Bones have different shapes and sizes to suit the jobs they have to perform. For example, bones that have to bear a great weight will be different in shape from those that have only a light weight to support.

Bones are not made of just one material. Many bones are made of a hard tube, inside which is soft, spongy bone containing a honeycomb of chambers filled with red marrow.

The parts of a bone
All bones consist of many parts. The main part of the bone is a strong tube made of thick compact bone. This kind of bone is as strong as iron, but a third of the weight.

At the very end of the bone is a layer of tissue called the cartilage. This acts like a rubbery cushion and protects the bone from sudden jolts.

Bones are alive
At first sight, bones may seem to be just ‘dead bone’. But they also contain living cells, as do all other parts of the body. These living cells allow the bones to grow or to mend. The centre of the bone contains marrow where blood cells are produced.

Bones make up the rigid framework that supports and protects the soft parts of the body. This is called the skeleton.

At the top is the skull – a set of curved plates that are wrapped around, and protect, the entire brain. Below it is the rib cage, which protects the lungs and heart.

People, like all large land animals, have an internal skeleton which grows with the rest of the body.

How a bone grows
When your skeleton first formed, the bones were soft. However, they soon began to change as well as grow. Soft bone became hard bone. Your bones will continue to grow in length until you are about eighteen years old.

Bones can regrow if they are broken, no matter what your age.

A store of minerals
Bones store calcium and other minerals in their thick walls. If the body has too little of a particular mineral, some will dissolve out of the bones in order to supply the loss.

Shells
Skeletons do not just have to be inside an animal. Most animals have skeletons on the outside. These kinds of skeletons are called shells, and the soft parts of the animal are attached to the inside of the shell.