122 THE HOUSE IN MOTION. 



Plate LXXXI. represents the animal in the greatest degree of 

 extension he reaches in the run. The posterior extremities have suc- 

 cessively performed their functions as supporters and propellers, the 

 anterior limbs are extended to relieve them, and for the instant 

 the diagonal feet are upon the ground, but it is only for an instant ; 

 the weight of the body is already on the fore leg, and the only pro- 

 pulsive force left in the hind one is derived from the reaction of the 

 suspensory ligament and its reinforcing tendons. This position 

 nearly corresponds with that in Fig. S, page 93, though a little in 

 advance of it. 



Plate LXXXV. illustrates the run in the position shown in Fig. 

 ID, page 95. The fore leg must be straight from the elbow to the 

 foot when it makes contact with the ground, as only in that rela- 

 tion of the bones forming the columns of support could the weight 

 suddenly thrown upon them be borne. A moment's consideration of 

 the mechanical construction of the knee-joint will suffice to convince 

 one of this, and a weakness at that point which renders the animal 

 liable to stumble is a very serious defect, and where it exists it in- 

 dicates the loss of the balance of power between the flexors and 

 extensors of the foot. This inflexible position of the knee-'joint will 

 be found to be universal in all the paces when the limb is sustain- 

 ing weight. 



