If you look for information on cat claws, most of what you find is devoted either to the mechanics of the retractable claw or to the pros and cons (or evils, depending on your degree of zeal) of the de-clawing operation. Google up pictures of the claw anatomy, and you get stuff like this. But, to quote (sort of) Arlo Guthrie, "...that's not what I came here to talk about."
People often talk about a cat "sharpening" its claws. You have to wonder how that works. You know that the cat is not sitting there with a whet-stone, working the claw down to a needle-sharp point. On the other hand, you also know that the claws generally do have a needle-sharp point... or should I say razor-sharp? How does that work?
Just walking around doesn't require sharp claws. In fact, it doesn't require claws at all. They walk on the next bone up, with the third phalanx retracted up out of the way (as in the top picture). The needle-sharp point is taking no wear. The cat does benefit from sharp claws when climbing trees. They climb trees to escape bigger predators, as well as to gain access to prey themselves. This is bound to put some wear and tear on the claws, dulling them down.
Then, when you move in for the kill (or to slap an unwary veterinarian), and out pops the switchblade, you'll be wanting some claws that can really penetrate deeply with no trouble. Dull claws just won't get it when you are the deadly feline hunter/fighter. If you're hunting, climbing and fighting, they are bound to wear down. So, how do you sharpen them?
We see the cat on the scratching post (or your furniture, alas) just working away. "He needs to sharpen his claws." I see how that destroys the furniture, but how could it be sharpening his claws?
After all that hunting, climbing, slapping and destruction of the furniture, it would appear that the claw would be worn down like this. And it is, and it would stay that way if the cat's claws grew lengthwise like a dog's claws or a person's fingernails. That's not the way cat claws grow, though. They don't grow lengthwise. They grow in layers like an onion (surely you don't need a picture of an onion).
Thus, as the claw is wearing down on the outside, a brand-new, needle-sharp (or razor-sharp) claw is growing inside. This is why trimming cat toe-nails doesn't give you a long-lasting short, smooth nail. It won't be long before the outer sheath you trimmed is shed, revealing the brand-new weapon, ready for action.
The "sharpening" of the claw is actually the removal of those worn outer sheaths. As the new inner claw reaches full development, the outer sheath becomes a little loose and a little uncomfortable. The cat keeps working on something like tree bark, or your furniture, until he gets it pried off, and he's ready to rumble again. Of course, some cats just tear up your furniture for the hell of it, like Garfield, but the cat actually does need to scratch.
Which brings us to Bob. Bob used to be outside a lot. Unfortunately, with advancing age and a plague of stray dogs, his owners felt like he might not be safe outside anymore, and he had to become a total house-cat. He came in recently because he was chewing on his feet all the time. The owners just couldn't understand what was bothering him. He never used to do this.
While Bob was outside, he loved to scratch on tree-bark. Since he's been inside, he doesn't scratch on anything. Not on the furniture, not on the lovely scratching-posts they bought him. With older cats, those old claw sheaths don't shed as easily as with younger cats. Occasionally, we'll find an old cat where several layers have accumulated. Bob is the champion in this department.
This pile of claw sheaths (and pieces thereof) are the product of one toe-nail trim... and a lot of picking away at the outer sheaths by me. His feet looked pretty normal afterward, and he quit chewing on them. I can't really imagine what that must feel like with all those extra claw-layers jammed up in your toe. (Ingrown toenails, anyone?)
Bob is scheduled for monthly toe-nail trims (or inspections at least). I've recommended getting a log with the bark still on for his new scratching post. We'll see if that takes care of it.