Muscle that is under the control of the will and is generally attached to the skeleton. An individual muscle consists of bundles of long muscle fibers, each containing many nuclei, the whole muscle being covered with a strong connective tissue sheath (epimysium) and attached at each end to a bone by inextensible tendons. Each fiber contains smaller fibers (myofibrils) having alternate light and dark bands, which contain protein filaments responsible for the muscle’s contractile ability and give the muscle its typical striped appearance under the microscope. The functional unit of a myofibril is the sarcomere. The end of the muscle that is attached to a nonmoving bone is called the origin of the muscle; the end attached to a moving bone is the insertion. As a muscle contracts it becomes shorter and fatter, moving one bone closer to the other. Since a muscle cannot expand, another muscle (the extensor) is required to move the bone in the opposite direction and stretch the first muscle (known as the flexor). The flexor and extensor are described as antagonistic muscles.