Take these arms

The bones of the lower arm perform one of the great tricks of the human body, which we do every time we do something like turn a key in a lock or look at our wrist watch - the twisting motion that, even though we do it without thinking, involves some of the most interesting bone shapes in the body. There are two bones in the lower arm, the ulna and radius, and each is responsible for a different action - the ulna carries the lower arm up and down, like when one lifts a weight, and the radius, as the name suggests, twists the lower arm.

The joint of the humerus at the elbow has an interesting spool shape that the ulna moves up and down on - and the ulna has an answering shape that has been called the 'parrot's beak.' The radius pivots on a shape on the humerus joint that's called the capilum, a half ball shape that fits into the radius's half round joint.

These shapes don't express themselves in figure drawings, of course, but their action does, and the differing shapes the forearm makes as it moves. When the palm is up and the thumb is pointed away from body's centerline, ulna and radius are parallel - in the supine position. When the palm is down and the thumb is pointed towards the body, the radius is crossed over the ulna, which we call the pronate position. The radius attaches at the wrist on the thumb side, and we can imagine the radius carrying the thumb this way and that as it moves.

These two side by side, or crossing, bones, give the forearm its distinctive shape at the wrist, twice as wide as high. The bones at the elbow, upper arm and shoulder are essentially buried under the thick cords of crisscrossing muscles that perform the many actions of our expressive arms.