As with everything else in the human body, the human eye is astounding. Instead of being a camera, the eye registers every new image, one immediately after the other (about 20 per second). In addition, because there are two eyes, in-depth (binocular) vision is accomplished. Because the eyeballs can move separately, and the lens can change thickness, you can see things both closer and at a distance. The eye lies in a circular cavity within several bones which, before birth, fused together. Each eye has optical equipment, muscles, conjunctiva, tear apparatus, and eyelids. Here is a very simplified description of the many wonders in the eye:
Eyeball: As you look at the eye, the white of the eye is the sclera. Inside that is the colored part, or iris. Inside that is the black spot, the pupil. Light passes into the eyeball through the pupil, which can enlarge (dilate) or shrink (contract) in size. (A camera has a similar mechanism, which is called the diaphragm or “stop”).
Behind the pupil, the light travels through the lens, double convex in shape. Behind that is a clear fluid throughout the middle of the eyeball (vitreous humor). At the back of the eyeball, the light strikes the retina, which contains nerve fibers of theoptic nerve and the nerve cells sensitive to light. Two types of cells are there: rods and cones. There are 100 million rods in each retina, which can see things as light and dark (black and white), even in very dim light. There are less cones; they see color, but only in brighter light. This is why, at night, you only see objects as dark and light, without any color to them.
The lens bends thicker or thinner in order to focus the light into a sharp image. This focusing is called accommodation. The light image is then carried to the cells and nerves in the retina and is sent through the optic nerve, to the sight center in the brain.
Eye muscles: Smooth muscles in the eye control the size of the pupil and focusing action of the lens. Three pair of eye muscles outside the eyeball move the eyeball. Another muscle holds the eyelid open (or relaxes and lets it shut).
Tear glands: The lacrimal (tear) glands are above the eyeball. They secrete tears which keep the eyeball moist. They drain into the conjunctival sacs and then drain into the inner corners of the eyelids. From there, they drain through small ducts into the nose. When they flow rapidly, tears run down the cheeks.
Eye problems: When a lens becomes cloudy, the condition is called cataract. If the eyeball is too short, the light rays focus behind the retina instead of on it. This causes far-sightedness (hyperopia). Corrective convex lenses are needed. In near-sightedness (myopia), the eyeball is too long or the lens too strong, and the light rays are focused in front of the retina. Corrective concave lenses are needed. Presbyteria is lack of lens elasticity, which tends to occur in old age. If the eye has a defective shape, the rays do not exactly focus on a point on the retina. This is astigmatism. Corrective lenses must be slightly cylindrical. Cross eyes converge excessively. Wall eyes diverge too much.