lol, most of the time they do....

Its not hard to tell if a horse gets injured from their back...their gait changes, it becomes less even and you will probably loose the groove that you've been in because something changed. Most (good) jockeys will stop the horse at this point...however it is sometimes hard because the horses dont want to stop running....you should watch it...they have an amazing amount of heart, and sometimes they just dont realize it.

I would worry...horses are more likley to break legs in a pothole ridden field than on the track.

But then again, horses can break their legs doing anything....they just have so little muscle on those lower legs...everything is operated by stored and released energy, they are like rubber bands, so there is alot of tension and therefore it is not difficult for things to break in general.