We have several different ways of looking at the human eye. We can consider it to be one single strong plus lens, or we can consider it to be an optical system comprised of a series of refracting surfaces.

In the early 1900's a Swedish Ophthalmologist, Alvar Gullstrand, described the eye globe using representative values for the curvatures and indices of refraction for the various parts of the eye. The result is what is considered the average eye, or the schematic eye. For now we are going to look at the major parts of the eye. When you take the Anatomy and Physiology of the Eye course you will learn these parts in great detail.

The eye globe itself is approximately spherical, with an almost spherical protrusion at the front. This front part, which is clear, comprises about 1/6th of the surface of the eye. The length of the eye, from the front to the back of the eye, is approximately 24mm (just under 1 inch) long. From top to bottom, or from side to side, the globe is slightly less -- about 23 mm.

We will follow the path of a ray of light as it enters the eye.

Starting from the front, the clear part is the cornea. The cornea contains a lot of nerves, so it is extremely sensitive. It is avascular, which means that there are no blood vessels in it. It is made of living cells, which need regular nourishment. Since blood vessels are how the human body normally nourishes its cells, the lack of vessels in the cornea means it must receive oxygen and nutrients some other way. The very thin tear film which covers the front of the eye is one of these sources of oxygen and nutrients.

Once the ray of light enters the eye through the clear cornea, it next encounters the aqueous. Does that sound like a word for water to you? The aqueous is indeed a watery liquid containing a variety of chemicals. It is the second source of nutrients for the cornea, and it is also clear. It is constantly being created by the ciliary body (which we will discuss in a moment) and it is constantly draining into an area between the iris and the cornea. When something goes wrong with this plumbing system the eye develops the condition glaucoma, one symptom of which is an increase in pressure in the eye.

After travelling through the aqueous the ray of light next encounters the crystalline lens, or just the lens. This is a bi-convex lens, also made of cells. This lens is actually capable of changing shape. In its relaxed state it has 17-20 diopters of power. (Different text books give different values for the dioptric value of the crystalline lens at rest.) The front surface of this lens can bulge out, adding a few more diopters. We call the changing of the shape of the lens accommodation. Very young children's lenses are capable of adding 10 or more diopters. As we age the center of the lens slowly becomes denser, and the amount of change that it is capable of decreases. By the age of 40, the normal eye can only add about 2.50 diopters of power. By the age of 70 the lens is no longer able to add extra diopters at all. This is a perfectly normal progression, does not change the ability of the ray of light to pass through the eye, and is not something that can be changed by diet, exposure to radiation, meditation, or exercise. From the moment that a baby is born, the lens is constantly adding cells to its outer layer, and pushing the existing cells toward the center. So, although the infant's eye has the consistency of Jell-O, as the child ages to an adult the new cells are pushing the old cells toward the center of the lens. The old cells have nowhere to go - they just pack in tighter and tighter. The cells that were in your lens when you were born are still there - in the center of your lens.

After passing through the lens the ray enters the vitreous. This is a transparent jelly that was present when you were born and is neither renewed or removed (except surgically).

Finally, the ray meets the retina, a thin layer of nerve cells that react to particular frequencies of the electro-magnetic spectrum. Unlike a photographic film, this sensitivity is 'renewable': about every 1/16th of a second the nerve cells in the retina can re-react to the light reaching them.

At the back of the eye is an area called the macula, the center of which is the fovea. This is a specialized part of the retina, and is where we get our best, central, in-focus vision.

Also at the back of the eye is an area called the optic disc or optic nerve head or blind spot. This is where the electrical impulses that are the result of the absorption of rays of light by the special nerve cells in the retina leave to enter the brain. These electrical impulses travel on the optic nerve into the brain and back to the visual cortex at the very back of the brain. The result of all of this is the processing of information in the brain that we interpret as sight.

It can be argued that we do not 'see' with our eyes -- we 'see' with our brains!. The eye is just the series of refracting surfaces and light-sensitive nerve cells that result in informational input to the visual cortex at the back of the brain.

That was the path of a ray of light that actually results in sight. Some rays will not enter the cornea - they will fall on the 'white' of the eye, also called the sclera This is a tough fibrous coating of the eye that gives it its shape. The junction between the sclera and the cornea is called the limbus. When you look at a person's eye from the front the limbus is an invisible circle where the 'colored' part meets the 'white' part. The blood vessels that nourish the rest of the eye globe stop at the limbus, and do not enter the cornea. The limbus is critical in contact lens fitting, and is used as a landmark in many eye surgeries.

Some rays will also enter the cornea, traverse the aqueous, and then fall on the iris. The iris is a round (sphincter) muscle that is capable of opening and closing, thus controlling the amount of light that gets past it to enter the back of the eye. The opening at the center of the iris is the pupil. When the iris relaxes (dilates) the pupil is larger and allows more light to reach the retina. When the iris constricts the pupil becomes smaller and allows less light to reach the retina. This iris is what we call the colored part of the eye - it is what is brown or blue or hazel or green. The pupil is what appears black to us. It is not really black. The light that goes into the pupil does not come back out again, so we do not see color or lightness when we look at it.

Not every ray of light that enters the pupil will result in sight. Rays that reach the retina but not in the exact position of a photosensitive cell will be absorbed by other cells. If this did not happen these extra rays would reflect diffusely and cause a blurring of vision.

The choroid is the layer between the retina and the sclera, and it is comprised mostly of blood vessels.

Near the front of the eye globe the ciliary muscle is another round or sphincter muscle that controls the crystalline lens, giving it the ability to change shape. The aqueous is secreted at the front of this muscle.

That is about as simplistic a description of what the eye globe is as I know how to give! For this course you will need to be able to identify each of these structures by name on a diagram similar to this one.

On to today's promised subject: refractive errors, or 'why we all have jobs."!