척골, or the ulna, is one of the two long bones in the forearm, the other being the radius. It is located on the inner side of the forearm, opposite the thumb. The ulna is slightly longer than the radius and plays a crucial role in forming the elbow joint, connecting with the humerus at the upper end. The ulna has a distinctive shape, featuring a prominent projection called the olecranon at its upper end, which forms the point of the elbow. This bone helps in the movement of the arm and provides stability during various activities, such as lifting and pushing.