MS-3400

You are in the emergency department and a patient has just come in after falling off his bicycle onto an outstretched hand. On examination of his hand, there is significant pain in the anatomical snuffbox. The medial border of this region is formed by the tendon of a muscle that attaches to the distal phalanx of the thumb and causes extension of the metacarpophalangeal joint and interphalangeal joints.

What is the name of this muscle and which nerve is it innervated by?