The strength of normal, healthy human bones is amazing. The saying that our bones are “twice as strong as oak” is not far from wrong.

Bone needs to be strong because it forms the framework, or skeleton, that supports the whole body. Bones vary in shape and size according to the type of animal to which they belong. Fish and small birds have tiny bones. Elephants have bones that weigh several hundred pounds!

All bones have similar composition. Bone is a hard, grayish-white substance of which about two thirds are inorganic, or mineral matter, especially phosphate of lime This gives the bone hardness, but at the same time, it makes the bone more brittle.

The remaining third of the bone is organic, or animal matter. This gives the bones the toughness which helps them resist breakage. In certain types of bones, there is a fatty substance called marrow, which is organic matter with a very high food value.

There is also a small amount of water in bone, which seems to dry out as the body grows older. As this drying takes place and as the mineral matter in the bone increases, the bones become more breakable and slower to knit and heal.

When you break a bone in your arm, for example, it must be set. This means it must be fastened firmly in its natural place so that the ends cannot move. The bone must knit before you can use your arm again.

The knitting is done by tiny cells known as osteoblasts. They secrete a limey substance that makes the bones hard and firm again. These cells also help in the natural growth of bones. Other cells called osteoclasts tear down old tissue so that growth is possible. This double process of building up and tearing down is going on in the bones all the time.