THE HORSE IN MOTION. 59 



After passing the perpendicular, and the angles of the extremity are 

 increased, the semi-tendinosus ceases to act, and the extension is con- 

 tinued by the vastus, gluteus, triceps, and muscles of the calf, to the 

 end of the stride. In the flexion of the limb that takes place as it 

 shortens in order to give uniform support, and not be itself crushed, 

 the flexion is effected by the weight borne, in which the flexors proper 

 bear no part ; their action could have no other effect than to bring the 

 body to the ground, but it is effected by the gradual giving way of the 

 triceps and the suspensory ligament. 



It will be seen that but a small part of the immense power of the 

 extensors, or propellers of the posterior extremity, is spent in the act 

 of propulsion, even when the animal is in full motion, but in supporting 

 weight ; and as the extension of the leg increases and the burden is 

 assumed by another limb, it is the better enabled to exert its propelling 

 power. As the limbs are successively relieved of that duty by their 

 alternates, they are in better position to exercise their functions as 

 propellers. 



This analysis of the mechanism of the posterior extremity will be- 

 come of importance when we come to apply it to the run or greatest 

 speed of the horse. The reader who has not had the patience to fol- 

 low us through the study to the end of this chapter will not be able 

 to master the next, and we would advise him to pass it over, and take 

 up the fifth chapter, where we will endeavor to apply the demonstra- 

 tions contained in these two; but such must take the facts on which 

 the theory of motion is based for granted. 



