Sclera

The sclera is the tough white sheath that forms the outer-layer of the eyeball. It is also referred to by other names, including the sclerotic and the sclerotic coat (both of which have the same meaning as the word 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.

Colour of the sclera

Although the sclera is commonly described as the 'white of the eye', its colour tends to change somewhat with age. The sclera is thinner and more translucent in children, enabling the underlying tissue to show through enough to give the sclera a slightly 'bluish' hue. As people age, the sclera of their eyes tend to take on a slightly yellow hue. In some cases very small blue-grey flecks or spots can appear on the sclera, which is a condition called scleral melanocytosis.

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.

The sclera is supplied by many nerves and vessels that pass through the posterior scleral foramen, the hole in the sclera formed by the optic nerve. Blood supply to the sclera is via small interlinking capillaries.

For further detail see Gray's Anatomy, which is informative yet inexpensive.