The Human Eye Part 1:
By William E. Steinman:
September 19, 2005:

In this work, it may be that I will tell you more about the human eye than you want to know. As a man who is facing the probability of blindness, there is not much I don't want to know. I must discover all I can because I have no idea where the clues to healing will be. The human eye is a very complex organic mechanism that yields its secrets reluctantly. Our ophthalmologists have learned much in recent years, but I fear there is still much to learn.

Let us consider the eye. If there is a God, he is some kind of a heck of a super engineer to have invented and developed the human eye. It's enough to make video engineers weep with jealous rage. In fact, the eye can be thought of as the very first video camera. It does function in much the same way that a video camera functions. However, the human eye evolved long before 1923, which is when Vladimir Zworykin invented the world's first iconoscope. This iconoscope became the first video camera pickup device which was used to produce the early black and white television signals. I know you don't remember, but I do! In fact I built a black and white televison receiver as my final exam when I attended Electronics Institute in Detroit in the good old Milton Berle days.

Like any other camera the human eye has a lens in the front which focuses light rays onto the retina. The retina is a very delicate, multilayered, light-sensitive membrane which lines the inner eyeball. It is very much like the light sensitive receptors on the image plane in a modern digital camera or televison camera. In modern cameras the light sensitive receptors are solid-state semiconductors called charged-coupled devices (CCD). When light from an image is focused on a CCD, a pattern of electrical charges, which vary in proportion to the amount of light, is created. These charges are then transmitted to an amplifier. Our eye works in pretty much the same way. The receptors in the camera are connected to the camera's electronics (brain) through wires. The light sensitive membrane of the eye is connected to the human brain through optic nerves. We can notice that the eye will have an off center blind spot where these optic nerve bundles pass out of the eye.

Also like the camera the human eye has a diaphragm which controls the amount of light which is admitted to the inside of the eye. In a camera this is usually a disk with a variable sized orifice. In the eye, it is a muscle that controls the size of the pupil. The pupil is that black hole in the center of the iris that we see when we gaze into our true love's eye. We also have a variable focusing mechanism in the eye. This is achieved by a muscular distortion of the lens so as to change it's focal length. One of the problems with cataract surgery is it replaces this variable focal length lens with a fixed focal length plastic lens. That happened to me and it does limit my visual control. Recently there has been some progress with this. Doctors are starting to implant a new variable focal length plastic lens as an alternative. It's too late for me, but good for many others.

Now, this human eyeball has a lot of built in protection, even more that a good camera. While the camera must keep out coincident light in order to work, the eye also keeps out air to protect its very sensitive inner workings. At the perimeter of defense are the eyelids with their lashes which are designed to keep out debris and usually do that well. Around the eyeball itself we have a layer called sclera. We see it as the white of the eye of which we are not supposed to shoot until we see it. This is a tough layer which covers and protects everything except the cornea at the front of the eye. The cornea, of course, cannot be covered as it is the window that we see through. Now under the sclera layer we have the choroid layer which is composed of pigment and blood vessels. This pigment is what we see as the eye color where it appears in the iris. Near the front of the eye this choroid layer thickens out to form the ciliary muscle. This is the muscle which actually changes the shape of the lens for focusing. This muscle also merges with the iris that controls the pupil size.
A sketch will be useful here.

The eye actually has three liquid filled chambers, not just one. There is an outer layer in front of the lens called the anterior chamber. Anterior means, of course, in front. This chamber, between the lens and the cornea is filled with a clear, watery fluid called aqueous humor. The lens itself constitutes a chamber that is filled with something called a crystaline humor and the full true name of the lens is crystaline lens. Finally, the larger chamber between the lens and the retina, called, of course, the posterior chamber, is filled with a clear gelatinous substance called vitreous humor.

On the inside of all of this protection and control stuff is the retina which lines the inside of the eye. This is the actual light sensitive surface that responds to light and sends images to the brain. The retina is a complex tissue consisting of several layers, only one of which contains light-sensitive cells. Actually light must pass through the covering layers to reach this layer. The light-sensitive cells are of two types, rods and cones, which are identified by their unique shapes and by their sensitivity to different kinds of light.

Rods are most sensitive to reduced light intensities, hence they provide night vision and aid in visual orientation. Cones are more prominent in the human eye, and provide detailed vision, as for reading and color perception. In general, the more cones per unit area of retina, the finer the detail that can be discriminated by that area. Rods are fairly well distributed over the entire retina, but cones tend to concentrate in two sites.

The fovea centralis is a pit at the rear of the retina, which contains no rods and has the densest concentration of cones in the eye, The surrounding area called macula lutea, is a circular patch of yellow-pigmented tissue about one centimeter in diameter. All of this is a minute area near the center of the retina of the eye at which visual perception is most acute. It controls our central vision which allows us to see fine detail. Hence we can read and write and notice nice things like what our true love looks like. When that part of the eye goes to pot, we end up with a condition called macular degeneration. That is where a dark spot appears in the center of our vision blocking out the area of focus. As this disease, or whatever it is, progresses the dark spot grows and eventually we become legally and functionally blind. I am not there yet, but I seem to be approaching that condition.

I will have much more to say about the functional details of the human eye and macular degeneration in a follow up essay.