Complex sentence: When the pterygium is removed the tissue that covers the sclera known as the conjunctiva is also extracted, auto-grafting replaces the bare sclera with tissue that is surgically removed from the inside of the patients upper eyelid.
Split into simpler sentences: When the pterygium is removed, the tissue that covers the sclera known as the conjunctiva is also extracted. Auto-grafting replaces the bare sclera with tissue that is surgically removed from the inside of the patients upper eyelid.