I was wondering the same thing about internal damage. I watched a colie-shepherd cross of mine get 'rolled' under a 4WD pickup on the highway. I grabbed the shovel and went out to pick up the carcass; on the way out I saw he had somehow made his way back to the porch. Couldn't detect anything broken, but he was pretty sore; didn't move for a week or more. He wasn't interested in food, just a sip of water. Couple weeks later he was back to roaming. About a year later, he showed he didn't learn his lesson and ran out of ives.