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HANDBOOK OF PHYSIOLOGY. 



gradually come into contact, and in the third month of pregnancy the 

 cavity between them has quite disappeared. Henceforth it is very diffi- 

 cult, or even impossible, to distinguish the two layers. 



The Placenta. During these changes the deeper part of the mu- 

 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, push- 

 ing the thin wall of the sinus before them, and so come into intimate 

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



FIG. 464. 



FIG. 466. 



FIG. 464. Diagrammatic view of a vertical transverse section of the uterus at the seventh or 

 eighth week of pregnancy, c, c, c', cavity of the 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; dr, decidua reflexa, with the sparser villi im- 

 bedded 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 yelk sac, which lies in the cavity between the amnion and chorion. (Allen Thom- 

 son.) 



FIG. 465. Extremity of a placenta! villus. a, lining membrane of the vascular system of the 

 mother ; b, cells immediately lining a ; d, space between the maternal and foetal portions of the vi- 

 lus; e, internal membrane of the villus, or external membrane of the chorion;/, internal cells of the 

 villus, or cells of the chorion; g, loop of umbilical vessels. (Qoodsir.) 



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

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

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

 of matters between them. Thus the villi of the chorion containing foatal 



