The pelvis is a basin bridging spine, legs, and birth
The pelvis is the trunk’s lower story between abdomen and thighs—bony ring, cavity, floor, and perineum together. Hip bones meet the sacrum and coccyx behind and each other in front, then take the femurs at the hips. In mammals the central gap is wider in females because offspring must pass.
Anatomists apply the word pelvis both to the lowest region of the trunk, below the abdomen and above the thighs, and to the skeleton set inside it. The region stacks four layers: the bony pelvis, the pelvic cavity it encloses, the pelvic floor beneath that cavity, and the perineum below the floor. The cavity mainly holds the reproductive organs and the rectum.
The bony ring is built from the sacrum and coccyx at the back and a pair of hip bones at the front and sides. The hip bones are the bridge between spine and legs: they attach to the sacrum behind, meet each other in front, and take the thigh bones at the hip joints. Anatomists call the hip bones the pelvic girdle and the sacrum and coccyx the pelvic spine. Each hip bone begins life as three separate pieces, the ilium, ischium and pubis, held together in childhood by a Y-shaped triradiate cartilage and fused into one bone at puberty.
Definitions of the cavity differ. Narrowly, it is the space between the pelvic brim above and the floor below. Broadly, it is everything the skeleton encloses, split at the brim into the greater or false pelvis above and the lesser or true pelvis below. The entrance to the lesser pelvis, the inlet, is framed by landmarks such as the sacral promontory and the top of the pubic symphysis, while the outlet lies between the pubic arch, the ischial tuberosities and the coccyx. Another scheme simply names three planes: inlet, midplane and outlet.
The pelvic floor has a built-in conflict. It must close off the pelvic and abdominal cavities and carry the weight of the organs above, yet it is pierced by the rectum and urogenital openings it has to control, which weakens it. Overlapping sheets of muscle and connective tissue let it do both. In mammals the central opening of the bony pelvis is markedly larger in females, since the young pass through it at birth.
Source: Pelvis
