- Ilium (bone)
Bone: Ilium of pelvis Overview of Ilium as largest bone of the pelvis. Capsule of hip-joint (distended). Posterior aspect. (Ilium labeled at top.) Latin os ilii Gray's subject #57 236 MeSH Ilium
The ilium is the uppermost and largest bone of the pelvis, and appears in most vertebrates including mammals and birds, but not bony fish. All reptiles have an ilium except snakes, although some snake species have a tiny bone which is considered to be an ilium.
The ilium of the human is divisible into two parts, the body and the ala; the separation is indicated on the top surface by a curved line, the arcuate line, and on the external surface by the margin of the acetabulum.
Body (corpus oss. ilii)
The body enters into the formation of the acetabulum, of which it forms rather less than two-fifths.
Its external surface is partly articular, partly non-articular; the articular segment forms part of the lunate surface of the acetabulum, the non-articular portion contributes to the acetabular fossa.
The internal surface of the body is part of the wall of the lesser pelvis and gives origin to some fibers of the Obturator internus.
Ala (ala oss. ilii)
The wing of ilium (or ala) is the large expanded portion which bounds the greater pelvis laterally. It presents for examination two surfaces—an external and an internal—a crest, and two borders—an anterior and a posterior.
In humans, biiliac width is an anatomical term referring to the widest measure of the pelvis between the outer edges of the upper iliac bones.
Biiliac width has the following common synonyms: pelvic bone width, biiliac breadth, intercristal breadth/width, bi-iliac breadth/width and biiliocristal breadth/width.
In the average adult female, it measures 28 cm (11 in). It is best measured by anthropometric calipers (an anthropometer designed for such measurement is called a pelvimeter). Attempting to measure biiliac width with a tape measure along a curved surface is inaccurate.
The biiliac width measure is helpful in obstetrics because a pelvis that is significantly too small or too large can have obstetrical complications. For example, a large baby or a small pelvis often lead to death unless a caesarean section is performed. 
It is also used by anthropologists to estimate body mass.
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Bones of pelvis / pelvic cavity (TA A02.5.01–03, GA 2.231–241) General Iliumarcuate line Ischiumno substructures Pubis Compound
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