The eyes are nearly indispensable organs, used more than almost any other; although you may have admired them in the mirror, few people give much thought to how intricate and complex they are.
The eye is a sphere, although not a perfect one – each is slightly asymmetrical. They measure about one inch in diameter and are located in the orbit, the round hollows in the skull between the forehead and cheekbone. The orbit is cushioned by fat and a network of muscles, nerves, and blood vessels that connect to the eyeball and hold it in place. The gland that produces tears is also located here, providing lubrication and helping to flush out debris; the eyelids and eyelashes also serve this protective purpose.
The first thing you notice about someone’s eyes are the iris, the colored disc in the front. The iris gets its color from the amount of pigment you have, which is determined by genes. More than being decorative, the iris acts as a sort of shade, regulating the amount of light that enters the eye: it opens the pupil to allow more light in when it’s dark, and closes it to shut out excess light. The pupil is the dark center.
The cornea is the transparent disc that covers the front of the eye; it focuses light and directs it into the eye, affecting the clarity of the images we get. This focus can be changed with surgery, by altering the shape of the cornea itself.
The lens is a transparent disc located behind the iris, focusing light onto the retina, the lining on the back of the eye covered in light-sensing cells that translates the amount of light into electrical signals. It then sends these signals through the optic nerve (which is actually made up of more than a million nerve fibers) to the brain, which produces images. We actually receive images upside down, but the brain translates and flips them. The lens deteriorates over time, leading to weakened eyesight and conditions like cataracts, but it can be surgically replaced.