The forearm contains two long bones, the ulna and the radius, which facilitate hand and wrist movement. Understanding their precise location requires standard anatomical terminology. Determining whether the ulna is positioned medially or laterally is foundational to grasping the mechanics of the forearm.
Defining Medial and Lateral
Anatomical descriptions rely on the anatomical position, a standard reference frame where the body stands upright, arms hang at the sides with palms facing forward, and feet are parallel. All directional terms are applied as if the body is in this fixed position.
The terms medial and lateral describe a structure’s position relative to the body’s midline, an imaginary vertical line dividing the body into equal halves. Medial indicates a direction toward the midline. Conversely, lateral refers to a direction away from the midline, toward the side of the body. For example, the nose is medial to the eyes, and the ears are lateral to the nose.
The Ulna’s Position Relative to the Forearm
The ulna is located on the medial side of the forearm, meaning it is the bone closer to the body’s midline in the anatomical position. This placement corresponds to the side of the little finger, while the radius is positioned laterally on the side of the thumb.
Proximally, the ulna forms the large, hinged part of the elbow joint by articulating with the humerus via the trochlear notch and the olecranon process. This connection makes the ulna the more stable of the two forearm bones, primarily responsible for hinge-like flexion and extension. The radius is smaller at the elbow but wider at the wrist, where it forms the wrist joint. This structural difference allows the radius to pivot around the ulna, enabling the movements of supination (turning the palm up) and pronation (turning the palm down).
A Quick Reference Guide to Remembering the Bones
A simple way to recall the positions of the two forearm bones is to associate them with the digits they align with. The ulna is on the side of the little finger (the pinky side). The radius is on the side of the thumb, which can be remembered because the gesture for “thumbs-up” suggests something “rad” or “radius.” Visualizing the thumb as a radio antenna, which radiates out to the side, is another helpful technique for remembering the lateral position of the radius. The ulna is firmly established as the medial bone of the forearm, running from the elbow to the wrist.