The severer forms of prolapse of the uterus are often very difficult to relieve by mechanical means. The pessaries commonly used are of two forms: (1) those supported externally by belts or straps, like the O'Leary: (2) those which are self-retaining, which are in the form of discs, rings or balls. The self-retaining theoretically should be the most convenient and comfortable, but practically they are often failures because they cannot be made to stay in place.
When it is found that the ring, the disc, or the ball will not stay in place, it is natural to ask why, and the accompanying diagrams have been worked out in an attempt to show the reason.
If in preparing to fit a pessary we examine the vaginal cavity, we will find on either side a shelf (Fig. 1). This is on the upper or rectovesical surface of the levator ani muscle and corresponds