Our bones, being hidden away inside our skins, are not something that we normally think about much. But once you break one, it’s hard to think about anything else.

I had an early introduction to the subject when I was seven years old and my big brother broke my arm. Not deliberately: we were rolling down the back steps of our house in Tulia, Texas, inside a big cardboard box a refrigerator had come in, and he landed on my arm . The result was a two broken bones and an L-shaped wrist.

Although my first reaction was indignation (I remember trying to say, “Now look what you’ve done!”, although I think it actually came out “Ubull-glub-ulp!-ulp!”), curiousity followed. Twenty-six years from now, I told myself, I’m going to write a science column about bones.

We think of bones as hard, dead matter, like hair or fingernails, but they’re actually organs consisting of living cells embedded in a matrix of calcium phosphate and other calcium minerals, held together by collagen, the tough fibrous protein we also use to make ligaments, tendons and skin. Bone tissue constantly renews itself, some cells dissolving old tissue and others depositing new tissue, of which there are two major types: compact, which is very hard and solid, and spongy, which isn’t.

Our bones’ most important function is to keep us from collapsing into shapeless lumps. They also enable us to move: the contraction of muscles repositions the bones they’re attached to, carrying the soft tissue along for the ride.

A second important function of bones is to protect vital organs such as the brain, spinal cord, lungs and heart: the brain inside the hard shell of the cranium, the spinal cord inside the 26 vertebrae, and the lungs and heart inside the curving bars of the ribcage.

A third function is the production of blood cells. Spongy bone tissue contains marrow, the source of all blood cells. Young marrow is red because many new blood cells are forming, while older marrow is yellow because of fat cells deposited in it.

Bones also store calcium, vital to the proper functioning of cell membranes, and phosphorus, required for our metabolism. On a much smaller scale, tiny bones in the middle ear conduct sound waves and make hearing possible.

Bones are classified as long, short, flat, irregular or sesamoid. The long bones are the big ones in the limbs. They’re the classic “doggy-bone” shape, a central shaft with lumpy ends that form joints with other bones. The long bones are made of compact bone tissue outside and spongy bone and marrow inside. The ends are capped by cartilage, a rubbery tissue which acts as a cushion and forms a smooth surface where bones come together. (It also forms a temporary skeleton in human embryos, a template for the permanent one.)

The short bones, in the wrists and ankles, consist of a spongy core within a shell of compact bone, while the flat bones, which include the ribs and many skull bones, consist of two plates of compact bone with a spongy layer between. Sesamoid bones are small, rounded bones like the kneecap that develop in certain joints to provide special support or reduce friction. Irregular bones are everything else: the vertebrae, for example.

Humans are born with about 275 bones, but later many of these fuse together. Adults have about 206 name-bearing bones (not “Fred” and “George” — names like “zygomatic” or “left intermediate cuneiform”) and a variable number of largely unnamed sesamoids.

Despite being hidden away, bones are subject to a variety of problems, including infections, nutritional deficiencies like rickets (a failure of the bone cells to deposit enough calcium salts to make the bones rigid), toxic diseases like lead poisoning (bones store bad minerals as well as good ones) and tumors (the bones are particularly vulnerable to metastatic tumors, which arise elsewhere and are spread to the bones by the blood).

Still, the most common problem is the fracture, whether caused by falling down a ski-hill or rolling down the back steps in a box with your brother. Again, because bones are living organs, they are capable of healing themselves. Get the broken bone lined up straight and immobilize it, and it will grow back together good as new.

In the meantime you’re stuck hobbling around on crutches, but hey, them’s the breaks!

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