Tell me about the normal human eye. How does it work?

The human eye has many similarities to a camera both in its structure and function. There is a clear window at the front of the eye called the cornea, a coloured iris to control the amount of light, a lens for changing focus, and a retina or film to make the images. The eye is connected to the brain's computer via a very high speed cable (the optic nerve) so that images are downloaded in real time. Remember that your retina is actually an outgrowth of the brain itself.

Visible light enters the eye by passing through the clear cornea. The cornea actually does the majority of focussing of light. It also filters out most harmful UV radiation. Your cornea needs to be properly lubricated by the tear film secreted by tiny glands in your eyelids. Once the light enters the lens of the eye it can be focussed at will by contraction of a small muscle (ciliary muscle) so that we can look from far to near and back again. It functions like the focus ring on your camera's lens. The light finally is absorbed by photoreceptors in the retina. These are known as rods and cones. You have about 170 million of them in each eye! Rods provide your side and night vision. Cones provide colour, detailed central vision. You are using your cones to read this. The rods and cones turn light into an electrical impulse which is modified by other nearby cells and in turn sent along the optic nerve to the back of the brain to be turned into pictures.

By using special instruments it is possible to look directly at the retina, the optic nerve and other structures within the eye. The optic nerve is the pinkish/yellow oval structure on the left side. Radiating from it are retinal arteries and veins. The macula is the slightly darker region in the centre and is relatively free of blood vessels. The macula contains the cone cells and is the part of the retina responsible for detailed vision. Underneath the retina is a pigment layer which has a crucial role in the maintenance of the photoreceptors. It is similar in some ways to the black insides of a camera. Underneath the pigment layer is a layer of blood vessels called the choroid. The choroid keeps the photoreceptors alive by allowing nutrients to diffuse to them. Finally the eyeball itself is made of a tough white collagen tissue called the sclera. This is the "white of the eye". The sclera maintains an optimal shape of the eye and protects the sensitive internal tissues from damage.

This is a schematic cross-section of the retina and underlying tissues. Light passes from top to bottom, through the retina to the rods and cones. The retinal pigment epithelium rests on Bruch's membrane, under which is the choriocapillaris. All these structures are intimately related in the normal eye but are subject to numerous disease processes. Commonly, fluid or blood can collect within or between layers causing dysfunction. Abnormal cells can grow into the layers. Even new blood vessels can appear either under the rods and cones or under the retinal pigment epithelium. Also, all of the normal structures can be affected by genetic mistakes causing slow degeneration over time.