A “pachyderm” is an elephant. Quite a large sort of animal in most people’s opinions. And although elephants don’t, strictly speaking, have toes, the term “pachydactyl” means a type of animal (often a bird) that has large toes or talons. An ostrich, for example. Or the Road Runner from the cartoons. Another group of animals are the “pachyglossates” — they have large tongues.
There are some other “pachy-“ terms describing either animal or plant characteristics, and they’re all focused on the size of this or that — fruit, toes, tongues, and the like. Then there is the “pachymeter.” It’s a tool; one used in science and medicine in the same way a machinist uses a micrometer, to measure something very, very thin. Perhaps just a fraction of a millimeter.
But wait, what’s going on here? Everything “pachy-“ in the animal and vegetable kingdoms is distinguished by large size, but a “pachymeter” measures something very small? The secret here is that “pachy-“ doesn’t really mean “large size” at all — it means thick. There’s a clue in the term for elephant — the second half is “derm”, which means skin. The scientific name for elephants means “thick skin,” not “enormous beast” or “prehensile trunk,” or even “holy toledo, what is that enormous beast?!?”
The other “pachy-” terms also have to do with thickness — “dactyl” in “pachydactyl” means “digits” (and could apply to fingers as well as toes), and “glossate” in “pachyglossate” means “tongue.” All of these are Greek derivations, by the way. So a “pachymeter” simply measures thickness, and isn’t named for any particular amount of thickness.
In practice, a pachymeter is used to measure very thin things, and in its medical applications the goal is generally to detect whether one of those things, while exceedingly thin, is nevertheless thicker than it ought to be. There’s an ultrasonic pachymeter used to measure the thickness of the cornea of an eye, because some vision problems are associated with a cornea that’s too thick. You want to get conditions like those treated so you can see when a large thick-skinned animal in a bad mood might be headed in your direction.
