Ilium (bone) in the context of "Legless lizard"

The sacrum (pl.: sacra or sacrums), in human anatomy, is a triangular bone at the base of the spine that forms by the fusing of the sacral vertebrae (S1–S5) between ages 18 and 30.

The sacrum situates at the upper, back part of the pelvic cavity, between the two wings of the pelvis. It forms joints with four other bones. The two projections at the sides of the sacrum are called the alae (wings), and articulate with the ilium at the L-shaped sacroiliac joints. The upper part of the sacrum connects with the last lumbar vertebra (L5), and its lower part with the coccyx (tailbone) via the sacral and coccygeal cornua.

In vertebrate anatomy, the hip, or coxa (pl.: coxae) in medical terminology, refers to either an anatomical region or a joint on the outer (lateral) side of the pelvis.

The hip region is located lateral and anterior to the gluteal region, inferior to the iliac crest, and lateral to the obturator foramen, with muscle tendons and soft tissues overlying the greater trochanter of the femur. In adults, the three pelvic bones (ilium, ischium and pubis) have fused into one hip bone, which forms the superomedial/deep wall of the hip region.

The hip bone (os coxae, innominate bone, pelvic bone or coxal bone) is a large flat bone, constricted in the center and expanded above and below. In some vertebrates (including humans before puberty) it is composed of three parts: the ilium, ischium, and the pubis.

The two hip bones join at the pubic symphysis and together with the sacrum and coccyx (the pelvic part of the spine) comprise the skeletal component of the pelvis – the pelvic girdle which surrounds the pelvic cavity. They are connected to the sacrum, which is part of the axial skeleton, at the sacroiliac joint. Each hip bone is connected to the corresponding femur (thigh bone) (forming the primary connection between the bones of the lower limb and the axial skeleton) through the large ball and socket joint of the hip.

The sacroiliac joint or SI joint (SIJ) is the joint between the sacrum and the ilium bones of the pelvis, which are connected by strong ligaments. In humans, the sacrum supports the spine and is supported in turn by an ilium on each side. The joint is strong, supporting the entire weight of the upper body. It is a synovial plane joint with irregular elevations and depressions that produce interlocking of the two bones. The human body has two sacroiliac joints, one on the left and one on the right, that often match each other but are highly variable from person to person.