These two pages concentrate on the mechanism by which the arm twists to direct the palm upwards (supination) or downwards (pronation). Leonardo demonstrates with great clarity how this is achieved – whether the arm is flexed or extended – by crossing the radius and ulna (the bones of the forearm) but without rotating the humerus (the bone of the upper arm). Leonardo discovered that the biceps muscle (in the upper arm) has two actions, both flexing the arm at the elbow and supinating the forearm. It would be more than two centuries before this observation was repeated. The animated sequence alongside shows the opposed roles of the muscles biceps and supinator (which supinate the arm) and pronator teres and pronator quadratus (which pronate the arm). The muscles are coloured green when contracting and red when relaxed.