The head of the humerus forms the ball of the ball-and-socket shoulder joint, with the glenoid cavity of the scapula acting as the socket. The rounded shape of the head of the humerus allows the humerus to move in a complete circle (circumduction) and rotate around its axis at the shoulder joint. Just below the head, the humerus narrows into the anatomical neck of the humerus. Two small processes, the greater and lesser tubercles, extend from the humerus just below the anatomical neck as attachment points for the muscles of the rotator cuff. The humerus narrows below the tubercles again in a region known as the surgical neck before extending toward elbow joint as the body of the humerus. About a third of the way to the elbow, the humerus swells into a small process known as the deltoid tuberosity, which supports the insertion point of the deltoid muscle.