This muscle arises from the anterior surface of the radius below the anterior oblique line and above the insertion of pronator quadratus, and from an adjoining strip of interosseous membrane. Some fibers, arising in common with flexor digitorum superficialis, join the muscle from either the medial or the lateral border (or both) of the coronoid process of the ulna. The oblique cord represents phylogenetically degenerate fibers of the upper part of the muscle. The tendon forms on the ulnar side of this unipennate muscle; it should be noted that the tendon receives fleshy fibers into its radial side to a point just above the wrist, a useful point in distinguishing it from the flexor carpi radialis in window dissections or in wounds above the wrist. The tendon passes in the carpal tunnel deep to that of the flexor carpi radialis, then spirals around its ulnar side to become superficial. It extends into the thumb to be inserted into the base of the distal phalanx.
Nerve supply of flexor pollicis longus:
By the anerior interosseous branch of the median nerve (C7, 8).
Action of flexor pollicis longus:
As indicated by its name, it flexes the thumb.