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.
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.