cornea vs arcus

cornea and arcus both are nouns.

Word	Noun	Adjective	Verb	Adverb
cornea	Yes	No	No	No
arcus	Yes	No	No	No

As nouns, arcus is a hyponym of cornea; that is, arcus is a word with a more specific, narrower meaning than cornea:

cornea: the transparent dome-shaped anterior portion of the outer covering of the eye; it covers the iris and pupil and is continuous with the sclera
arcus: a whitish deposit in the shape of an arc that is sometimes seen in the cornea

Other hyponyms of cornea include arcus senilis.

cornea (noun)	arcus (noun)
the transparent dome-shaped anterior portion of the outer covering of the eye; it covers the iris and pupil and is continuous with the sclera	a whitish deposit in the shape of an arc that is sometimes seen in the cornea