CHAPTER XI. 



On PROGRESSION. 



X HE successive alternation of the limbs in 

 the motion of quadrupeds is so transitory, as to 

 render it very difficult to discern, at the same 

 view, their action separately and collectively. 

 The horse may be considered as a machine sup- 

 ported by four perpendicular columns, each of 

 which has its own center of gravity ; but dur- 

 ing progression, each of these columns must 

 shift its center, and obtain a new basis succes- 

 sively. In order to effect this, the alternate 

 actions of flexion and extension of the limbs are 

 carried on by means of joints acted upon by 

 muscles in different directions. 



O Thus 



