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.
 

Fig. 10.   The Flexor Tendons And Extensor Pedis

Fig. 10. - The Flexor Tendons And Extensor Pedis. (After HaÜBner.) 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.

Fig. 11.   The Flexor Perforans And Flexor Perforatus Tendons

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).

Fig. 12.   The Flexor Perforatus And Flexor Perforans Tendons

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.

Fig. 13.   Median Section Of Foot

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.

Action. - The action of the perforans is to flex the third on the second, and the second on the first phalanx. The latter it flexes in turn on the metacarpus. It also assists in the flexion of the entire foot on the forearm, and in supporting the angle of the metacarpo-phalangeal articulation when the animal is standing.