The os coxae, commonly known as the hip bone or pelvic bone, is a fundamental component of the human skeleton. It is an irregularly shaped bone that plays a significant role in connecting the upper body to the lower limbs.
Location and Basic Definition
The os coxae is one of two large, irregular bones that form the pelvic girdle. These two hip bones join together anteriorly at the pubic symphysis and posteriorly with the sacrum, which is part of the axial skeleton. This arrangement creates the bony pelvis, a basin-shaped structure.
The hip bone is also referred to as the innominate bone or coxal bone. Its position allows it to transfer weight from the upper body to the lower limbs. This bone’s robust nature makes it a stable base for various bodily movements and postures.
Detailed Anatomy: Its Component Parts
The os coxae is not a single bone but rather a fused structure composed of three distinct bones: the ilium, ischium, and pubis. These three parts are separate in childhood, connected by cartilage, and fully fuse into one bone during puberty, typically by the late teenage years. The point where these three bones meet and fuse is at a deep, cup-shaped socket called the acetabulum.
The ilium is the largest and most superior portion of the hip bone, resembling a blade or wing. Its superior border forms the prominent iliac crest, which can be felt through the skin and serves as an attachment site for muscles. The ilium contributes to the upper part of the acetabulum.
The ischium forms the posteroinferior part of the hip bone. It includes the ischial tuberosity, often called the “sit bone,” which bears body weight when seated. The ischium also features the ischial spine and the greater and lesser sciatic notches. This bone contributes to the lower and side boundaries of the acetabulum.
The pubis is the most anterior and smallest component of the hip bone. It consists of a body and two rami, a superior and an inferior, which extend from the body. The body of the pubis articulates with the opposing pubic bone at the pubic symphysis. The pubis forms part of the acetabulum and helps enclose the obturator foramen.
The acetabulum is a concave socket on the lateral surface of the os coxae that articulates with the head of the femur, forming the hip joint. This articulation allows for a wide range of motion in the leg. Anteroinferior to the acetabulum is the obturator foramen, a large opening bordered by the ischium and pubis. This foramen is mostly covered by a membrane, but a small canal remains open, allowing the passage of nerves and blood vessels to the thigh.
Core Functions
It provides substantial support for the upper body and trunk, acting as a stable base. The two hip bones, along with the sacrum and coccyx, form the pelvic cavity, which protects internal pelvic organs such as the bladder, lower colon, rectum, and reproductive organs.
The hip bone is also responsible for transmitting the weight of the upper body to the lower limbs when standing or moving. It serves as an extensive attachment point for numerous muscles involved in locomotion, posture, and stability. These muscles enable a variety of movements, including walking, running, and maintaining balance.