Independent movement is a unique characteristic of animals. Most animal movement depends on the use of muscles. Together, muscles and bones make up what is known as the musculoskeletal system. This combination provides protection for the body's internal organs and allows for many kinds of movement. Whether the movement is as simple as opening the eyes or as complex as flying, each is the result of a series of electrical, chemical, and physical interactions involving the brain, the central nervous system, and the muscles themselves.

Muscle is the flesh, minus the fat, that covers the skeleton of vertebrate animals. Muscles vary in size and shape and serve many different purposes. Large leg muscles such as hamstrings and quadriceps control limb motion. Other muscles, like the heart and the muscles of the inner ear, perform specialized involuntary functions. Despite the variety in size and function, however, all...