The pelvis is the lowest part of the trunk and is mounted on the thighs. The pelvis is a rigid part of the body, despite the contortions some so-called dancers are able to go into with this part of the body .it is made the two hipbones (innominate bones) rigidly joined together in front at the pubes, behind, these two bones are joined just as rigidly to the rigid lower part of the spine,the scarum, with the coccyx hanging down stiffly below it, like a chopped-off tail end.
The upper and the larger opening of the funnel-shaped pelvic cavity is in communication with the abdominal cavity, while the narrow lower outlet or bottom of the pelvic cavity is covered by muscles and fascia, forming a strong floor, the "perineum" or pelvic floor which supports the pelvic organs.
In the male the perineum is pierced by the openings of the urethra and anus, in the female the opening of the vagina is between the two former openings.
Under great strain and during childbirth the rigidly joined pelvic bones give way slightly at the sacroiliac joints,causing what is called sacroiliac pain in women who have borne children