A coxal bone, also known as a hipbone or innominate bone, is an integral part of the pelvis and hip socket in many animals including humans. Humans are typically born with six coxals, three on the right and three on the left. These are known scientifically as the ilium, the ishium, and the pubis, though they usually fuse together during puberty so that by adulthood each person has just two total bones, one on each side. They are important primarily because together they form the pelvis, which connects the legs to the torso. The pelvis also does a lot to support and distribute a person’s standing weight and helps protect many of the internal organs from impact and damage. The precise shape and size usually differs a little bit between the genders, with males having broader, more tightly fused bones and women having slightly more flexible jointing, most likely as a means of facilitating childbirth.
Fusing
Humans as well as some other animals are usually born with pelvises that haven’t fully fused together, but rather are joined with flexible cartilage. The cartilage holds the bones in place and provides freedom for movement during growth. Scientists aren’t always entirely sure why this happens, but most speculate that it is because the pelvis plays such an important role when it comes to anchoring the legs and providing stabilization during walking, running, and other upright activities. If it was stiff and fixed when people were born, growth and development might be a lot choppier. In most cases all of the bones have fully fused by the time people are about 20 years of age.
Location and Attributes
The bones make the receiving sockets of the hip and leg joints, creating a cup-like space called the acetabulum. This space receives the ball end of the femur, which is the main upper bone in the leg.
Even when they’re fused, the individual coxals still retain their own shapes and are usually identifiable when looking at the pelvis as a whole. The ilium forms the upper part of the bone, and usually has a crest-like structure at the top. People can often feel this part when they put their hands on their hips; it’s the fan-like bone that juts out. The ishium is the back part of the acetabulum, and is one of the bones a person sits on. The pubis is located in the front, and forms the frontal part of the acetabulum opening, as well as offering protective coverage to the organs in the pubic area.
Main Role and Function
The shape of the bone as a whole tends to be somewhat irregular and flattened. It is wider at the top and bottom than in the center. It supports the upper body’s weight and provides structural integrity of the vertebral, or spinal, column. In addition to offering support to the upper body, another primary function of the coxal bones is the protection and support of the bladder and reproductive organs.
These bones also form the basic pelvis, and together with the sacrum and the coccyx make up what is known as the pelvic girdle and form the pelvic cavity. On each side of the human body a coxal bone joins the sacrum in the back, and ultimately anchors it to the tailbone or coccyx. In the front, a muscle known as the pubic synthesis joins everything together.
Differences Between Men and Women
The shape of these bones tends to differ between men and women. This is primarily because of the role they play in human reproduction. Male coxals tend to be bigger and closer together, and as a result the pelvis is a bit broader and fans out in a more pronounced fashion. In many respects this is to support a man’s typically broader stance and longer stride. Females, on the other hand, typically have narrower pelvises that are more widely spaced.
In a pregnant woman, these bones have the added responsibility of supporting the fetus inside the uterus. They usually loosen a bit as the pregnancy progresses, which both supports the woman’s shifting center of balance and allows for ultimate delivery of the baby through the birth canal.