Ask Dr. K: Describing sight is not the same as knowing how it works

The eye is like a camera. It has a lens that continuously focuses to sharpen the picture. Then the eye sends the picture to the brain, which processes the picture and does the seeing.

Let’s start with some eye anatomy. (I’ve put a corresponding illustration on my website, AskDoctorK.com.) The eye’s surface is made up of the sclera (the white part of the eye that protects its interior) and the cornea. The cornea is a clear, dome-like window at the front of the eye that helps focus light.

The middle layer contains the iris. The black hole in the center of the iris is the pupil. By changing the size of the pupil, the iris controls how much light enters the eye (more in dim light, less in bright light). The iris has the color that defines you as blue-eyed or brown-eyed.

Just behind the pupil and iris lies the lens. The flexible lens alters its shape, allowing the eye to focus on objects at varying distances. The shape the lens takes when you are looking at a sunset is very different from the shape it takes when you read a book.

The lens focuses light rays on the retina, the light-sensitive layer at the rear of the eye. The macula is a small part of the retina that gives us sharp central vision. Within the retina are millions of specialized cells: Rod cells perceive changes in light and dark, while cone cells perceive color.

So how do all of these parts work together to help us see? Consider what happens when you walk through a parking lot and spot your car. What you are actually seeing is the light reflected off the car.

The light thrown off the surfaces of your car hits your cornea, where it is bent inward and passed through to the lens. The light rays bend further and get projected onto the retina.

The retina absorbs the light and turns it into electrical energy. Those electrical signals travel along a nerve — the optic nerve — through your brain to the very back part of it, called the occipital lobe. When those electrical signals reach the back of the brain, the brain interprets the image — the size, shape, color and distance of your car. Other parts of the brain also are involved in interpreting the images from your eyes.

We understand what parts of the eye and brain are involved in seeing. But we only understand dimly how it really works. How does your brain know where a tennis ball speeding toward you will be one second from now so that you can position your racquet to hit it? The answer to that and similar questions remains a marvelous mystery.

Dr. Komaroff is a physician and professor at Harvard Medical School. To send questions, go to AskDoctorK.com, or write: Ask Doctor K, 10 Shattuck St., Second Floor, Boston, MA 02115.