The ischium is a part of the pelvic bone, specifically the lower and posterior portion. It is one of the three bones that fuse together to form each half of the pelvis, along with the ilium (the upper and largest portion) and the pubis (anteriorly).

The ischium has a thick, robust structure because it supports our body weight when we sit. Its main parts include:

1. The ischial tuberosity (sitting bone): This is the roughened, weight-bearing portion where you typically feel discomfort after sitting for long periods.
2. The ischial spine: A thin bony projection that serves as an attachment point for various muscles and ligaments.
3. The ramus of the ischium: The slender, curved part that extends downwards and joins with the pubis to form the inferior (lower) portion of the pelvic ring called the obturator foramen.

Together with the other components of the pelvis, the ischium plays a crucial role in providing stability, supporting the lower limbs, and protecting internal organs.

The pelvic bones, also known as the hip bones, are a set of three irregularly shaped bones that connect to form the pelvic girdle in the lower part of the human body. They play a crucial role in supporting the spine and protecting the abdominal and pelvic organs.

The pelvic bones consist of three bones:

1. The ilium: This is the largest and uppermost bone, forming the majority of the hip bone and the broad, flaring part of the pelvis known as the wing of the ilium or the iliac crest, which can be felt on the side of the body.
2. The ischium: This is the lower and back portion of the pelvic bone that forms part of the sitting surface or the "sit bones."
3. The pubis: This is the front part of the pelvic bone, which connects to the other side at the pubic symphysis in the midline of the body.

The pelvic bones are joined together at the acetabulum, a cup-shaped socket that forms the hip joint and articulates with the head of the femur (thigh bone). The pelvic bones also have several openings for the passage of blood vessels, nerves, and reproductive and excretory organs.

The shape and size of the pelvic bones differ between males and females due to their different roles in childbirth and locomotion. Females typically have a wider and shallower pelvis than males to accommodate childbirth, while males usually have a narrower and deeper pelvis that is better suited for weight-bearing and movement.