Sclera

The sclera is the tough white sheath that forms the outer-layer of the ball.
It is also referred to by other terms, including the sclerotic and the sclerotic coat (both having exactly the same meaning as the sclera).

In all cases these names are due to the the extreme density and hardness of the sclera (sclerotic layer). It is a firm fibrous membrane that maintains the shape of the eye as an approximately globe shape. It is much thicker towards the back/posterior aspect of the eye than towards the front/anterior of the eye.

The white sclera continues around the eye; most of which is not visible while the eyeball is located in its socket within the face/skull. The main area of the eye that is not covered by the area is the front part of the eye that is protected by the transparent cornea instead.

The Structure of the Sclera

The sclera is composed of white fibrous tissue intermixed with fine elastic fibers and corpuscles of flattened connective-tissue. These fibers are grouped together in bundles.
Blood supply to the sclera is via small (but not very numerous) interlinking capillaries.
The nerves connected to the sclera are from the ciliary nerves.

For further detail we recommend Gray's Anatomy - an excellent general text that is informative yet inexpensive.