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A MANUAL OF VETERINARY PHYSIOLOGY 



abductor femoris, or long vastus) to the calcis (see Fig. 192). 

 It is seldom that the eye can witness extreme extension of the 

 limb — i.e., when the tibia and metatarsus form their nearest 

 approach to a straight line (Fig. 198) ; the movement is 

 altogether too quick during the gallop. Its nearest representa- 

 tion is obtained when a horse ' stretches ■ ; the limb is then 

 thrust out behind, rigid and taut, and the hock fully extended. 

 From the position of backward extension the limb is brought 



forward by flexion of the 

 tibia on the femur, and 

 the femur on the pelvis, 

 by mechanisms already 

 discussed, and flexion of 

 the hock -joint follows 

 (Fig. 197). The limb 

 now comes forward under 

 the body, and the foot, 

 which was previously 

 flexed, is extended, in 

 order to come to the 

 ground heel first or flat, 

 depending on the pace. 

 The leg is then in the 

 position of forward exten- 

 sion (Fig. 199). While 

 the extensor of the foot 

 is producing this action 

 with the assistance of 

 the lateral extensor, the 

 flexor metatarsi has to 

 relax in order to unbend 

 the hock. The flexor of 

 the hock and the ex- 

 *i__ tensor of the foot have 

 a common tendon of 

 origin (Fig. 190), so that 

 a remarkable physio- 

 logical peculiarity now occurs, in that a pair of muscles 

 having a common tendon of origin are enabled to function 

 in a diametrically opposite manner, the extensor of the 

 foot contracting, the flexor of the hock relaxing. The flexor 

 of the metatarsus, apart from the common tendon at the 

 femur, has a muscular addition from the tibia (Fig. 190), 

 and it would be quite competent for this muscular portion 

 to relax or contract independently of its neighbour. The 

 difficulty lies in explaining the behaviour of the common tendon 



Fig. 198. — Extension Backwards of the 

 Hind-Limb. 



