The term meatus is a weird one. The plural form is not what you might think if you've never heard it before.
In anatomy, the term meatus refers to a passageway or opening into (or through) a tube.
For example, the term external acoustic meatus refers to the tubelike opening in the temporal bone that allows passage of the tubelike canal of the external ear bearing the same name.
There are many examples of meatus throughout the body.
What?! Was that a typo? Did I just mean to state, "examples of meatus throughout the body."? Yes. Yes, I did.
Considering the many nouns in science and medicine that end in -us and are pluralized by changing the -us to -i, one would think that the proper plural form of meatus is meati. Nope.
The term meatus is a weird one, remember? It's proper plural form is either meatus or meatuses.
Why the weirdness?
The plural form meatus comes from the fact that it in Latin, it belongs to the fourth declension class of nouns—and as such, meatus is the proper plural form in Latin.
The plural form meatuses adopts the English form of pluralization, which happens frequently when Latin terms are imported into English.
The combining form of meatus is meato-, as in meatoscope (for looking into the urinary meatus).