What are our skeletons for?
If your skeleton were taken away, your organs would be in an untidy heap on the floor. But your skeleton is much more than a simple support for your softer parts – by transmitting force and providing leverage, it allows you to move.
The centres of the long bones (such as those in the arms and legs) are hollow, which makes them strong yet light. The cavity inside the bone is filled with bone marrow, where blood cells are made. In childhood, the ends of the long bones in our arms and legs, which normally go on growing for 17 years or so, are made mainly of cartilage. This softer tissue gradually becomes calcified as it turns into the solid, but still spongy, tissue of mature bone.
By then, cartilage is left only at the ends, where it eases joint movements. Even when calcified, bones are still living tissue. Bone – particularly the protein and mineral of the bone matrix – is continually remodelled and replaced in response to the stresses and strains of movement.