The ischium ( ) forms the lower and back part of the hip bone (os coxae).
Situated below the ilium and behind the pubis, it is one of these three bones whose fusion creates the hip. The superior portion of this bone forms approximately one third of the acetabulum.
The ischium is made up of three parts–the body, the superior ramus and the inferior ramus.
The body contains a prominent spine, which serves as the origin for the superior gemellus muscle. 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) and can be felt simply by sitting on the fingers. Additionally, it serves as the origin for the inferior gemellus muscle and the adductor magnus muscle.
The superior ramus is a partial origin for the internal obturator and the external obturator muscles. The inferior ramus serves partially as origin for part of the adductor magnus muscle and the gracilis muscle.
The ischial ramus joins the inferior ramus of the pubis anteriorly and is the strongest of the hip (coxal) bones.
