Some of the most poignant moments in the visual arts involve arms. I think immediately of Jahweh’s extended arm from Michelangelo’s Sistine Chapel ceiling, about to transfer the spark of life to Adam’s exanimate one; or of Daphne’s arms beginning to morph into tree branches, as she flees Apollo in terror in Bernini’s astounding statue. Our arms are how we engage physically with the world, either to enact our will upon it, or to protect ourselves from it. To use arms expressively in our work, we have to be able to draw them in a multitude of positions and from every possible angle. In order to do this, we must first understand the structure of the arm, both inside and out.
Form and Function
The best way to learn anatomical structure is to think about the body functionally. Rather than trying to memorize a series of seemingly disparate shapes, thinking about how the body operates will make the concepts much more meaningful. The body is a complex machine and the arm, in particular, has proved capable of executing a huge number of tasks, from gathering food to assisting the hand in performing heart surgery. Perhaps the major functional principle to remember in drawing the arm is that the movement of any part – the shoulder, upper arm, forearm, wrist or hand- is controlled by the part preceding it anatomically. Therefore, the muscles of the shoulder and back control the upper arm, the muscles of the upper arm move the forearm and the muscles of the forearm control the wrist and hand.
Bones and Joints
The type of bones that constitute the arm, and the nature of the joints that connect them, are crucial to understanding the structure of the arm. The upper arm has only one bone, the humerus, while the forearm has two, the radius and the ulna. These three long, thin bones allow us to extend our reach outward into space, and act as levers to help us lift, push, pull and hold onto objects.
The upper arm is longer than the forearm. The radius and ulna are approximately 5/6th the length of the humerus, which means that the distance from the shoulder to the elbow is greater than from the elbow to the wrist.
The arm has three major joints- the shoulder, elbow and wrist- all of which offer varying degrees of movement. The shoulder is a true ball and socket joint, affording extensive movement in all directions. This great degree of flexibility allows us to do things like throw high-speed baseball pitches and do summersaults in gymnastics.
The elbow is a hinge joint, which, because of its more limited action, acts as a stabilizing brace for the arm as a whole. The articulation between the humerus and the ulna allows only for bending and straightening the forearm, which is called flexion and extension. To make up for this limitation, the second bone of the forearm, the radius, has a shallow ball and socket joint which allows it to rotate around the ulna. This is called pronation and supination. Supination is when your arms are at your sides and your palms are facing forward; pronation is when your palms are directed backwards. In supination, the radius and ulna run parallel to each other; in pronation, the radius crosses over the ulna.
Finally, the wrist is made up of eight small bones bound together in a shallow arch, and their relatively loose articulation with the radius and ulna allows for a good deal of flexion, extension, rotation and lateral movement.
There are close to two dozen different muscles in the arm proper. Some of them are so deep that we do not really see them, except to the extent that they enlarge the muscles that lie above them. But there are several significant muscle groupings that comprise the surface anatomy of the arm that are of particular importance to us in drawing.
Joining the arm to the shoulder is the mass of the deltoid, named after the Greek letter delta for its flat triangular shape. The deltoid completely covers the shoulder joint and has three distinct segments- front, side and back- all of which join together in the triangle’s point of insertion into the middle of the upper arm. The middle section has secondary fibers that run obliquely to the mass of the muscle and can often be seen on well-developed models. The deltoid raises or abducts the arm, as well as moves it forwards and backwards. The three distinct segments can contract to greater or lesser degrees depending on which direction the arm is to be moved. The bottom point of the triangle is valuable to locate because it serves as a landmark that separates the other two significant muscle masses of the upper arm, the biceps on the front and top, and the triceps located towards the back and bottom of the arm.
The biceps, joined visually with the brachialis muscle, is the most prominent muscle in the arm, and many a strong man’s claim to fame. Its major job is to flex the forearm. Opposing it is the triceps, named for its three distinct heads. Varying in size, the posterior head is the longest and all three end in a flat common tendon observable going into the elbow. Its major function is to extend the forearm.
Crossing the elbow joint are two muscle masses that help with both flexing and rotating the forearm. Moving from the outside of the elbow towards the inside of the wrist in a long diagonal is a mass composed of the brachioradialis and the extensor carpi radialis longus. This muscle grouping forms a very distinct oblique angle that defines the upper contour of the forearm from most positions. Going in the opposite direction and visible only up close to the elbow itself, is the smaller mass of the pronator teres. The opposition of these two angles forms the distinctive shape of the inside of the elbow.
The forearm proper can be divided into muscles that either flex or extend the wrist and hand, although both aid in rotation as well. The flexors are the more powerful group and take-up about two-thirds of the mass of the forearm. That is why you can close your hand much more powerfully than you can open it. The extensor group, although smaller, is situated slightly higher up on the forearm, due to the fact that some of its muscles cross the elbow joint. Since both the flexors and extensors have their fleshy masses closer to the elbow, the upper portion of the forearm is fuller and rounder, while the wrist is slimmer and more squared-off where the muscles have become thin and tendonus.
Much of the bony structure of the arm is subcutaneous, or observable directly beneath the skin. Unlike other parts of the body, this makes it relatively easy to find and draw. The whole elbow joint- made up of the inner and outer epicondyles of the humerus and the olecranon process of the ulna- are readily observable, as is the entire length of the ulna, from the elbow all the way to its small head on the top of the wrist. This crest-like ridge conveniently divides the flexor and extensor groups. If you can locate this landmark, you know immediately where you are on the form, no matter which way the arm is rotated. Like the ulna, the radius has a small head observable on the side of the wrist, slightly lower than the ulna. Because the radius can run either parallel to, or cross over the ulna, remembering that the radius is always on the thumb side will help you keep the bone structure straight.
Remarkably powerful and agile, our arms are also infinitely graceful. One of the things that makes them so is their tapering movement, both in the whole and in the parts. The arm as a whole tapers supplely from the shoulder to the wrist. But in addition, the upper arm and the forearm each taper individually from top to bottom, setting up an undulating rhythm of swelling and narrowing that is amazingly beautiful.
Because the different parts can bend, twist and rotate in so many different directions at once, the gestures of the arm are infinite; striking, embracing, shielding, the arms express the sweep of complex human emotions.