THE PLACENTA.
The placenta is a soft,•vascular organ, varying in size and shape, and ranging, in color from a bright to a dark red. Its function is to form a band of communication between the foetus and the mother.
The ovum, at first free within the uterine cavity, soon becomes fixed to the mucous membrane of the uterus, the villosities that spring from the vitelline membrane forming the first chorion.
This primitive chorion, however, is soon replaced by a second chorion formed from the external layer of the blastoderm. As soon as the allan this comes in contact with this second definite chorion, the umbilical ves sels which it carries penetrate the villi of the chorion. These villosities are rooted deep in the uterine mucous membrane, and establish an inti mate connection between the mother and the fatus.
While, however, those villosities that are in contact with the old or re flected decidua atrophy, those, on the contrary, that are in contact with the utero-placental mucous membrane branch and grow very rapidly. The mucous membrane itself begins to develop, and its blood-vessels be come profoundly modified; so that the villi of the chorion and the utero placental mucous membrane together form a vascular plexus, which is the placenta.
Thus the placenta is both a maternal and a foetal organ, and we may speak of a fatal and of a maternal placenta.
By about the fourth month, the atrophy of the villi in contact with the decidua is complete, and the placenta is definitely formed. At birth the foetal placenta, together with the superficial layer of the utero-pla cental mucous membrane is expelled.
1. Shape.
Although usually oval, the placenta is sometimes round, or even kidney shaped. It is thickest at the centre, save in cases of so-called mem branous placenta, when it is flattened, and of equal thickness throughout. Usually it forms a simple mass, though it may be bi-lobed, or even tri lobed. In exceptional cases there are found, besides the ordinary pla cental mass, several completely detached cotyledons, united to the central body only by the vessels. These are the placenta succenturice. (Fig. 128.) The average placenta is 61 to 7 inches long, 51 to 6i inches broad, I to 11 inches thick at the centre, and 2 to 21 inches thick at the edges. Its weight is very variable, and may be on an average between 16 and 19 ounces. .Thus, in the first table given by Bustamante, and containing 107 cases, the mean weight was 7530 grains, while in his second table of 92 cases the mean weight was 8016 grains. The smallest placenta
he observed weighed 5025 grains, and the largest 20100 grains. In a general way the weight of the placenta varies with the weight of the child.
These figures refer only to the placenta of simple pregnancies; those of twin or triplet births will be considered later.
Although usually implanted at the fundus, the placenta may attach itself to any point of the uterine cavity. When it is attached to the in ferior segment of the organ it forms what is known as placenta prwvia.
The internal or foetal face of the placenta is smooth and moderately convex. It is covered by the chorion and the amnion, which form its most superficial layer. Here we see the implantation of the cord, the umbilical vessels, and, according to Bustamante, a number of little white bodies, the vascular corpuscles. (Fig. 130.) The external or uterine face of the placenta is red, spongy, irregular and bleeding; it is divided by furrows into lobes or cotyledons. A layer of greyish glutinous matter covers the lobes and fills up the furrows.
The circumference is continuous with the chorion and the decidua, up to the point where the great coronary or circular sinus occurs.
II. Structure.
Our knowledge of the structure of the placenta is due to the researches of Hening, Jassinsky, Friedlander, Langhaus, Winkler and Leopold. Two parts are to be distinguished, the one foetal and the other maternal.
1. The Fetal Placenta (Fig. 131).—The festal placenta is essentially composed of the chorionic villi, and the umbilical vessels that ramify in them. A close examination of the organ will show us that each cotyledon is a tuft composed of the ultimate ramifications of the chorional Each villus ends in a cul-de-sac, and most of them are provided with Teasels. Some few, however, according to Langhaus, are not vascularized, and are so solidly implanted in the maternal tissue that they cannot be detached without tearing the uterine wall. Winkler states that the ar chitectural framework of the placenta is furnished chiefly by the mater nal portion of the organ, and that the villi of the frets] placenta penetrate its meshes. Weber, Sharpey and Jassinsky claim that the villosities penetrate the uterine glands. Ercolani has even gone so far as to describe a special glandular organ of new formation in the utero-placental deci dua, into which the villosities plunge, but his description is rejected by all other authorities.