, c, c', cavity of uterus, which becomes the cavity of the decidua, 

 opening at c, c, the cornua, into the Fallopian tubes, and at c' into the cavity of the cervix, 

 which is closed by a plug of mucus ; d v, decidua vera ; d r, decidua reflexa, with the sparser 

 villi imbedded in its substance ; d s, decidua serotina, involving the more developed chorionic 

 villi of the commencing placenta. The foetus is seen lying in the amniotic sac ; passing up from 

 the umbilicus is seen the umbilical cord and its vessels, passing to their distribution in the villi 

 of the chorion ; also the pedicle of the yolk-sac, which lies in the cavity between the amnion 

 and chorion. (Allen Thomson.) 



blood of the one and of the other offer no obstacle to a free interchange 

 of matters between them by diffusion and osmosis. Thus the villi of the 

 chorion containing foetal blood, are bathed or soaked in maternal blood 

 contained in the uterine sinuses. The arrangement may be roughly 

 compared to filling a glove with foetal blood, and dipping its fingers 

 into a vessel containing maternal blood. But in the foetal villi there is a 

 constant stream of blood into and out of the loop of capillary blood-vessels 

 contained in it, as there is also into and out of the maternal sinuses. 



