If the vagina were an open tube admitting air, so that the uterus above was in contact with the contents of the pelvic vessel and below with atmospheric air, then the condition of things would be altered. In this case the uterus would in reality become part of the floor of the vessel, and would be subjected to a pressure from above equal to the intra-abdominal pressure, and to this pressure would be opposed only the strength of the uterus and its attachments. Such a state of things occurs when the perineum is torn and the vagina becomes a patulous open canal, and not a closed slit. Therefore when the opening of the vagina is torn and air constantly enters the vaginal canal, the normal hydrostatic equilibrium of the pelvic contents is destroyed, the resultant of the forces acting upon the uterus is downward, and the organ has a tendency to fall or to prolapse (Fig. 65). The normal perineum and vagina do not sustain the uterus by furnishing a mechanical support from below, any more than the bottom of a vessel sustains any single particle of fluid floating in it. When the uterus tends to fall down or to prolapse, its progress is opposed at a certain level by its various attachments. The ligaments become suspensory in character as soon as their uterine attachments are below their pelvic attachments. The cellular tissue, fat, blood-vessels, etc. connected with the uterus restrain its downward motion. And, finally, this motion is restrained by what has been called the “retentive power of the abdomen,” which is merely the atmospheric pressure acting from below on the contents of a vessel the top and sides of which are closed. [Illustration: FIG. 65.--Diagram representing the direction of the intra-abdominal pressure in the woman with a laceration of the perineum.] Refer again to a simple physical example: If a glass tube be filled with water, a finger placed over one end, and the tube inverted, the water will not run out: it is sustained by atmospheric pressure acting from below. If the finger be removed, atmospheric pressure also acts from above, and the water will fall. If a hole be made in the side of the tube, atmospheric pressure will act through it, and the water below the hole will fall. In order that the column of water be sustained, the sides of the tube must be rigid or unyielding. If the sides of the tube yielded slightly to atmospheric pressure, they would sink in and a certain amount of water would escape.