The ligaments of the uterus act as guys. They steady it, and prevent too great lateral and fore-and-aft movement; they do not, when the uterus is in its normal position or at its normal level, sustain it against the force of gravity. When, however, the uterus, for any reason, falls an inch or more below its normal level, the uterine ligaments become suspensory in character. In the normal woman the vagina is always closed. As has already been said, it is a slit in the pelvic floor, valvular in character; consequently the abdominal and pelvic viscera may be considered to be contained in a closed vessel, in woman as well as in man. The uterus floats in this closed vessel at a level which is consistent with its own specific gravity. If, for any reason, the specific gravity of the uterus were increased, it would sink below the level at which it is normally situated. Since, normally, there is no tendency in the uterus to change its position, the pressure upon it must be equal in all directions. The subject may perhaps be better understood by referring to a few simple facts in hydrostatics. If a fluid contained in a closed vessel be in a condition of equilibrium so that its various particles are at rest, then the pressure upon any particle is equal and opposite in all directions (Fig. 63); otherwise the particles would not be in equilibrium, but would move. The bottom of such a vessel, however, is not, like the particles of the fluid, surrounded on all sides by the fluid, but above it is the fluid, and below it is the atmospheric air. Any point upon the bottom of the vessel is subjected to a downward pressure equal to the weight of the column of fluid above the point; this downward pressure is resisted by the strength of the material composing the vessel. If this material be yielding or elastic in character, the pressure above will make the bottom protrude to a certain extent. A particle within the fluid (like X immediately above the bottom of the vessel) will be subjected to a downward pressure equal to the weight of the column of fluid above it; but this pressure will be counterbalanced not by any strength in the particle, but by a counter-force acting from below equal and opposite to that acting from above. [Illustration: FIG. 63.--Vessel containing fluid in equilibrium. The arrows indicate the direction of the pressure at various points.] A similar state of things exists in the female pelvis. The uterus floats at a certain level, and the intra-abdominal pressure acting from above is counterbalanced by an equal force acting from below, while the floor or bottom of this vessel (part of which is the perineum) is subjected to a force from above equal to the intra-abdominal pressure, and this force is opposed only by the strength of the perineum (see Fig. 64). [Illustration: FIG. 64.--Diagram representing the directions of the intra-abdominal pressure upon the uterus in the uninjured woman.]