Apparently, Big Brown has had a problem with "quarter cracks" in his hoofs so he has been lightly raced. Quarter cracks are cracks in the hoof wall that run up and down or parallel to the angle formed at the front of the hoof. If they get deep enough they can cause lameness. I'm not a farrier or an expert on horses hooves, but I have had horses with quarter cracks and you do have to take them seriously. They take a long time to heal because if they're deep the entire hoof has to grow out until the crack is healed. Meanwhile, the entire time they are growing out, stress on the hoof is tryin to make the crack widen. I'm really impressed with Big Brown--I thought he looked really confident and ready during his workout this week. He also looked like he had a lot of gas left after running the derby!! Having said that, a horse is only as good as his worst foot. If he would develop quarter cracks again, he could be done racing for this year no matter how ready he is otherwise. That would be to bad because to me he looks like the most likely candidate in some time to be called "the next Secretariat"!!
As far as the injury to Eight Belles--the best analogy I heard once was that a race horse at one points in their stride puts all his weight and momentum on a single joint approximately the size of an average human wrist (check the video Knabe posted a few days ago). The ankle area of the horse is made up of several smaller bones, ligaments, etc. The slightest mistep can be fatal to these finely tuned running machines because the great ones also have inherited the will to run and will keep trying even when they are hurt. Terrible tragedy and I'll bet no one understands or empathizes more with the horse than the owners, trainers, etc. that work with them day to day.