How, then, is the nerve to be protected, and yet the sight not obstructed?
If it were covered with skin, as the other nerves are, you could not see through it.

For thousands of years after men had eyes and used them, they knew no substance, at once hard and transparent, which could answer the double purpose of protection and vision.

And to this day they know none hard enough for protection, clear enough for vision, and elastic enough to resume its form after a blow.

But men did the best they could, and put a round piece of brittle but transparent glass in a ring of tougher metal for the protection of the hands of a watch; and he who first invented the watch crystal thought he had made a discovery.

Now, observe in the eye, that forward part is the watch glass; the cornea, made of a substance at once hard, transparent and elastic--which man has never been able to imitate--set into the sclerotica, that white, muscular coat which constitutes the white of your eye, acts as a frame for the cornea, and answers another important purpose, as we shall presently see.
[Illustration: Structure of the Human Eye] But, supposing the end of the nerve protected by the glass, we might have had it brought up to the glass without any interposing lenses or humors, as, in fact, is nearly the case with some crustacea.