A protein that is the main structural component of the cornea in the eye and similar tough tissues in other organisms.
From Latin 'corneus' meaning horn-like, combined with the chemical suffix '-in' used for proteins. The term evolved in the 19th century as scientists identified the primary protein in transparent tissues.
Your eye's cornea is so tough because it's made of cornein—the same type of protein that makes up horn and nail, which is why transparent eye tissue can withstand being poked and still let light through!
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