There are some muscle names in New Latin that seem to be nouns as far as I can tell, such as flexor and extensor. However, according to several Wikipedia articles for these muscles, they behave as if they are attributive nouns with a nominative form. For example, in the name musculus extensor digitorum, the digitorum part seems fine to me because it's in the genitive; but the extensor part seems weird because it just gets tacked on to musculus even though it looks like a nominative form, and from what I've heard you're not supposed to just string nominative forms of nouns together in Latin.
So, are phrases like musculus extensor digitorum grammatically correct in proper New Latin?