A healthy frog is triangle shaped and full-fleshed, without thrush.
To understand what the sole should look like you need to see the corium again:
The concave shape that you see on the bottom of the corium should be reflected by the external sole. Why are soles concave? Because then they form an arch, one of the strongest weight bearing shapes possible. When weight is placed on an arch it is evenly distributed throught the entire structure. That means each bit of the arch is supporting the smallest amount of weight possible. Think of ancient bridges, there's a good reason why the basic shape on most of them is an arch- it's darn strong.
Can you think of a better shape to hold up a couple hundred pounds of horse? I sure can't.
|Coriander's right hind, check out the concavity|
The frog serves as the shock absorber at the back of the hoof. Frog tissue grows from the corium and gets compacted into a very dense, yet springy mass. Want to know something interesting? There are people that refer to the frog as the horse's pads, like a dog or cat. Thing is, I have to agree. If they were called pads instead of frogs, I think people would be a lot less confused about their purpose.
Mrs Mom just left an excellent comment that I want to add here: "the shock absorbing ability of the frog is not just limited to up and down as we have all been taught-- but also in it's ability to allow for lateral movement, acting as a wide rubber band. (ie: load the hoof, walls flex, frog allows for X amount of flexion before bringing things back into line. Combined with the up and down compression, the side to side action also allows for wonderful energy distribution/ dissipation.)." Thanks Mrs Mom!
Surprisingly frogs are actually quite delicate, if the hoof health is compromised, frogs are one of the first things to go. Sadly, the majority of horse hooves are so compromised that most people don't even know what a healthy frog looks like- my horses don't even have healthy frogs yet :(
So here you go, the healthiest frog I could find. Take particular note of the color, healthy frogs aren't gray, they are brown.
|Photo from Heike Bean|
The Affect of Shoes on the Sole and Frog
Now lets imagine you take a hoof and nail a shoe to it- what happens to the sole and frog? They don't touch the ground anymore. This is key. Without contact and abrasion from the ground the tissues are no longer stimulated to grow, the frog will atrophy and the sole loses its callus. I believe this is what actually causes heel contraction in shod hooves: atrophy. As the saying goes: Use it or lose it. This is aggravated if your farrier routinely trims sole- if sole growth has slowed due to lack of ground pressure and what's left gets cut off- you end up with extremely thin soles.
Think about your own feet: If you've worn shoes and boots all winter long and then rush outside barefoot the first chance you get in the spring, what happens? Your feet hurt. Why? Because you don't have the callus on the bottom of your feet to protect you. But if you steadily keep going out barefoot and acclimate, it's not long before you can wander around on rocks in fair comfort. Horse hooves are no different.
This is one reason why so many people fail in taking their horses out of shoes, the sole and frog need time to grow and build a callus, something that doesn't happen overnight. This is even harder for those horses that routinely get their soles cut off, for that kind of foot you desperately need to have boots and pads on hand to keep the horse out of pain while their sole and frog grows in.
Then there's thrush, if your horse has infected frogs he won't be comfortable no wonder what the sole looks like.