What structures make up the midline of the pelvic girdle?

The midline of the pelvic girdle is constituted by the sacrum and coccyx. The sacrum is a triangular bone formed by the fusion of several vertebrae and sits at the base of the spine, connecting to the iliac bones of the os coxae, which are the hip bones that make up part of the pelvic girdle. The coccyx, commonly referred to as the tailbone, is located at the very end of the sacrum. Together, these structures form the posterior aspect of the pelvis and provide crucial support for the pelvic organs, while also serving as an attachment point for various ligaments and muscles that stabilize the pelvic region.

The other structures mentioned in the options play different roles; the os coxae or hip bones comprise the lateral and anterior aspects of the pelvis, the femur and tibia are bones found in the leg, and the ilium and ischium are parts of the os coxae but do not comprise the midline. The sacrum and coccyx's central positioning along the body's axis makes them the defining structures of the pelvic midline.