MOUNTED INSTRUCTION 147 



THE PHYSICAL CONFORMATION OF THE HORSE, 

 AS A MECHANICAL DEVICE 



The power of the horse to move loads is accomplished by the animal 

 pushing against the collar which is connected to the carriage by the 

 traces. The greater part of the force of propulsion comes from the 

 hind legs, through the back to the shoulders, and a smaller amount 

 from the front legs to the shoulders. 



A certain amount of push, depending upon the weight of the animal, 

 acts through the center of gravity by the horse leaning forward against 

 the collar. This last force is that due to the animal's weight and is not 

 muscular force. We thus hav^e three forces acting, the hind legs, the 

 fore legs, and a component of the animal's weight acting through the 

 center of gravity. The center of gravity of the horse is approximately 

 at the intersection of the vertical line passing through the eighth 

 dorsal vertibrae and a horizontal plane passing through the points of 

 the shoulders. 



From this we can see that when the horse leans forward in draft the 

 center of gravity is in the front of the mean position of contact of the 

 feet with the ground. 



A line from the mean position of the hind legs with the ground to the 

 center of the shoulder blade will represent the direction of the resultant 

 force exerted by the hind legs and a line from the mean position of the 

 fore legs to the center of the horse's shoulder blade, the resultant force 

 exerted by the fore legs. From an inspection of the skeleton of the 

 horse, it will be seen that the resultant direction of these three forces 

 is approximately perpendicular to the line of the shoulder blade. 



Considering the horse alone the line of draft should then be perpen- 

 dicular to the line of the shoulder at its middle point. The shoulder 

 blade rocks back and forth as the horse moves his fore legs. The center 

 of motion of the shoulder blade is about at its center. If the trace is at- 

 tached above or below this point, whenever that half moves forward 

 there is a heavy pull trying to limit its motion. It is the same principle 

 as if you tried to pull some object with the strap around both knees. 

 When fastened to the center of the shoulder, however, the pressure is 

 still there, but there is nothing tending to limit the forward movement 

 of the lower part of the shoulder. 



While we see that the construction of the carriage compels almost 

 horizontal draft, the conformation of the horse and the necessity of pro- 

 tecting the shoulders from injury must control the direction of pull 

 from the shoulder. This is accomplished by the "holding down strap" 

 used on swing and lead teams and by the "holding up strap" on the 

 wheel horse. This applies to steel collar. Due to the manner of mak- 

 ing the breast collar and the necessity for preventing interference with 

 movement of shoulders and sore shoulders due to any downward pull, 

 the pull should be horizontal. The lead and swing pairs are usually 

 about the same size, making the pull on these two pair practically hori- 

 zontal and since this is true, no holding down straps are required on the 



