Basic Anatomy of the Eye
This is a basic description of the anatomy of the human eye. It is written by a teacher, not by a doctor. I hope that more expreienced people will be patient with the simple descriptions, and with any inaccuracies which I written.
Click here to jump down to the image of the eye.
When light enters the eye, it passes first through the cornea. This is shaped like slice of a sphere, and is clear and solid. It bends the light about seventy percent of the amount needed for good vision. It now can be transplanted from one person to another, as it contains no blood vessels.
Light next passes through the pupil. This is simply a name for a hole. The size of the hole (pupil) is controlled by the iris. This is like a camera shutter. It is attached to the eye's outer wall by muscles and ligaments. It constricts (makes a smaller hole) and dilates (makes a larger hole) automatically in response to the amount of light in the environment. A human cannot voluntarily control the adjustments of the iris, like some birds can. The iris is colored, and thus we describe people by their eye color: blue, brown, hazel. If the iris is missing, or stays dilated, then too much light may get into the eyes and make seeing more difficult.
The lens is next and provides the remaining thirty percent of the light bending needed for good vision. It is shaped like a lens (surprise!) and is also attached with muscles and ligaments. When the muscles contract, the lens gets a wider diameter and become thinner.
When the muscles relax, the lens gets a smaller diameter and becomes thicker. With age, the lens becomes less flexible, so that the muscles are unable to adjust the lens to view things up close. (Thus, most adults require reading glasses somewhere in middle age.) If the lens becomes cloudy (called a "cataract"), it can now be removed and replaced with a plastic one. The plastic one is not flexible like the original, but it does mean that the prescription lenses needed are much smaller, often just reading glasses, rather than the "coke bottle glasses" of past generations.
Vitreous humor (a clear, jelly-like substance) fills the cavity between the cornea and the lens, passing through the pupil, and filling the whole rear of the eye. This substance gives the eye its ball shape and helps keep the retina intact. The body is continuously making more vitreous humor, and the old leaks out through ducts to the outside of the eye, where it follows the path of tears (drying out into the air, running out through the lacrimal duct into the nose, or dripping down the face). Glaucoma is a condition which blocks the exit ducts, so that the pressure builds up inside the eye. If the vitreous humor becomes cloudy due to internal bleeding, it can now be removed and replaced with an artificial fluid.
The retina coats most of the inside of the eye. It is a very delicate structure, composed of many thin layers. (Here's an analogy: if the eye were a basketball, the retina would be like one thickness of Saran wrap along the inside.) It houses blood vessels, rods, and cones. The optic nerve also starts here. The light entering the eye ends up here, as on a camera's film.
The center of the retina is the macula. This region is responsible for most of the eye's ability to distinguish small details and color. The fovea is the most sensitive part of the macula, and it is here that the image entering the perfect eye "lands".
The rest of the retina is good at distinguishing movement.
Rods are cells which help the brain interpret images seen in low light. There are a few in the macula, but most are located around the periphery of the retina.
Cones are cells which distinguish color. They are located only in the macula. They do not function well in environments with low lighting.
The optic nerve goes from the eye to the visual center of the brain (in the occipital cortex, which is at the center in the back of the head, about where the spine enters the skull). It doesn't go straight there. A short way from the eyes, each optic nerve splits in half. The parts that carry the information from the left halves of the two retinas join up, as do the parts that carry the information from the right halves. The two new nerves then go to the brain, to the right and left sides of the occipital cortex, respectively. The result of this is that if someone has trauma in the left side of her head, she doesn't lose the vision in the right eye (even though she may lose muscle control on the right side of her body). She loses the vision in the right half of each eye.
Here is a graphic which shows a cross section of the human eye, with some parts labeled. I borrowed it from the web site Understanding Vision. (I didn't ask first.) Click here to visit their site.