The forearm contains two long bones, the radius and the ulna, which work in tandem to provide structure and facilitate the complex movements of the hand and wrist. Understanding the location of these bones is fundamental to grasping how the arm functions. Each bone is responsible for different aspects of arm motion and stability, which depends entirely on its anatomical position.
Locating the Ulna
The ulna is located on the side of the forearm closest to the pinky finger, which is referred to as the medial side of the limb. This bone extends from the elbow down to the wrist, forming a parallel structure with the radius. The ulna is prominent at the elbow joint.
The upper end of the ulna features a large, C-shaped indentation that wraps around the end of the upper arm bone, the humerus, to form the elbow joint. The lower, or distal, end narrows to a small, rounded structure that can be felt as a bump on the wrist on the side of the pinky finger. While the ulna contributes to the wrist structure, it does not directly articulate with most of the small wrist bones.
The Radius and Its Position
The radius is the second long bone of the forearm and is positioned opposite the ulna, aligning with the thumb. This location is known anatomically as the lateral side of the forearm. The radius is thicker at the wrist, contrasting with the ulna, which is thicker at the elbow.
In contrast to the ulna, the radius bears most of the load transferred from the hand to the arm. At the wrist, the flared end of the radius directly connects with the small carpal bones of the hand, forming the main joint of the wrist. This arrangement allows the hand to move in multiple directions.
Key Roles in Arm Movement
The distinct positioning of the ulna and radius dictates their roles in arm movement. The ulna’s thick upper end includes a large bony projection called the olecranon process, which forms the recognizable tip of the elbow. This process acts as a large lever for the triceps muscle, facilitating the powerful extension, or straightening, of the arm at the elbow joint.
The elbow joint itself is a hinge joint, primarily formed by the articulation of the ulna and humerus, allowing for simple flexion and extension. The radius, however, is the bone responsible for the forearm’s rotational movements, known as pronation and supination. When a person rotates their palm, the head of the radius pivots and crosses over the ulna.
This crossing movement is facilitated by radioulnar joints, located at both the elbow and the wrist. The radius essentially rotates around the fixed ulna, allowing for the wide range of motion necessary for tasks like turning a doorknob or twisting a screwdriver. Thus, the ulna provides the stable axis for the hinge movement of the elbow, while the radius handles the complex rotation.