The forearm contains two long bones, the radius and the ulna, which extend between the elbow and the wrist. These bones allow for the complex movements of the hand and arm, including rotating the palm. Understanding the precise relationship between the radius and ulna is fundamental to comprehending the anatomy and mechanics of the upper limb.
Understanding Anatomical Terminology
Scientists and medical professionals rely on the “anatomical position” to create a consistent reference point for the entire body. This standard posture involves standing upright with the feet parallel, arms hanging at the sides, and the palms facing forward. By establishing this one static pose, the relative location of any two structures can be described without confusion, regardless of the body’s actual movement or orientation.
Two terms used to describe horizontal position are “medial” and “lateral.” Medial refers to a position closer to the imaginary midline of the body, which runs vertically from the head down to the feet. Conversely, lateral indicates a position that is farther away from this central line, toward the side of the body. For instance, the torso is medial to the arms, while the arms are lateral to the torso.
The Static Position of the Forearm Bones
When the forearm is placed in the anatomical position, the radius is positioned lateral to the ulna. This means the radius is on the side of the forearm farther away from the body’s midline. The radius runs along the thumb side of the forearm, which is often called the radial side.
The ulna, in contrast, is situated on the medial side of the forearm, placing it closer to the body’s central axis. The ulna is located on the side of your little finger, or the ulnar side. This fixed, side-by-side arrangement is maintained by a sheet of connective tissue called the interosseous membrane, which connects the two bones along their shafts.
Movement and Function in the Forearm
The positional difference between the radius and ulna is directly tied to the forearm’s unique ability to rotate the palm. While the ulna remains relatively stable, forming a major part of the elbow joint, the radius is designed to pivot. It articulates with the ulna at both the elbow and the wrist, allowing for complex rotation.
This rotation is called pronation and supination, which turn the palm up and down. During pronation, the radius crosses over the ulna, shifting its wider end at the wrist from the lateral side to a more medial position. When the palm is turned back up during supination, the radius uncrosses, returning to its parallel, lateral position next to the ulna.