The structure indicated is the abductor pollicis longus muscle of the posterior forearm.

The abductor pollicis longus muscle is one of the muscles located in the deep layer of muscles in the posterior compartment of the forearm. There are 5 muscles in the deep layer of the posterior forearm:

1. Supinator
2. Abductor pollicis longus
3. Extensor pollicis brevis
4. Extensor pollicis longus
5. Extensor indicis

The abductor pollicis longus muscle, as the name suggests abducts the thumb.

Origin: Posterior surface of proximal ulna, radius and interosseous membrane

Insertion: Base of 1^st metacarpal

Innervation: Posterior interosseous nerve

Action: Abduction and extension of the thumb at carpometacarpal joint

Learn more about the muscles of the forearm in this tutorial.