[Illustration:  FIG. 12.—­THE FLEXOR PERFORATUS AND FLEXOR PERFORANS TENDONS.  The metacarpo-phalangeal sheath and the ring of the perforatus laid open posteriorly, and the cut edges reflected; the flexor perforans cut through at about the region of the sesamoids, and its inferior portion deflected. 1, Superior end of severed perforans tendon; 2, inferior end of severed perforans tendon; 3, insertion of flexor perforans into semilunar crest of os pedis; 4, the cut and reflected edges of the metacarpo-phalangeal sheath and perforatus ring; 5, the bifid insertion of the flexor perforatus into the lateral surfaces of the os corona; 6, the capsular ligament of the pedal joint; 7, the navicular bone; 8, the posterior surface and glenoid fibro-cartilage of the os coronae.]

Action.—­This muscle flexes the second phalanx on the first, the first on the metacarpus, and the entire foot on the forearm.  Mechanically, it acts as a stay when the animal is standing by maintaining the metacarpo-phalangeal angle.

[Illustration:  FIG. 13.—­MEDIAN SECTION OF FOOT. A, Os suffraginis; B, os coronae; C, os pedis; D, navicular bone; E, tendon of the extensor pedis; F, insertion of the extensor pedis into the pyramidal process of the os pedis; G, the tendon of the flexor perforatus; H, insertion of perforatus into the os coronae; I, tendon of the flexor perforans; J, its passing attachment to the os coronae; K, its final insertion into the semilunar crest of os pedis; a, section of coronary cushion; b, section of plantar cushion; c, semilunar sinus of os pedis.]

THE FLEXOR PEDIS PERFORANS, OR THE DEEP FLEXOR OF THE PHALANGES.—­This muscle consists of three easily-divided portions:  an ulnar, a humeral, and a radial, and has for points of origin the olecranon process of the ulna, the inner condyloid ridge of the humerus, and the posterior surface of the radius.  These portions are continued by a common tendon which enters the carpal sheath with the tendon of the perforatus, and continues with it through the synovial sheath of the metacarpo-phalangeal region.  Like the last-named tendon, it receives a supporting check-band, in this case from the posterior ligament of the carpus.  Passing down between the suspensory ligament in front, and the perforatus tendon behind, it glides over the sesamoid pulley and passes through the ring formed by the perforatus.  Continuing its course, it passes between the bifurcating portions of the extremity of the perforatus, glides over the smooth posterior surface of the supplementary glenoid cartilage of the articulation of the first and second phalanges, plays over the inferior surface of the navicular bone, and finally becomes inserted into the semilunar crest of the os pedis.  On reaching the posterior border of the navicular bone it widens out to form the plantar aponeurosis.

In connection with the lower portion of this tendon must be noticed the Navicular Sheath.  This is a synovial sheath lining the deep face of the tendon, and reflected on to the navicular bone and the interosseous ligament of the pedal joint.  This will be of particular interest when we come to deal with cases of pricked foot from picked up nails.  Above, it is in connection with the synovial membrane of the pedal articulation and that of the metacarpo-phalangeal sheath.