The elbow consists of a joint between the arm bone, called the humerus and two forearm bones. The forearm bones are called the radius, which is on the thumb side, and the other is the ulna, which is on the little-finger side of the forearm. The prominent part of the ulna, at the tip of the elbow, is called the olecranon. The elbow can bend (flex) and straighten (extend), but the joint between the top of the radius and the ulna called the proximal radioulnar joint is also involved in forearm rotation. There are supporting structures around the elbow consisting of the capsule, which is like a thin ligament around the elbow and larger individual ligaments. The elbow is moved by muscles that cross the joint, many of which are attached via their tendons close to the elbow, such as the biceps and triceps tendons. There are also muscles controlling wrist and finger motion, whose tendons attach into the bone around the elbow. Some attach into the prominent bony areas on the inner (medial) and outer (lateral) aspect of the humerus, just above the elbow (epicondyles).