The abdominal and pelvic cavities in the erect woman may be considered as a tube filled with fluid contents. The top of the tube is closed by the diaphragm; the sides are the more or less rigid abdominal walls and the back; the floor is the perineum. When the floor is destroyed a hole is made in the bottom of the tube: the contents tend to fall, but the fall is resisted by atmospheric pressure acting from below. If the diaphragm and the parietes were rigid as glass, there would be no prolapse, any more than there is prolapse of the water in the glass tube. If the parietes yield somewhat, the amount of fall or prolapse is proportional. Thus the retentive power of the abdomen is dependent upon the strength or rigidity of the abdominal walls. CHAPTER IX. PROLAPSE OF THE UTERUS. Prolapse of the uterus means a falling of that organ below its normal level. The condition is popularly spoken of as “falling of the womb.” There are an infinite number of degrees of prolapse of the uterus, between the slightest descent on the one hand and complete protrusion of the organ from the body on the other hand. The term “complete prolapse” should properly be applied to the entire protrusion of the uterus outside of the vulva. This condition, however, is most unusual. The term is generally used to designate those cases in which the cervix alone, or the cervix and part of the body of the uterus, protrude from the vulva (Fig. 66). In any case of prolapse of the uterus it is best to describe in detail the extent of the prolapse and the other conditions present. Thus, some of the various kinds of prolapse may be described as follows: “Prolapse of the uterus, the cervix resting on the pelvic floor;” “prolapse of the uterus, the cervix presenting at the vulvar cleft;” “prolapse of the uterus, the cervix protruding about two inches from the ostium vaginæ, with elongation of the supra-vaginal cervix,” etc. Injury to the pelvic floor that allows air to enter the vagina destroys the normal equilibrium of the pelvic contents and exposes the uterus to a direct abdominal pressure from above, which is not counterbalanced by an equal force from below, but is opposed by the strength of the uterus and its attachments and the retentive power of the abdomen. Most cases of prolapse occur in women in whom the perineum has been injured at childbirth. [Illustration: FIG. 66.--Prolapse of the uterus, the cervix protruding from the vulva. There is a bilateral laceration of the cervix.]