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Bone: Ischium of pelvis
Pelvic girdle
Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis. (Ischium labeled at bottom left.)
Latin	os ischii
Gray's	subject #57 234
MeSH	Ischium

The ischium forms the lower and back part of the hip bone (os coxae). The word comes from the Greek ἰσχίον ischion, meaning "hip". (Taber's, 1985).

Situated below the ilium and behind the pubis, it is one of these three bones whose fusion creates the coxa. The superior portion of this bone forms approximately one third of the acetabulum.

It is divisible into three portions:

• Body of ischium - Contains a prominent spine which serves as the origin for gemellus superior. The indentation inferior to the spine is the lesser sciatic notch. Continuing down the posterior side, the ischial tuberosity is a thick, rough surfaced prominence below the lesser sciatic notch. This is the portion that supports weight while sitting (especially noticeable on a hard surface). It can be felt simply by sitting on one’s fingers. Additionally, it serves as the origin for gemellus inferior and adductor magnus.
• Superior ramus of the ischium - Serves as a partial origin for obturator internus and obturator externus.
• Inferior ramus of the ischium - Serves as a partial origin for gracillis and adductor magnus.

The ischial ramus joins the inferior ramus of the pubis anteriorly. It is the strongest of the coxal bones.

Clinically, avulsion fracture of the ischial apophysis may occur (Wootton 1990).^[1]

Avulsion fractures of the hip bone (avulsion or tearing away of the ischial tuberosity) may occur in adolescents and young adults during sports that require sudden acceleration or deceleration forces, such as sprinting or kicking in football, soccer, jumping hurdles, basketball, and martial arts. These fractures occur at apophyses (bony projections that lack secondary ossification centers). Avulsion fractures occur where muscles are attached: anterior superior and inferior iliac spines, ischial tuberosities, and ischiopubic rami. A small part of bone with a piece of a tendon or ligament attached is avulsed (torn away) (Moore 2006).

Etymology [link]

The word ischium dates back to c. 1640, from Greek ἰσχίον iskhion meaning "hip joint", in plural, "the hips", and most likely comes from ισχύς iskhys "loin", which is of unknown origin.^[2]

In dinosaurs [link]

The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ischium.^[3]

• 

  Ornithischian pelvic structure (left side)

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  Saurischian pelvic structure (left side).

Additional images [link]

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  Right hip bone. External surface.

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  Right hip bone. Internal surface.

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  Plan of ossification of the hip bone.

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  .

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  The Obturator externus.

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  Pelvis

External links [link]

• SUNY Labs 44:st-0722 - "The Male Peniel: Hip Bone"
• ischium at eMedicine Dictionary
• Cross section at UV pelvis/pelvis-e12-15

References [link]

• Moore, Keith L., Arthur F. Dalley, and A. M. R. Agur. Clinically Oriented Anatomy. Philadelphia: Lippincott Williams & Wilkins, 2006. Print.

• Saladin, Kenneth S. Anatomy and Physiology The Unity of Form and Function. 5th ed. McGraw-Hill Science Engineering, 2009. Print.

This article was originally based on an entry from a public domain edition of Gray's Anatomy. As such, some of the information contained within it may be outdated.