A small muscle situated on the outer part of the fore-arm, between the extensor pedis and the flexor metacarpi externus.

Origin, - From the external lateral ligament of the elbow-joint, and from the external tuberosity and outer side of the radius and ulna.

Insertion. - The tendon of this muscle, after passing over a groove on the inferior and outer part of the radius or fore-arm, and through a synovial sheath on the outer side of the knee, becomes united with the outer branch of the tendon of the extensor pedis and a fibrous slip from the external side of the carpus. Thus strengthened, it passes along the outer front part of the canon-bone to the fetlock-joint, where it plays over a synovial bursa and becomes inserted into the anterior and upper part of the first phalanx or large pastern.

Action. - To extend the fetlock on the canon and the knee on the fore-arm.