PLACENTA.
The placenta is the organ provided in each pregnancy for the nutrition and respira tion of the fcetus. To this it is connected on its free side by the umbilical cord, while its opposite or attached surface is united to the fundus, sides, or lower part of the body of the uterus by a lay er of the decidua. A placenta exists only in the mammalia and in some of the cartilaginous fishes. It is composed of structures derived partly from the ovum and partly front the uterus. The fcetal or embry onal portion is not always furnished by the same portion of the ovum. It is sometimes constructed from the yolk sac, as in certain sharks, and the vessels which ramify in it are then the branches of the omphalo-mesenteric artery and vein. In other cases, as in the mammalia, the chorion supplies the fcetal portion, which is here rendered vascular by the umbilical vessels derived from the allan tois. The maternal or uterine portion of the placenta is furnished by the decidua or lining membrane of the uterus. These two portions, viz., the fcetal and the maternal, originally dis tinct, and, even in their subsequent union, pre serving a certain independence, become more or less closely connected together by interdigi tating the one with the other. Their union may be one of mere contact, the fcetal portion forraing numerous projecting vascular folds which in the form of 'minx or tufts, or single villi, are received into corresponding depres sions or sulci, equally vascular, formed in the lining membrane of the uterus. Or it may consist in a more intimate conjunction of these parts, such as takes place in man, where the decidua or maternal portion forms a lamina which is spread over and united to the groups of villi that constitute the fcetal portion. In the former case at the time of parturition the two portions are separated, the fcetal processes being simply drawn out of the recesses which contained them without laceration of either of the tissues. But in the latter, the one part cannot be expelled without carrying a con siderable portion of the other with it.
Form.—The mammalian placenta exhibits numerous varieties of form. In most Rumi nants it is composed of numerous detached placentulm constitutmg groups or bosses of vascular villi that project from the surface of the chorion, and are received into correspond ing cotulm upon the inner surface of the uterus.
In the Carnivora the placenta encircles the fcetus in the form of a broad flat belt. In Pachydermata, Cetacea, and many other fami lies, the villi are nearly evenly distributed over the whole surface of the chorion, so that the fcetus is everywhere surrounded by placenta. In some Rodentia and Quadrumana the pla centa is double, In man the placenta forms a single discoid organ, which in its natural position is slightly convex upon the outer, and concave upon the inner superficies. Its outline is generally cir cular or oval; it is sometimes reniform,cordate, or more or less triangular. It is rarely bilobed or nzultilobed.
Dimensions and weight.—The size of the placenta is exceedingly variable, bearing usually a certain proportion to the bulk of the child. A full-sized oval placenta measures 7-7i" in its shorter, and 8-9-3/4" in its longer diameter, and measures 23-24" in circumference. The thickness is generally greatest opposite to the point of entrance of the funis, where the organ measures commonly 1-11", but it becomes gradually attenuated towards the margin, which is slightly rounded, measuring here only 2-4!" in thickness. The weight of the plazenta ranges from I5--30 oz. or more.
Retal szalace.—Upon the fcetal surface of the placenta are observed portions of the amnion and chorion, together with the root of the funis and the principal branches of the umbilical arteries and vein.
Amaion.—The amnion (fig. 484. am), after furnishing the outer covering of the funis, passes off in all directions at the root of the cord, and spreads in a thin opaline lamina over the fcetal surface of the placenta, to which it slightly adheres. In some cases, especially when the umbilical vessels divide before entering the placenta, the amnion has no attachment at all to the latter. The am nion of the placenta does not differ in any respect from the rest of the amniotic sac, of which the placental portion constitutes about one third. Upon its fcetal surface is a single layer of flattened polygonal cells filled with delicate fat granulations.