In the Cervix.—The internal os is defined by a muscular ring separa ting the cavity of the cervix and the body. The internal layer is made up of vertical fibres, which form the arbor vita, and of interlacing fibres forming incomplete rings, that are well marked about the external orifice.
The uterus is thus formed of three layers, the external and internal layers of which, longitudinal and transverse fibres, are disposed to expel the fcetus, opposing the annular fibres of the cervix. The middle layer acts on the circulation. Pinard says each ring seems to be a living lig ature about the uterine vessels, limiting the blood supply and preventing hemorrhage.
IV. Changes in the Mucous Membrane.—Decidua.
The uterine mucosa suffers great changes. It is to cover the ovum and with it to sever its connection with the internal surface of the uterus: hence the name decidua. Coate and Robin were those who finally over threw the ancient theory [Hunter's] and established the true one.
The existence of a mucous membrane of the uterus was not known to older writers, who, however, described a third envelope of the festal cover ings. Hunter, Moreau, Breschet and Velpeau thought that, under the influence of fecundation, a layer of coagulable lymph covered the internal surface of the uterus.
The ovum, coming from the tube, met this membrane, pushed it in front of it, producing two folds: one the parietal or true decidua, ad herent to the uterus; the other the ovular or reflected decidua, in contact with the ovum. But as they saw that the ovum was separated from the muscular layer of the uterus by a membrane identical with these two, they said the ovum induced an irritation which led to the secretion of lymph, where the ovum began to push out the parietal decidua to trans form it into the reflected decidua. As this membrane was between the ovum and the uterus, and as it appeared late, it was called the inter mediate decidua, or decidua serotina (serotina tardiva). (Fig. 88.) As it developer, this serotina joins the fold formed by reflection of the parietal decidua and becomes one with it. Thus the three decidute are one. But as the ovum grows, it pushes ahead of it the decidua reflexa, which also tends to unite with the parietal decidua. But the two de cidute formed of lymph are always separated by a layer of fluid lymph: • the hydroperion. Thus, from without inwards, we find parietal decidua, hydroperion, decidua reflexa, chorion and amnion.
This theory was exploded by Costs, who proved that the orifices of the tubes are open as the ovum arrives at the uterus, that the hydroperion does not exist, and that what was formerly believed to be coagulable lymph, was only the changed uterine mucosa, modified in all its elements.
Costa's explanation is as follows: As the fecundated ovum reaches the uterus, the mucosa is swollen and thrown into folds, so that, when the ovum falls into the uterus, near the tubes as a rule, it is caught in one of these folds. In this depression the ovum induces phenomena like that
caused by issue-peas. The mucosa bulges up around it and covers it. The point where it still communicates with the uterine cavity is the de cidual umbilicus. This soon disappears, and then the ovum is wholly invested. The membrane covering the uterine wall is the parietal de cidua; that about the ovum is the ovular or decidua reflexa; the third, where the ovum was primarily implanted, where the villosities of the chorion, and, later, the placenta are to form, is the inter-utero-placental decidua, or decidua serotina. Each of these deciduas undergoes important changes.
1st. The growth of the ovum brings the ovular nearer to the parietal decidua. Thus, at the third montk, the ovular decidua is everywhere in contact with the parietal mucosa covering the orifice of the tubes, and blending with the inter-utero-placental mucous membrane. The uterine mucosa seems to be one complete whole, but the ovular and parietal de eidiue are only in simple contact; they can be separated. The hydro perion, however, does not exist. At the fourth month, the two decidna3 become one membrane; and now the ovular decidua adheres to the chorion. Now also the villosities of the chorion reach their highest de velopment at the inter-utero-placental mucous membrane.
The placenta forms, and the ovum has the definite structure that it presents when expelled.
Now also appears the new mucous membrane, loosening the old mucosa, from the muscular layer of the uterus, thus truly changing it into de cidua.
Structural modifications attend these changes.
The normal mucosa consists (Robin) of the following: 1st. Numerous follicles.
2d. A small number of special cells.
3d. Numerous embryo-plastic nuclei.
4th. Laminated fibres, at times embryonic (fibro-plastic bodies); at times, completely developed filaments.
5th. Amorphous matter. • 6th. Vessels, nearly all capillaries.
7th. Prismoidal epithelium.
Now us let see what occurs in pregnancy.
Up to the formation of the placenta (second month), there is hyper trophy of the uterine decidua, every element partaking. Thereafter, hypertrophy concentrates in the inter-utero-placental mucosa, the place where the placenta is to form, while the remainder of the mucosa (ovular and parietal decidua) undergoes atrophy, so that the new membrane can easily replace it.
After the fourth month, the parietal and ovular mucosa form one membrane, shed with the ovum at birth, and replaced by a new uterine mucous membrane. (Fig. 88.) In studying how these changes occur in the different elements we will follow Ch. Robin, who was the first to investigate them.