Cornea

A clear membrane that lies in front of the iris (colored part of the eye)
and pupil. It is commonly referred to as the “front of the eye.”
It is literally the window to the eye and has tremedous focusing power
for the eye. It determines 80% of the refractive power of the eye. It
also greatly magnifies the internal eye structures that allows an examiner
to view the inside of the eye in great detail. It is rich in sensory nerves
which explains the extreme sensitivity of the eye to scratches and foreign
bodies. (See Eye Anatomy and
How The Eye Works for more details.)