CHUNK OF JUICY MUSCLE is the predator's prime meal in the natural world, from a lion gnawing at a gazelle to a human eating a steak. On average most animals, including humans, are one-half muscle. Muscles are the body's movers. This is because a muscle is a lump of tissue specialized to do a simple job: to shorten, or contract. Every movement, from the blink of an eye to sprinting a race, is driven by the body's muscular system. Muscles, along with the bones they pull, were among the first parts of the body to be drawn, described and named, because of the general interest in the body's movement and eating meat. Galen of Ancient Rome identified about 300 individual skeletal muscles, just under half the body's total. Leonardo da Vinci noted and named more, and his extraordinary engravings and paintings of living people reflected this knowledge in the supple bulges and curves under their skin. The volume, Epitome, of Andreas Vesalius' monumental work De Fabrica (1543) is a classic of science and art, depicting yet more muscles in carefully dissected and posed bodies.