There seem to be two factors that result in the production of inversion: a degeneration or atrophy of part of the uterine wall, and traction, as from the drag of a uterine polyp or of the umbilical cord. These causes may act together or independently. If a portion of the uterine wall has lost its strength or tonicity, it may be depressed toward the uterine cavity. The depression is increased by the traction of a tumor or of the umbilical cord. The inversion having been started in this way, may be rapidly increased by uterine contractions. Emmet says that inversion usually takes place between the birth of the child and the delivery of the placenta. A consideration of the subject of acute inversion following labor belongs to obstetrics. It is very important that reduction should be accomplished immediately. The delay of a few hours greatly increases the difficulty of replacement. Emmet says: “The uterus is generally well contracted in twelve hours, and with many cases it would be then quite as difficult to effect a reduction as if a year had elapsed.” If the placenta is still attached to the inverted uterus, it should be removed before reduction is attempted. Inversion of the uterus when seen by the gynecologist is usually of the chronic form. It has existed for a few weeks or for several years. Various degrees of inversion are met with. Rarely inversion of one horn of the uterus is seen. In the case of fibroid polyp there may be a slight depression of part of the uterine wall, resulting from local atrophy and traction. In other cases inversion of the fundus as far as the internal os exists. The most usual condition is one of complete inversion, in which the body of the uterus protrudes from the external os into the vagina (Fig. 138). The cervix may or may not be inverted. Sometimes the inversion is complicated by vaginal prolapse--or, rather, by inversion of the vagina--so that the whole genital tract becomes turned inside out and protrudes from the vulva. The exposed endometrium becomes congested and bleeds easily. Ulceration or gangrene may result. [Illustration: FIG. 138.--Complete inversion of the uterus.] If the inversion is extensive, the Fallopian tubes and the ovaries are drawn in the cup formed on the upper aspect of the uterus. Intestines or omentum may also lie in this cup. In cases of long standing the rim of the cup formed by the muscular cervix becomes very much contracted, and adhesions may take place between the peritoneal surfaces. These complications offer great, sometimes insurmountable, difficulty to reduction in old cases. Inversion of the uterus is not a common disease. It is very rarely seen at the present day. By far the most frequent form is that which follows labor; it is much less often caused by fibroid polyp. It seems especially likely to occur in sarcoma of the uterus.