HOW NORMAL VISION WORKS

Outer eye:

The light reflected from objects travels through the air. What we see are the light rays which strike the retina in the back of our eyes. In order for the vision process to produce a good quality image for the brain to interpret, a lot of things have to function properly to correctly focus the light rays onto the retina. The process is called refraction.

First, the light goes through the conjunctiva, which is a thin membrane that covers the cornea. The main purpose of the conjunctiva is to provide a seal which keeps debris from entering the eye.

Anterior Chamber:

The next group of parts make up the anterior chamber, or the frontmost structures in the eye. Right behind the conjunctiva is the cornea, which is the first stage of refraction in the eye. The normally smooth hemispherical shaped cornea is responsible for starting the process of refraction by directing light rays toward the lens so that the lens can more easily focus them on the retina. The cornea has more refractive ability than any other part of the eye.

Right behind the cornea is aqueous humor fluid. This is a fluid that nourishes the cornea and helps to protect the iris, pupil, and lens. The fluid body is constantly being recirculated with entire replacement taking about four hours. It is delivered behind the iris, flows through the pupil, and leaves between the iris and cornea.

After light passes through the aqueous humor, it goes through the pupil to the lens. The purpose of the pupil is to control the amount of light which is allowed to strike the lens just behind it. The size of the pupil is controlled by the iris which detects intensity of incoming light and adjusts the size of the pupil accordingly. The iris contracts the pupil to limit intense light and expands the pupil to allow maximum light entry in darker instances. The iris is a sphincter (ring-shaped) muscle.

The iris is visible as the circle of color in the eye, while the pupil is the black circle in the center of it. The color of the iris depends upon the pigment deposits within it combined with the purple to black surface of the rear of the iris. The pigment deposits range from yellow to reddish brown and can sometimes include some white. When there is little pigment, the eye color appears as bluish.

The lens refracts the light rays that pass through it so that the light is focused on the retina at the back of the eye. When viewing distant objects, the lens is almost flat. While viewing close objects, the lens becomes more curved as the distance range becomes narrowed.

Posterior Chamber:

The adjusted light travels from the lens through the vitreous fluid to the back of the eye. The vitreous fluid is the gel like material which gives the eye it's sphere shape and holds the retina in place.

Lining the back of the eye is the retina, which is the term for the collective group of rod and cone cells. At the center of the retina is the macula which is used for more reliable vision in bright light. The macula only contains cone cells. Due to the natural flow of light through the eye, the image as it strikes the retina is a reversed copy of the actual image being viewed.

The rod and cone cells contain chemicals which are activated by light. These cells convert the light which strike them into electrical impulses, which will be transmitted to the brain for processing. It is important to note that Vitamin A is used by these cells to produce the chemical conversion. If a person has Vitamin A deficiency, their vision in darker situations may be compromised.

The rod and cone cells get their names due to their shapes. The rod cells measure about 1/400th of an inch in length and are about 1/100th of an inch thick. There are about 120 million rod cells in each eye. These cells are responsible for the black and white part of the image production and work with dim light.

The cone cells are shorter and thicker than rod cells. There are about 7 million cells in each eye, which are responsible for the color portion of the image creation. Cone cells work with brighter light than the rod cells do.

Beyond the eyeball:

Once the light has been converted to a stream of electrical impulses, the information flows from the eye to the brain via the optic nerve. Because the opening of the optic nerve is located within the retinal area, a tiny blind spot naturally occurs. This blind spot is not normally noticeable within the image.

Because the sharpest vision occurs at the macula, the brain is constantly adjusting the eyeball to have the object of attention to be in the center of the image.

All during the time our eyes are open, there is a steady stream of light flowing in and being processed. The eye is also constantly being positioned for target and the lens adjusted for focus. It is simply overwhelming to try comprehending the amount of work our eyes do to provide us with vision!

Other parts of the eye:

We have covered the parts of the eye that are directly involved in the flow of light and conversion to electrical impulses. Now we will touch upon a few other parts which deal with maintenance and positioning of the eyes. These also play a huge role in the overall health of the eye and therefore the quality of the image you perceive. The names may be helpful in case s doctor mentions one of them in a discussion.

The retinal arteries are responsible for blood flow to and from the eye. They enter the eye through the optic nerve and terminate within the vitreous body.

The sclera is basically the casing of the eyeball. It is made up of tough white tissue and contains tiny blood vessels. The white part of the eye surrounding the colored iris is part of the sclera. This whiteness can appear to be bloodshot if the blood vessels within become expanded due to eye irritations (caused by fatigue, dust particles, some medications or drugs, and alcohol).

The outside of the eye is cleaned with a fluid produced at the lacrimal gland, which is located just above the outer corner of the eye. The fluid is distributed over the surface of the eye during blinking, helping to wash dust and particles away. The eye has a glossy appearance when the fluid is washed over the conjunctiva. The tear fluid exits the eye at the lacrimal sac, located in the lower corner of the eye nearest the nose. It drains through the nasolacrimal duct, in the nose.

The eyelids are comprised of skin which covers the eye while it is closed. They help to protect the eye during sleep or if the brain instructs them to close if it senses objects are too near to the eyes. They also help to distribute the tear fluid during blinking, which is a rapid closure and reopening of the eyelids.

The eyelashes are the short hair groups on each eyelid. They help protect the eyes from particles and dust. These only live for a few months, but new lashes are constantly replacing old ones. There are about 200 lashes on each eye.

Eye muscles:

The eyes are positioned for best viewing by a group of muscles that move in unison in both eyes. Positioning the eye itself is more of a fine tuning operation because more often we turn our entire head in the direction of the object(s) which we want to view. However, the job of eye positioning is still very important since making the constant slight adjustments with the eye saves a lot of wear and tear on neck muscles.

The rectus inferior moves the eye downward, the rectus lateralis moves the eye to the side, and the rectus superior moves the eye upward. Also, the superior oblique moves the eye sideways with some slight rotation as does the inferior oblique muscle. The eyelids are controlled by muscles also, with the levator palpebrae causing the lid to open and the orbicularis palpebrarum which closes the lid.

That wraps up the basics of how the eyes work. There is a lot more detail that isn't covered here though. Hopefully you have learned something in the text above and it will help build a foundation upon which you can further the quest for information.