Dogs have a much more flexible spine. Horses have 17 or 18 ribs, and a large intestine full of hay in various stages of digestion. As a result, horses have minimal ability to bend their spines against these forces. In contrast, a dog has only 13 ribs and a low-volume intestine. In addition, dogs have a comparatively longer loin and strong abdominal muscles. Thus, the dog is able to flex and extend its spine producing a great deal of power for forward drive.