Drum, you might also want to look at
http://muextension.missouri.edu/...agguides/ansci/g02742.htm,
or Google up some other hits on "cannon bone" or "splints" and "horse." Modern horses have these two little skinny bones, the 2nd and 4th metacarpals, alongside the main bone above the hoof. These bones taper off to nothing 2/3 of the way down this cannon bone (third metacarpal) - they don't connect to anything. As a horse ages, ligament between them and the cannon bone turns to bone and fuses them together - older horses don't have splint bones.
Contrast this with fossil horses which had three functional toes. Remember that approximate age at burial of a fossil can be determined by tooth wear - like the expression "don't look a gift horse in the mouth". Why do you think our modern horses have these little bones which appear to serve no purpose other than to cause terribly sore feet? Is that due to The Fall, or is it a little more parsimonious to think that it's because
Equus had three-toed ancestors?