The innominate bone in humans is the hip bone, made up of the union of three initally separate bones - the ilium, ischium and the pubis.

Should you be the visual sort, refer to http://ect.downstate.edu/courseware/haonline/labs/l44/st0701.htm, where there are photos pointing out all of the features mentioned here. Yahoo! also provides a public-domain version of Gray's Anatomy with some drawings and ridiculously detailed description with which I will not attempt to compete here. http://education.yahoo.com/reference/gray/subjects/subject?id=57#1

Innominates are pelvic bones, so named (innominate in Latin means "no name") because they do not exactly fit into any bone shape category. The pelvis as a whole is comprised of the two innominates and the sacrum-coccyx. Three major features you may notice on an innominate are a large concave fan-like area, which is the ilium, a large socket, which is called the acetabulum, and with which the femur articulates, and a hole, the obturator foramen.

Certain aspects of an innominate can be useful in determining the age or sex of an unknown specimen.

The innominate can be classified as either a flat bone or an irregular bone. A subadult innominate is comprised of three bones: the illium, ischium, and pubis. These three bones fuse between the ages of 12 and 17. Other epiphyses are located on the iliac crest, the anterior inferior iliac spine, the pubis, and the ischial tubersoity. These appear about puberty, and unite between the ages of 16 and 23.

Ilium

The portion of the innominate above the acetabulum, it articulates with the first sacral vertebrae at the sacroiliac joint. The ilium can be described as fan-shaped, or wing-shaped; it is sometimes called an ala, which means wing.

The concavity of the ilium is the iliac fossa.

On the anterior medial aspect of the ilium is a large articular surface to which the sacrum is attached in life. The sacrum is specifically connected to the ear-shaped auricular surface.

The iliac crest is the ilium's superior border. The crest is created by traction of abdominal muscles. It ends anteriorily in the rounded anterior superior iliac spine, to which Rectus Femoris attaches, and posteriorily in the sharp posterior superior iliac spine. The most lateral point of the crest is a tubercle.

The posterior border of the ilium is subdivided by the posterior inferior iliac spine.

The gluteal surface of the ilium is known as the dorsum ilii. This surface is crossed by the posterior, anterior, and inferior gluteal lines, which arch upward and forward from the greater sciatic notch. The Glutei muscles (Maximus, Medius, and Minimus) inhabit these areas in life.

Ischium

The lower part of the innominate, below and behind the acetabulum. It is an irregular V shape. The ischium has three parts: the body adjoins the ilium, the tuberosity projects inferiorly, and the ramus unites in an adult with the ramus of the pubis. The empty space surrounded by all of these ramii is the obturator foramen, which in life is filled by a membrane. Nerves and blood vessels inhabit this region in life.

The greater and lesser sciatic notches extend from the posterior inferior iliac spine to the ischial tuberosity. The notches are separated by the ischial spine, which is the point of attachment of the sacrospinous ligament. The sacrotuberous and sacrospinous ligaments cross the mouths of the sciatic notches, making them foraminae in life. In males, the greater sciatic notch is narrower than in females.

The ischial tuberosity is the origin of one's hamstrings, and supports the body when in a sitting position. The upper part of the tuberosity's medial border forms part of the lesser sciatic notch; the lower part is a rough crest to which the sacrotuberous ligament attaches.

The ramus of the ischium unites with the inferior ramus of the pubis in about the eighth year of life to form the conjoint ramus of the ischium and pubis. The conjoint ramii of both innominates together are the pubic arch. At the junction of the innominates there is an articular surface on each. The joint is a symphysis.

Pubis

The anterior part of the innominate, in front of and below the acetabulum. Like the ischium, it is an irregular V shape, and its articulation with the opposite pubis is called the pubic symphysis. The pubis is separated from the ischium by the obturator foramen.

Like the ischium, the pubis has three parts: the body, and the superior and inferior ramus. The border between the two ramii is at the pubic symphysis. The superior ramus is pyramidal, and its base forms one fifth of the acetabulum.

The body is an elliptical symphyseal surface which is covered in cartilage in life. The lateral end of the body, very corner-like, is the location of the pubic crest and pubic tubercle. The body can be useful in aging unknown specimens.

Two lines extend from the pubic tubercle: the pecten pubis, or pectineal line, and the spiral margin of the obturator foramen.

The pectineal line extends to the iliopubic eminence, and is the anterior half of the iliopectineal line. The posterior half of that line, called the arcuate line, is formed by the ilium; the union of these two halves is the iliopubic eminence.

The spiral margin begins at the pubic tubercle and ends near the iliopubic eminence; the ends of the spiral bound a groove on the underside of the superior ramus for the obturator vessels and nerve.


Sources

Almost entirely from Dr. Donaldson's lectures in Anthropology 451: Human Osteology at the University of Victoria
Some reference made to the Wikipedia article on the pelvis

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