NORMAL STRUCTURE OF THE EYE

The eyeball can be divided into three coats or tunics. The outer white coat, or sclera, forms a dense, fibrous outer covering. In the front part of the eye the sclera becomes continuous with the cornea, the transparent structure through which we see. The innermost lining of the eyeball, is a thin layer known as the retina (image-gathering tissue in the back of the eye much like the film in a camera), which contains the nerve endings whose function it is to send visual impulses to the brain. These nerve fibers form the optic nerve, which exits through an opening in the back part of the sclera. The eyeball cavity is filled with a jelly-like substance called the vitreous.

The layer between the sclera and the retina is the uvea or uveal tract, the pigmented middle coat that is made up of three parts:
1. The iris is the colored portion in the front part of the eye; it has a circular opening in the middle, the pupil, which dilates and contracts in response to light.

2. The ciliary body is the middle part of the uveal tract, which produces the fluid inside the eye (aqueous humor). It comprises of very fine muscles and ligaments that attach to the capsule of the lens of the eye and help in varying the focus of the eye. The lens is a transparent semisolid body enclosed in a transparent capsule. The contraction and relaxation of the ciliary muscle are what enable the lens to modify its shape, allowing the eye to focus as it looks from one object to another.

3. The choroid is the back part of the uveal tract and lies just outside the retina. The blood vessels contained within the choroid provide vital nourishment for the eye.