Muscle · Forearm

Flexor pollicis longus

Musculus flexor pollicis longus

The only muscle in the body that bends the end joint of the thumb. It lies deep in the front of the forearm on the radial side, taking a broad origin from the front of the radius and the sheet of tissue joining it to the ulna, and narrowing into a single tendon that travels through the carpal tunnel and out along the thumb to the base of its last bone. Because it is the sole supplier of that one movement, an inability to bend the thumb tip is unusually informative — there is no second muscle to cover for it, and its nerve, the anterior interosseous branch of the median, is purely a motor nerve, so the problem shows as lost movement without any numbness.