C. THE TENDONS

In order to convey an intelligent understanding of the tendons it will be wise to briefly describe the course of their parent muscles from their commencement.

THE EXTENSOR PEDIS.—­The extensor pedis arises from the lower extremity of the humerus in two distinct portions of unequal size, a muscular and a tendinous.  These are succeeded by two tendons passing in common through a vertical groove at the lower end of the radius.  Lower in the limb these tendons separate, the outer and smaller joining the tendon of the extensor suffraginis, and the inner and main tendon continuing its course downwards.  With the exception of the navicular, it is attached to all the bones of the foot, and is covered internally by the capsular ligaments of the joints over which it passes, those with which we are concerned being the pastern joint and the pedal joint.  Before its attachment to the os pedis it receives on each side of the middle of the first phalanx reinforcement in the shape of a strong band descending obliquely over the fetlock from the suspensory ligament.  Widening out in fanlike fashion, it is inserted into the pyramidal process of the os pedis.

Action.—­The action of this muscle is to extend the third phalanx on the second, the second on the first, and the first on the metacarpus.  It also assists in the extension of the foot on the forearm.

[Illustration:  FIG. 10.—­THE FLEXOR TENDONS AND EXTENSOR PEDIS. (AFTER HAUeBNER.) 1, Tendon of flexor perforans; 2, its supporting check-band from the posterior ligament of the carpus; 3, tendon of the flexor perforatus; 4, ring and sheath of the flexor perforatus; 5, widening out of the flexor perforatus to form the plantar aponeurosis; 6, suspensory ligament; 7, reinforcing band from the suspensory ligament to the extensor pedis; 8, the extensor pedis.]

THE FLEXOR PEDIS PERFORATUS, OR THE SUPERFICIAL FLEXOR OF THE PHALANGES.—­In common with the perforans, this muscle arises from the inner condyloid ridge of the humerus.  It is reinforced at the lower end of the radius by the superior carpal ligament, passes through the carpal and metacarpo-phalangeal sheaths, and, arriving behind the fetlock, forms a ring for the passage of the flexor perforans.  Its termination is bifid, and it is inserted on either side to the lateral surface of the second phalanx.

[Illustration:  FIG. 11.—­THE FLEXOR PERFORANS AND FLEXOR PERFORATUS TENDONS.  The metacarpo-phalangeal sheath and the ring of the perforatus laid open posteriorly, and the cut edges reflected to show the passage of the perforans. 1, Reflected cut edges of the perforatus ring and the metacarpo-phalangeal sheath; 2, the perforans tendon; 3, point of insertion of the perforans tendon into the semilunar crest of the os pedis (this widened and thickened extremity of the perforans is known as the plantar aponeurosis).]