Each time a healthy person takes a step, bends a finger, or raises an arm, the ends of the two bones that come together in a joint slide across one another gracefully, with less friction than that of a skate gliding on ice. A smooth, lubricated layer of cartilage tissue that covers the ends of the bones makes that action possible. This tough, somewhat elastic material also acts as a shock absorber in the joint. continued...

Illustration: Richard Tuschman

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Reprinted from the HHMI Bulletin, June 2003, pages 8-13.
©2003 Howard Hughes Medical Institute