Pelvic Gridle: Sacrum, Coccyx and Os Coxae

The bony structure at the base of the front limbs is the shoulder gridle and the one at the base of the hind limbs is the Pelvic gridle. The pelvic gridles of terrestial vartebrates are connected to the vertebral column and are much larger than their homologs in fish. These adaptation are required for weight bearing and muscle attachment in terrestial forms. In early land-dwelling vertebrates, the right and left limb gridles joined dorsally with the sacral vertebrae to form a bony ring around the rear of the trunk.
The adult human bony pelvic comprises three main elemnts: the right and left Os Coxae and the Sacrum and Coccyx. The sacrum and coccyx are part of the axial skeleton and are actually variably fused vartebrae. The bony pelvis functions to support and protect the abdominal and pelvic organs. In additon, it anchor muscles of the abdomen leg. Unlike the shoulder gridle, which is movable platform, the pelvic gridle is firmly fixed to the axial skeleton via its vertebral element, the sacrum.

The pelvis is divided by an oblique plane passing through the prominence of the sacrum, the arcuate and pectineal lines, and the upper margin of the symphysis pubis, into the greater and the lesser pelvis. The circumference of this plane is termed the linea terminalis or pelvic brim.

The Greater or False Pelvis (pelvis major):
The greater pelvis is the expanded portion of the cavity situated above and in front of the pelvic brim. It is bounded on either side by the ilium; in front it is incomplete, presenting a wide interval between the anterior borders of the ilia, which is filled up in the fresh state by the parietes of the abdomen; behind is a deep notch on either side between the ilium and the base of the sacrum. It supports the intestines, and transmits part of their weight to the anterior wall of the abdomen.

The Lesser or True Pelvis (pelvis minor):
The lesser pelvis is that part of the pelvic cavity which is situated below and behind the pelvic brim. Its bony walls are more complete than those of the greater pelvis. For convenience of description, it is divided into an inlet bounded by the superior circumference, and outlet bounded by the inferior circumference, and a cavity.