Posteriorly, the superior ramus

Superior ramus of the ischium

The superior ramus of the ischium projects downward and backward from the body and presents for examination three surfaces: external, internal, and posterior.The external surface is quadrilateral in shape...

 of the ischium forms a large swelling, the tuberosity of the ischium (or ischial tuberosity or Tuber ischiadicum in latin, also known as the sitz bone,
or as a pair the sitting bones).

It marks the lateral boundary of the pelvic outlet

Pelvic outlet

The lower circumference of the lesser pelvis is very irregular; the space enclosed by it is named the inferior aperture or pelvic outlet.-Boundaries:It has the following boundaries:* anteriorly: the pubic arch* laterally: the ischial tuberosities...

.

When sitting

Sitting

Sitting is a rest position supported by the buttocks or thighs where the torso is more or less upright.- Sitting on the floor :The most common way of sitting on the floor involves bending the knees...

, the weight is frequently placed upon the ischial tuberosity. The gluteus maximus

Gluteus maximus muscle

The gluteus maximus is the largest and most superficial of the three gluteal muscles...

 covers it in the upright posture, but leaves it free in the seated position.

Divisions

The tuberosity is divided into two portions: a lower, rough, somewhat triangular part, and an upper, smooth, quadrilateral portion.

• The lower portion is subdivided by a prominent longitudinal ridge, passing from base to apex, into two parts;
  • the outer gives attachment to the Adductor magnus,
  • the inner to the sacrotuberous ligament
    Sacrotuberous ligament
    The sacrotuberous ligament is situated at the lower and back part of the pelvis. It is flat, and triangular in form; narrower in the middle than at the ends....
    
    .

• The upper portion is subdivided into two areas by an oblique ridge, which runs downward and outward;
  • from the upper and outer area the Semimembranosus arises;
  • from the lower and inner, the long head of the Biceps femoris and the Semitendinosus.

External links

- "The Female Perineum: Bones" - "Major Joints of the Lower Extremity: Hip bone (lateral view)"