74: THE ANATOMY OF THE HORSE. 



and are inserted into the comparatively smooth triangular area at the 

 upper part of the posterior surface of the tibia, and into a tubercle on 

 the inner edge of the bone at the same level. The terminal portion of 

 the popliteal artery is concealed by the muscle. 



Action. — It flexes the stifle, and to a slight extent rotates it inwards. 



The Deep Flexor of the digit (flexor perforans) (Plates 17 and 18). 

 This muscle is indistinctly divided into an outer and an inner division, 

 the former being the larger of the two. It arises from the ridged area 

 on the posterior surface of the tibia, from the external tuberosity at the 

 upper end of the same bone, from the fibula, and from the interosseous 

 membrane uniting the two bones. At the lower third of the tibia the 

 muscular divisions are succeeded by tendons, which soon unite ; and the 

 single tendon thus formed glides through the tarsal sheath at the inner 

 side of the os calcis, and then descends at the back of the metatarsus 

 and digit, to be inserted into the solar surface of the os pedis, in a man- 

 ner exactly similar to the flexor perforans of the fore limb. Like that 

 muscle, it receives, at the upper part of the metatarsus, a reinforcing 

 or check band — the subtarscd ligament, which is the downward continua- 

 tion of the posterior tarso-metatarsal ligament of the hock. This band 

 is not so strong as the subcarpal ligament of the fore extremity. 



Action. — It flexes successively from below upwards the interphalangfeal 

 joints and the fetlock, and finally extends the hock. 



The Tarsal Sheath, through which the tendon passes at the inner 

 side of the back of the hock, is formed outwardly by the grooved surface 

 of the os calcis, and in front by the posterior ligament of the tibia-tarsal 

 articulation and by the posterior tarso-metatarsal ligament, while it is 

 completed inwardly by a fibrous arch that converts the groove into a 

 complete canal. An extensive synovial membrane here invests the 

 tendon and lines the passage, extending upwards for a few inches at 

 the lower extremity of the tibia, and downwards below the middle of 

 the metatarsus. A dropsical condition of this synovial sac gives rise 

 .to the condition termed "thorough-pin." 



The Flexor Accessorius (Plate 17) is a somewhat slender muscle 

 extending obliquely downwards at the back of the leg, between the 

 popliteus and the perforans. It arises from the back of the external 

 tuberosity at the head of the tibia. Its tendon, which begins at the 

 lower third of the leg, descends first in a groove on the deep flexor, and 

 then through a synovial passage at the inner side of the tarsus, and 

 finally blends with the tendon of the deep flexor at the back of the 

 metatarsus. 



Action. — To assist the deep flexor. 



Directions. — The vessels and nerves of the region must now be 

 noticed, and it will be convenient to begin with the latter. 



The Internal Popliteal Nerve (Plates 17 and 18) is the continuation 



