Border Collies and Tombstones

So my dog, Blake, is not the most graceful of creatures. He's kind of big and goofy looking, especially for a border collie. His little sister, Callie, is tiny and sleek and Blake is just not lol So the other day we went for a walk down to the closest cemetery. We let the dogs go and they were *RUNNING!* having a marvelous time. Callie likes to try and cut past objects as closely as possible, hoping that Blake will run into something and slow him down. So she cut past a tombstone and Blake clipped it with his right rear leg.... I hear a crack and then watched Blake go head over heels howling all the way. He gets up crying and hobbles towards me as I run towards him. He looks up at me crying, I can see in his face, "Mom, it hurts!" He leans against my legs. I tell him to lay down. He is still crying, I feel his leg, nothing. Still crying he gets up and won't put weight on his leg. I start manipulating all his joints still no sign of pain. I have Blake walk with me for a while, he still won't put weight on it.... until Callie goes running by and amazingly the big wuss dog goes running off like nothing happened! He was fine of course, just gave himself a big ol' bruise lol