114 FARM ANIMALS 
capable of greater speed. These differences are not 
wholly due to shape and weight. It also depends upon the 
muscles, which are the source of all motion. Large mus- 
cles give power. Long muscles give speed. In running 
and driving horses the muscle lies more nearly in the 
same direction as the bones, while in draft horses the 
muscles act more nearly at right angles to the levers. In 
judging a horse, therefore, one considers not only the form 
of the animal, but the character of his muscles. 
WEIGHT aS A SOURCE OF POWER 
This horse is able to pull this great load of alfalfa hay because he is carrying part 
of the weight. The extra weight which he carries not only enables him to pull 
more, but he does not have so much to pull. 
