One-sixth of the outer layer of the eye (called "fibrous tunic") bulges forward as the transparent "cornea," which serves as the window of the eye and helps focus entering light rays. It is composed, for the most part, of connective tissue with a thin layer of "epithelium" on the surface. 

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. Along its circumference, the cornea is continuous with "sclera," the white portion of the eye. The sclera makes up the back five-sixths of the outer layer. It provides protection and serves as an attachment for the extrinsic muscles of the eye.