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If you look at the evolutionary anatomy of eyes, you’ll notice some major differences between humans and animals. For one, humans have their eyes set closer together than many other animals, which gives us wonderful depth perception. In exchange, we lose some of the great peripheral vision that protects cows. In a way, that makes us a species with blinders on—because we have binocular vision. The other interesting evolutionary element about our eyes is that little reddish-pink fleshy substance on the inside corner of your eye (it’s called a caruncle, and this is an acceptable word in Scrabble). That’s actually a remnant of the reptilian eye. Because reptiles have to see in fresh water (which can be irritating when it interacts with the salt in our bodies), a clear lid covers their eyes. Since humans don’t need that flap to live in air, we lost it through evolution, and it became the remnant that we have today.
HOW THE EYE WORKS First, let’s dissect your eyeball’s anatomy. To give them their squishy shape, your eyeballs contain a lot of fluid. Without fluid, your eyeballs would collapse—almost like a beach ball without air. The fluid constantly circulates in and out of your eye and is filtered through a meshlike covering, like a window screen. Behind your eyeball, there’s a lot of fat; that’s what pushes your eyes forward.

To see, your eye essentially takes information from outside sources and passes it along to your brain. Information travels through the cornea, the clear covering of the eye, to the iris, which is the colored part of the eye charged with regulating the light that can hit the retina. Behind the iris sits the lens, which is shaped like a camera lens. This remarkable setup not only changes shape to focus light, but filters out some parts of the light spectrum that may be harmful to the eye. The cornea and lens focus light to form an upside-down image on the back of the surface of the eye—your retina. Once it reaches the retina, it’s then sent through the optic nerve and rotated another 180 degrees so your brain can determine what you’re seeing right side up.

That’s actually only one of the ways that animals refract light to see. Flies, for instance, have multiple solar plates on their corneas, and each one of them focuses light. That gives them a very fragmented view of the world; they don’t see anything well, but they see everything panoramically. And that makes sense; flies don’t need to focus on calculators and classified ads; they just need to see well enough to avoid a flyswatter or horse’s tail. Spiders see like we do, but their systems work a little differently. They move their retinas to focus; if they want to see farther away, they push their retina closer, while the lens stays fixed. One of the key differences in animals is that hunters have binocular vision to focus on prey, while the hunted have peripheral vision to better locate potential predators.

What Can Go Wrong

Lots of things can make windows ineffective—they can break, they can fog up, they can be painted shut, they can come out on the losing end of a collision with a baseball. With your eyes, lots of things can tinker with the process of vision, too— some will fog them up and some will cause an all-out break. No matter what the cause of the problem, we usually think of the primary long-term effect of optical problems: blindness. Though Jose Feliciano shows his appreciation for his other senses, many of us still fear that losing sight would mean that you’d no longer be able to view the beauty in birds and beaches, in family and friends, in art and sports. So while you can certainly live without one of your senses, we also know that all of us would prefer to preserve our eyesight. As you age, these are the major things that can contribute to the decline in vision.

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