Clue to a pun in The Three Musketeers?

By motionGoose

At some point, when the Three Musketeers and d’Artagnan are heading to the Bastion Saint Gervais, Aramis drops this Latin bit: “Animadvertuntur in desertis,” and Porthos basically says something like, yeah, it would’ve been nice to have a desert around, but there isn’t one. I’m guessing it’s meant as a joke about Porthos not knowing Latin, but I can’t track down what that phrase actually means. Anyone know where it’s from or what it translates to?