330 THE MUSCLES. 



face of the principal navicular ligaments, that of the glenoid fibro-cartilage of the 

 first interphalangeal articulaticn, and by the posterior pulley of the second 

 phalanx (Fig. 183, 5). It is completed by a very wide membranous expansion 

 applied against the flexor tendons, closely adherent to the perforatus on the 

 median line of the digit, and fixed, laterally, to the phalangeal bones by 

 the aid of three special fibrous bands (Fig. 184, 6, 7, 8). A very extensive 

 vaginal synovial membrane covers the internal walls of this passage, and is 

 reflected on the flexor tendons ; it ascends along these tendons to the inferior 

 extremities of the lateral metacarpals (Fig. 183, 6), and forms, inferiorly, a 

 somewhat large cul-de-sac which envelops the perforans tendon (Fig. 183, 9), 

 and behind the second phalanx lies against the posterior cul-de-sac of the 

 articular synovial membrane of the foot, and also against the superior cul-de-sac of 

 the navicular sheath (Fig. 137, 14). The metacarpo-phalangeal sheath is also 

 named the ffreat sesamoid sheath ; but this designation is more frequently 

 applied to the synovial membrane lining its walls. When this bursa is distended 

 by the fluid it secretes, it projects at all those points where it is not supported 

 by the fibrous walls of the metacarpo-phalangeal sheath. Then are formed above 

 the metacarpo-phalangeal articulation, and between the suspensory ligament of 

 the fetlock and flexor tendons, as well as below the fetlock — between the fibrous 

 bands which attach the sheath to the first phalanx — those small tumours named 

 "windgalls" (Fig. 183, 6, 7, 8). The inferior cid-de-sac (Fig. 183, 9) seldom 

 appears externally, as it is sustained by the reinforcing sheath of the perforans 

 tendon. 



Action. — This muscle flexes the second phalanx on the first, that on the meta- 

 carpus, and the entire foot on the forearm. Its tendon, through the influence 

 of the fibrous band which attaches it to the posterior face of the radius, acts 

 mechanically as a stay while the animal is standing, by maintaining the meta- 

 carpo-phalangeal angle. 



5. Deep Flexor of the Phalanges, or Flexor Pedis Perforans 

 (Figs. 179, 181, 182, 183). 



Synonyms. — Cubito-phalangelis, or radio-phalangeus — Girard. (Humero-rndio-phdlangeus — 

 Leyh.) 



Situation — Composition — Extent. — This muscle is situated immediately behind 

 the radius, and is composed of three portions which unite at the carpus, to be 

 continued to the inferior extremity of the digit by a long and powerful tendon. 



Form, Structure, and Attachments of the muscular lioriion of the perforans. — 

 The three muscular portions may be distinguished, in regard to their origin, into 

 humercd, ulnar, and radial. 



The humeral portion is the most considerable, and lies beside the perforatus ; 

 being three or four times the volume of that muscle, it is easily divided into 

 several very tendinous fasciculi, which leave the summit of the epitrochlea along 

 with the superficial flexor. The ulnar portion, situated between the external 

 flexor and the oblique flexor of the metacarpus, is very short and conoid, thick 

 at its superior extremity, contracted at its inferior, to which succeeds a long flat 

 tendon, united below to the principal tendon ; it has its origin on the summit 

 and the posterior border of the olecranon. The radial jwrtion (or radialis 

 accessorius) ^ is the smallest, and is deeply concealed beneath the epitrochlean 



' It represents the flexor longus pollicis of Man. 



