The cornea, which covers the iris, pupil, and anterior chamber, is the transparent front section of the eye. The cornea, along with the anterior chamber and lens, refracts light, accounting for around two-thirds of the total optical power of the eye. Unmyelinated nerve endings in the cornea are sensitive to touch, temperature, and chemicals, and a touch induces an automatic response to close the eyelid. The healthy cornea does not have or require blood vessels because transparency is so important. Corneal and external diseases are ailments that affect the outside layer of the eye. Dry eye, corneal infections, blepharitis, allergies, conjunctivitis, and corneal dystrophies, all of which can produce clouding of the cornea, are some of the most frequent ocular surface diseases.