The ulna is the caudolateral bone of forearm, divide into three segments:
- Proximal extremity with the olecranon
- Body (shaft) of ulna
- Distal extremity with the head of ulna.
The development of ulna varies considerably between species due to the evolution of rotation movement in forearm:
- In human, the ulna is well developped due to an important rotational movement (pronation and suppination).
- In carnivores, the movement is quite limite but the radius and ulna remains separate.
- In the pigs, the rotation is prevented by firm soft tissue bridging the interosseous space.
- In horses and ox, there is no movement and the two bones are fused.