Ilium (part of the Os Coxae)
Large Flat Bone; part of the limb (appendicular) skeleton forming the pelvic girdle.
Also known as: Hip Bone, Wing of the Pelvis, Flank Bone
Classification
Large Flat Bone; part of the limb (appendicular) skeleton forming the pelvic girdle.
Dimensions
In humans, this is the largest portion of the hip, characterized by a broad, fan-like expansion. It is generally wider than it is tall, providing a large surface area for muscle attachment.
Key Features
The most striking feature is the wide, gently curved top edge, often felt as the top of the hip. The highlighted teal area is the 'iliac fossa,' a smooth, shallow bowl-like depression on the inner surface. Looking at the whole bone, it resembles a large, flattened ear or a butterfly wing with a thickened, rough junction where it meets the spine.
Similar Bones
It can be confused with the shoulder blade (scapula) due to its flat, leafy appearance. However, the hip bone is much thicker, more robust, and features a deep, distinct round socket for the leg bone, which the shoulder blade lacks.
Across Species
The shape of this bone varies wildly based on how an animal moves. In upright walkers like humans, it is short and broad to support internal weight. In four-legged runners like deer or horses, it is long and narrow, oriented towards the back to help generate forward speed. In birds, it is fused into a long, thin structure called a synsacrum to provide a rigid frame for flight.
Evolutionary History
Tracing back to the first land-dwelling tetrapods, this bone began as a simple rod-like structure. As animals transitioned from sprawling, lizard-like gaits to more efficient walking, the bone expanded into a 'blade' to accommodate powerful leg muscles. The transition to bipedalism in the human lineage forced the blade to flare outward and wrap around the sides of the body.
Photography Tips
To identify this bone, take one photo looking directly at the flat, broad 'wing' surface and another from the side to show the deep cup-like socket. Placing a standard ruler or a common object like a coin next to it will help viewers understand its scale.
Notes
what is the highlighted blue part