I just saw your post about the heave lines, and my gelding that I mentioned in my last post has the same lines; so I suspect that is what they are.
Here's one of her in 2009 though...and I can't really see it except in the action shot:
And then 2010...(after she was off the molasses and soy)
So now I can't tell if that's what it is, or whether it's just muscle definition in general with her being fit?
She never coughs and hasn't since I got her, and doesn't "roar" when being worked or anything like that either...