268 THE MUSCLES. 



aid of three special fibrous bands. A very extensive vaginal synovial mem- 

 brane 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, and forms, inferiorly, a somewhat large cul-de sac which, 

 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. The metacarpo-phalangeal sheath is also named 

 the great sesamoid sheath ; but this designation is more frequently applied 

 to the synovial membrane lining its walls. 



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

 metacarpus, and the entire foot on the fore-arm. 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 maintain- 

 ing the metacarpo-phalangeal angle. 



5. Deep Flexor of the Phalanges or Perforans. (Figs. 119, 121, 122.) 



Synonyms. Cubito-phalangeus, or rarlio-phalangeus Girard. (Flexor pedis perforans 

 Percivall. Humero-radio-phalangeus 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 

 powerful tendon. 



Form, Structure, and Attachments of tiie muscular portion of the perforans. 

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

 into epitrochlean, ulnar, and radial. 



The epitrochlean 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 portion, representing the flexor longus pollicis of Man, is the 

 smallest, and is deeply concealed beneath the epitrochlean muscular portion. 

 The fibres composing it are fixed to the posterior surface of the radius, 

 where they are slightly divergent, and collect upon a small particular tendon, 

 which is confounded with the common tendon after contracting adhesions 

 with the radial band of the perforatus. 



Course and Attachments of the tendon. The tendon which succeeds these 

 three portions enters the carpal sheath with that of the superficial flexor ; 

 Towards the middle of the metacarpal region, it receives a strong fibrous 

 band from the great posterior ligament of the carpus (Figs. 119, 23 ; 122, 18), 

 traverses the sesamoid annular apparatus of the perforatus tendon, passes 

 between the two terminal branches of that tendon, over the pulley on the 

 posterior face of the second phalanx, and afterwards widens to form a large 

 expansion designated the plantar aponeurosis. 



This aponeurosis glides, by its anterior face, over the inferior surface of 

 the navicular bone, by means of a particular synovial membrane, the 

 small navicular sheath, and is covered, posteriorly, by a fibrous layer, noticed 

 for the first time by M H. Bouley, who considers it as a reinforcing 

 sheath for the perforans tendon. It is finally inserted into the seinilunar 



