Epithelium is the type of tissue that covers all free body surfaces.
The transparency of the cornea is due to the fact that it contains hardly any cells and no blood vessels. On the other hand, it is well supplied with nerve fibers that enter on the margins of the eye and radiate toward the center. These fibers are associated with numerous pain receptors that have a very low threshold. Cold receptors are also abundant in the cornea, but heat and touch receptors seem to be lacking. The sclera makes up the back five-sixths of the outer layer.