Here are notes from my further research, these are the words of an opthalmologist: Option 2 is correct. Cornea is matrix of collagen. Cornea has maximum nerve endings, so maximum sensitivity. Though as some people are getting confused with retina, retina is ONLY photosensitive tissue in body, there's nothing for comparison so no question of more/most sensitivity.Option 3, cornea is NOT the major external covering, least protective/ weakest among coverings, and not totally proteinaceous