The pelvic cavity is really the inferior end of the abdominal cavity and is not separated from the abdomen with a muscular diaphragm like the thorax.

For descriptive purposes, anatomists agree that the most superior part of the pelvis begins on a plane running transversely between the two iliac crests. The pelvic cavity extends to the floor of the pelvis called the pelvic diaphragm. The pelvic diaphragm is composed of the levator ani and coccygeus muscles together with their fascia stretching across the bottom of the pelvic cavity like a hammock.

The pelvic cavity is further subdivided into a superior false pelvis partially surrounded by bone and extending from the crest of the ilia to the pelvic inlet or pelvic brim. The true pelvis is completely surrounded by bone and begins at the pelvic inlet to extend to the the pelvic diaphragm. The false pelvis primarily houses the lower portions of the gastrointestinal system, while the true pelvis mainly houses the organs of the reproductive system.

The pelvic inlet or the superior boundary of the true pelvis is defined as a plane that runs from the sacral promontory to the superior aspect of the pubic symphysis.

The pelvic outlet lies on a plane running from the inferior tip of the coccyx to the inferior aspect of the pubic symphysis. It lies between the two ischial tuberosities. The bony pelvic outlet provides the substrate for the hammock-like muscular pelvic diaphragm. Because of the significant role they play in childbirth, the pelvic inlet and outlet are clinically very important.

The actual floor of the pelvis is composed of the levator ani muscle. This muscle forms a sling running posteriorly from the pubis and ilium with the right and left halves meeting posterior to the rectum. It also makes connections with the coccygeal vertebrae. The levator ani forms the structural basis for the surface area between the legs known as the perineum.

The lateral wall of the true pelvis is formed by the obturator internus and the posterior wall is made of the coccygeus and piriformis muscles.

All of these muscles not only provide the physical floor for the entire abdominal cavity, but they also act as sphincters for the various external openings of the urinary, reproductive and gastrointestinal systems.