Muscles are tissues composed of bundles of fibers (fascicles) having varying lengths and diameters that can shorten, thicken, or lengthen depending on the location and the message sent by the controlling neurons. The change in the muscle fibers allows for the movement of body parts, whether it is involuntary (e.g., breathing by moving the lungs and blood circulation by the pumping of the heart) or voluntary where the primate can exert control (e.g., arm and leg muscles). The skeletal muscle is surrounded by a layer of connective tissue. Skeletal muscles, characteristically striated, attach to the bones by tendons that are strong, fibrous, non-elastic cords. The contraction or extension of the skeletal muscles causes the attached bones forming joints to move either by flexing or extending. Humans, one type of primate, have more than 600 muscles, accounting for approximately 40 percent of the average male's weight.