Anatomy Guide › Upper limb
Ulna
The ulna is the forearm's stabilizer, running along the little-finger side from elbow to wrist. Its upper end is unmistakable: a deep C-shaped notch that wraps around the pulley of the humerus like a wrench gripping a bolt, with a stout hook at the back — the olecranon — that you know as the hard point of your elbow. When you lean on a table, you are resting on your ulna.
That wrench-grip design makes the ulna the true hinge partner of the elbow. While the radius rotates freely to flip the palm, the ulna holds a steady line, giving the forearm something to rotate around. It is thickest at the elbow and tapers toward the wrist, exactly opposite to the radius, which is slender at the elbow and broad at the wrist — the two bones trade dominance from one end of the forearm to the other, joined along their length by a fibrous sheet that shares loads between them.
The ulna's exposed position along the back edge of the forearm makes it the classic 'defensive' bone: raise your arm to shield your face and a blow lands on the ulnar shaft. An isolated fracture there is called a nightwatchman's or parry fracture for exactly this reason. The bump on the little-finger side of your wrist is the ulna's head — its actual contact with the wrist bones is indirect, cushioned through a small disc of cartilage.
In Anamo, the ulna and radius form a paired lane; learning which side each occupies (ulna medial, radius lateral in anatomical position) instantly sharpens forearm guesses, and the ulnar nerve and artery running along it extend the same lane into the hand.
- Its olecranon hook forms the point of the elbow
- Acts as the stable hinge while the radius rotates around it
- The 'parry fracture' is named for its role in shielding the face
- Meets the wrist indirectly through a cartilage disc
Think you could find the ulna on a 3D model? Play Anamo and test yourself — every guess is color-coded by how close it lands to the mystery structure.