Your Guide to How the Eye Sees
Travel inside the eyes -- our window to the world -- and learn how they allow us to see objects both near and far.
In order to see, there must be light. Light reflects off an object and -- if one is looking at the object -- enters the eye.
The first thing light touches when entering the eye is a thin veil of tears that coats the front of the eye. Behind this lubricating moisture is the front window of the eye, called the cornea. This clear covering helps to focus the light.
On the other side of the cornea is more moisture. This clear, watery fluid is the aqueous humor. It circulates throughout the front part of the eye and keeps a constant pressure within the eye.
After light passes through the aqueous humor, it passes through the pupil. This is the central circular opening in the colored part of the eye -- also called the iris. Depending on how much light there is, the iris may contract or dilate, limiting or increasing the amount of light that gets deeper into the eye. The light then goes through the lens. Just like the lens of a camera, the lens of the eye focuses the light. The lens changes shape to focus on light reflecting from near or distant objects.
This focused light now beams through the center of the eye. Again the light is bathed in moisture, this time in a clear jelly known as the vitreous. Surrounding the vitreous is the retina.
Light reaches its final destination in the photo receptors of the retina. The retina is the inner lining of the back of the eye. It's like a movie screen or the film of a camera. The focused light is projected onto its flat, smooth surface. However, unlike a movie screen, the retina has many working parts:
- Blood vessels. Blood vessels within the retina bring nutrients to the retina's nerve cells.
- The macula. This is the bull's-eye at the center of the retina. The dead center of this bull's eye is called the fovea. Because it's at the focal point of the eye, it has more specialized light sensitive nerve endings, called photoreceptors, than any other part of the retina.
- Photoreceptors. There are two kinds of photoreceptors: rods and cones. These specialized nerve endings convert the light into electro-chemical signals.
- Retinal pigment epithelium. Beneath the photoreceptors is a layer of dark tissue known as the retinal pigment epithelium, or RPE. These important cells absorb excess light so that the photoreceptors can give a clearer signal. They also move nutrients to (and waste from) the photoreceptors to the choroid. Bruch's membrane separates the choroid from the RPE.
- The choroid. This layer lies behind the retina and is made up of many fine blood vessels that supply nutrition to the retina and the retinal pigment epithelium.
- Sclera. Normally light does not get as far as this layer. It is the tough, fibrous, white outside wall of the eye connected to the clear cornea in front. It protects the delicate structures inside the eye.
Signals sent from the photoreceptors travel along nerve fibers to a nerve bundle which exits the back of the eye The bundle is called the optic nerve. The optic nerve sends the signals to the visual center in the back of the brain.
Now light, reflected from an object, has entered the eye, been focused, converted into electro-chemical signals, delivered to the brain and interpreted or "seen" as an image.