尺骨, 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 plays a crucial role in forming the elbow joint and provides stability to the forearm. The ulna is longer than the radius and has a distinctive hook-like structure at its upper end, known as the olecranon, which forms the point of the elbow. This bone works in conjunction with the radius to allow for a wide range of motion in the arm, including bending and rotating.