Extensor pollicis longus

Ulna

Thumb, distal phalanx

Extensor pollicis longus extends the terminal phalanx of the thumb; in combination with the Extensor pollicis brevis, it helps to extend and abduct the wrist.

Posterior interosseus nerve (branching from the radial nerve)

The radial artery is crossed by the tendons of the Abductor pollicis longus and of the Extensores pollicis longus and brevis.

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More on Origin and Insertion:

It arises from the lateral part of the middle third of the dorsal surface of the body of the ulna below the origin of the Abductor pollicis longus, and from the interosseous membrane.

It ends in a tendon, which passes through a separate compartment in the dorsal carpal ligament, lying in a narrow, oblique groove on the back of the lower end of the radius.

It then crosses obliquely the tendons of the Extensores carpi radialis longus and brevis, and is separated from the Extensor pollicis brevis by a triangular interval, the anatomical snuff box in which the radial artery is found; and is finally inserted into the base of the last phalanx of the thumb.