Skeleton:

The structure that holds us together

Have you ever seen a real human skeleton? If you have been to a science exhibition or a museum, you definitely would have. Today, we will be discussing the human skeleton.

The human skeleton is made of individual or joined bones, supported and supplemented by a structure of ligaments, tendons, muscles, cartilage and other organs. The skeleton is not unchanging; it changes composition over a lifespan.

At the beginning, a foetus has no hard skeleton - bones form gradually during the nine months in the womb. At birth, all bones will have formed, but a newborn baby has more bones than an adult.

On average, an adult human has 206 bones (according to Gray's Anatomy, but the number can vary slightly from individual to individual). A baby is born with approximately 270 bones! How does a baby have more bones than an adult, and where do the bones disappear?

Though at birth a baby has a number of small bones, they fuse together during growth. They leave pockets of cartilage to allow more growth. The sacrum (the bone at the base of the spine) consists of six bones, which are separated at birth, but fuse together into a solid structure in later years. Growing is usually completed between the ages of 12 and 14. This means that the bones have no pockets of cartilage left, so there is no material left to allow more growth.

Not all bones are interconnected directly. There are six bones (three on each side) in the middle ear that are joined only with each other. Another bone, the hyoid bone in the neck, does not touch any other bones in the body, and is supported only by muscles.

The longest and heaviest bone in the body is the femur (thigh bone) and the smallest is the stapes bone in the middle ear. In an adult, the skeleton comprises 20 per cent of the total body weight.

The most obvious function of bone is to support the body. It is also the site of haematopoiesis, the manufacture of blood cells, that takes place in bone marrow (which is why bone marrow cancer is very often a terminal disease).

It is also necessary for the protection of vital organs. Movement in the vertebrae is dependent on the skeletal muscles, which are attached to the skeleton by tendons. Without the skeleton to give leverage, movement would be greatly restricted. Bone also serves as a mineral storage deposit in which nutrients can be stored and retrieved.

One way to group the bones of the human skeleton is to divide them into the axial skeleton and the appendicular skeleton. The axial skeleton consists of bones in the midline and includes all the bones of the head and neck, the vertebrae, ribs and sternum (breast bone).

The appendicular skeleton consists of the clavicles (collar bones), scapulae (shoulder blades), the arm bones, the bones of the pelvis (bony cavity at the base of the human trunk) and the leg bones.

There are many differences between the male and female human skeletons. Men tend to have slightly thicker and longer limbs and digit bones, while women tend to have larger pelvic bones in relation to body size. Women also tend to have narrower rib cages, less angular mandibles (lower jaw bones), and less pronounced cranial (skull) features such as the brow ridges and occipital protuberance (the small bump at the back of the skull).

Most striking is the difference in hip bones, owing to differences related to the process of reproduction.

There are also a number of smaller differences between human male and female skeletons. Men and women both have 12 pairs of ribs. Removed ribs can regenerate within 2-3 months of sectional surgery, as in the surgical procedure, rib thoracoplasty.

The skeleton can be affected by many diseases that compromise physical mobility and strength. Skeletal diseases range from minor to extremely debilitating (weakening) ones.

Bone cancer and bone tumours are extremely serious and are sometimes treated by radical surgery such as amputation of the affected limb. Various forms of arthritis attack the skeleton, resulting in severe pain and debility.

A fracture occurs when a bone is subjected to too much force. Fractures are divided into "simple" and "compound" fractures, a slightly confusing terminology: A "simple" fracture simply means that the broken bone has not broken through the skin, and does not imply a single break.

A broken bone is called a "compound fracture" when it has broken through the skin, also not implying that the bone has broken in more than one place.