PLACENTA (Lat. for a cake), in anatomy, the organ by which the embryo is nourished within the womb of its mother. When the young one is born the placenta and membranes come away as the " afterbirth." In human anatomy the organ is a circular disk about seven or eight inches in diameter and one and a quarter inches in thickness at its centre, while at its margin it is very thin and is continuous with the foetal membranes. It weighs about a pound.
In order to explain the formation of the placenta it is necessary to encroach to some extent on the domain of physiology. Before each menstrual period, during the child-bearing age of a woman, the mucous membrane of the uterus hypertrophies, and, at the period, is cast off and renewed, but if a fertilized ovum reaches the uterus the casting off is postponed until the birth of the child. From the fact that the thickened mucous membrane lining the interior of the uterus is cast off sooner or later, it is spoken of as the " decidua." The fertilized ovum, on reaching the uterus, sinks into and embeds itself in the already prepared decidua, and, as it enlarges, there is one part of the decidua lying between it and the uterine wall . (" decidua serrotina " or " basalis "), one part stretched over_the surface of the enlarging ovum (" decidua renexa " or " capsularis ") and one part lining the rest of the uterus (" decidua vera ")
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)