Brenda Morrison, age 10, of Stanley, North Carolina, for her question:

How do broken bones mend?

A broken bone mends itself, though it needs two kinds of assistance to do a proper job of it. A doctor must set it, and the patient must practice a long spell of patience. When you glue together a broken saucer, you make sure that the edges are fitted back to together in their proper places. This is somewhat like bone setting. Then the bone is kept in place with splints or a hard cast while the break cements itself together. The body repairs skin and flesh wounds in days or weeks. But it may need several patient months to finish cementing a broken bone.

Bones have special cells called osteocytes. They can produce collagen, a very tough protein material that can be made into long fibers. The body uses collagen to make hair, tendons and a firm web that connects all the soft cells of the flesh and skin. The collagen made to build bones has a special quality, though nobody knows how it works. It can attract calcium phosphate. This hard, durable chemical refuses to dissolve. It is just right for stiffening the cement to mend a broken bone. However, several things must be done before the fracture is ready to be cemented.

A hard, brittle bone has a soft center called the marrow and an outer sheath of double membrane. In the hard, bony material, nourishing blood seeps through a network of pores and pockets. The marrow and the membrane sheath are riddled with busier blood vessels. Those special..osteocyte cells are. present in the stiff bong, in the soft marrow and the inside layer of the sheath. When a fracture occurs, blood seeps from the broken bone, the marrow and the sheath. It soon clots to stop the bleeding. But it is too late to save the starved cells on both sides of the breakage. Some of their hard minerals are carried away and broken ends become rubbery. Meantime the blood clot forms a bumpy wad all around the breakage.

Now the osteocytes get busy producing collagen and more collagen. Fresh blood flows to the wounded area, bearing nourishment and minerals. The special bone collagen grabs all the hard calcium mineral it can and uses it to stiffen its tough fibers. This calcification job begins in the center of the blood clot, around the ragged, bony edges. Actually it fortifies the tough fibers of collagen to create new bone. This material forms a firm callus around the wadded region. Its fine fingers poke through the clotted blood, forming a hard lacy network. They pierce the soft rubbery edges of broken bone, stiffen them and cement them firmly together. The repaired bone is as good as new  ¬and sometimes a bit stronger.

The human skeleton is made of durable material, built to last hundreds of years. In the everyday world, it takes more time to build a sturdy house than a shoddy one. The body also needs lots of time to build and repair its sturdy bones. While a broken bone is mending, it needs help to hold it together. The patient wears a cast to keep the pieces in place until the bone grows strong and straight again.