52 Scats and Saddles. 



that of the other vertebrae, all of which evidently admit 

 of movement toward it, within certain limits, while 

 this one, not being adapted for this purpose, may so far 

 be considered to be intended for the centre of motion. 

 But the construction of the horse's legs, and the relative 

 position of the various bones composing them, furnish 

 us with very clear proof of this same vertebra being the 

 real centre of motion when the horse is in action. For 

 there is one bone in each of the hind and fore legs 

 through which the remainder of the limb acts as a lever 

 on the whole frame, either for the purpose of propelling 

 it (hind legs), or supporting and lifting it (fore legs). 

 These are the thigh-bone /, and the arm-bone v (fig. 4), 

 whose upper ends have their fulcrums or points of sup- 

 port in the hip-bones A and shoulder-blades D (Plate I.) 

 respectively, the power >/^ing applied through the me- 

 dium of the remaipJry 'portions of the legs at their 

 lower ends. Nov/ <"V"p -/rf atest result of lever action is 

 exercised at a righ': yrgle to the lever, and drawing the 

 lines P ^ and R S through the lower ends of these 

 two bones at right angles to them., we Jind that they 

 intersect (or cross) each other precisely at this sa^ne 

 fourteenth vertebra. The figure shows us, indeed, 

 the horse in a state of rest and not in action ; but it is 

 necessary to remember that the propelling action of the 

 hind leg comtnences., a7td is precisely the ?nost pozuer- 

 ftd., when the thigh-bone is in this position, diminish- 

 ing in intensity as the leg is stretched out toward i?, 

 and the angles become f itter ; whereas, on the con- 

 trary, the supporting and lifting action of the fore leg 

 ends., the arm-bone being /»-s shown in the figure, and is 

 also most intense, diminifV mg as the fore leg is stretched 

 out toward P., and the *».ngles become flatter. It is 

 therefore evident, that both in a state of rest and of 

 action the four teeiith vertebra is constantly the centre 

 of motion; and it is precisely from our practical know- 



