72 THE ANATOlfy OP THE HORSE. 



gastrocnemius. It arises from the head of the fibula, and its tendon 

 joins that of the preceding muscle, which it assists in extending the hock. 



Directions. — The inner head of the gastrocnemius is to be severed at 

 its origin, and turned downwards in the manner shown in Plate 17. 



The Superficial Flexor of the digit (flexor perforatus) (Plates 17 and 

 18) is remarkable in that, throughout nearly the whole of its extent, it 

 exists as a strong tendinous cord with a sparing admixture of muscular 

 fibres at its upper fifth only. It arises from the bottom of the supra- 

 condyloid fossa ; and, winding round the gastrocnemius tendon in 

 the manner already described, it gains the summit of the os 

 calcis, over the extreme posterior portion of which it plays by 

 means of a synovial bursa. At the os calcis it detaches on each side a 

 slip to be inserted into the bone. It is continued downwards in the meta- 

 tarsal and digital regions in the same manner as the perforatus of the 

 fore limb, becoming finally inserted by a bifid termination into the 

 second phalanx. In front of the tendons of the superficial flexor and 

 gastrocnemius there will be noticed a strong fibrous band, which is 

 united to these muscles above, and inserted into the os calcis below, 

 while laterally it is continuous with the deep fascia of the leg. 



Action. — It flexes successively the pastern and fetlock joints ; and, by 

 its insertion into the os calcis, it is also an extensor of the hock-joint. 

 It also plays an important part in mechanically maintaining the hock 

 ivi a state of extension so long as the hip and stifle joints are kept 

 extended by muscular contraction. 



The flexor perforatus of the horse is represented in man by two 

 distinct muscles — the plantaris and the flexor brevis digitorum. 



Directions. — The deep layer of muscles at the back of the leg consists 

 of the popliteus, the flexor perforans, and the flexor accessorius ; and 

 these should now be examined as far as possible without disturbance of 

 the vessels and nerves. The superficial muscles must therefore, in the 

 meantime, be allowed to remain in position. 



The Popliteus (Plate 17). This muscle is placed immediately behind 

 the stifle-joint, whose posterior ligament it covers. It arises by a tendon 

 from the lower and most anterior of the two pits situated on the outer 

 side of the external condyle of the femur. (The other pit is for the 

 attachment of the external lateral ligament of the stifle, the ligament 

 conceahng the origin of the tendon.) The tendon is partly invested by 

 the synovial membrane of the joint, and plays round the external semi- 

 lunar cartilage, and over the articular surface of the tibia. The fibres 

 of the muscle have an oblique direction downwards and inwards, and are 

 inserted into the comparatively smooth triangular area at the upper part 

 of the posterior surface of the tibia, and into the inner edge of the bone 

 at the same level. The terminal portion of the popliteal artery is 

 concealed by the muscle. 



