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|Ischium of pelvis|
|Left hip-joint, opened by removing the floor of the acetabulum from within the pelvis. (Ischium labeled at bottom left.)|
The ischium forms the lower and back part of the hip bone (os coxae). The word ischium dates back to c. 164o B.C.E., from Greek ἰσχίον iskhion meaning "hip joint", in plural, "the hips", and most likely comes from ισχύς iskhys "loin", which is of unknown origin.
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.
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).
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The clade Dinosauria is divided into the Saurischia and Ornithischia based on hip structure, including importantly that of the ischium. In the majority of dinosaurs, the ischium extends down from the ilium and towards the tail of the animal. The acetabulum, which can be thought of as a "hip-socket", is a cup-shaped opening on each side of the pelvic girdle formed where the ischium, ilium, and pubis all meet, and into which the head of the femur inserts. The orientation and position of the acetabulum is one of the main morphological traits that caused dinosaurs to walk in an upright posture with their legs directly underneath their bodies.
- Anatomy photo:44:st-0722 at the SUNY Downstate Medical Center - "The Male Peniel: Hip Bone"
- ischium at eMedicine Dictionary
- Cross section at UV pelvis/pelvis-e12-15
- Avulsion of the ischial apophysis J. R. Wootton, M. J. Cross, K. W. G. Holt - Journal of Bone and Joint Surgery, British Volume. Volume 72-B, No. 4. Date: 1990-07. Retrieved: 2010-05-16.
- Seeley, H.G. (1888). "On the classification of the fossil animals commonly named Dinosauria." Proceedings of the Royal Society of London, 43: 165-171.
- Martin, A.J. (2006). Introduction to the Study of Dinosaurs. Second Edition. Oxford, Blackwell Publishing. pg. 299-300. ISBN 1–4051–3413–5.
- 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.