This muscle is situated behind the lower arm, with which it is in contact. It is composed of three unequal portions - the humeral, the ulnar, and the radial. The humeral is much the largest of the three, and constitutes the chief bulk of the muscle.

Origin. - It arises (1) from a ridge on the inner and inferior part of the humerus; (2) from the upper and posterior part of the ulna; (3) from the posterior surface of the radius.

Insertion. - The three portions unite and form one common tendon, which proceeds downwards to the foot. In its course it passes, in company with the flexor pedis perforatus tendon, over the back of the knee, where it is bound down by the carpal sheath and lubricated with synovia. Below the knee it is joined by a strong fibrous band, the check ligament or inferior carpal ligament. Thence it continues downward behind the canon, over the fetlock-joint, where it is encircled by a tendinous offshoot from the perforatus. Behind the pastern it passes between the divided tendon of the flexor pedis perforatus, and ultimately enters the foot, where, after widening out beneath the navicular bone, it becomes inserted into the under surface of the os pedis.

Action. - To flex the pastern on the fetlock, and through it the knee also.