Which part of your body lets you read the back of a cereal box, check out a rainbow, and see a softball heading your way? Which part lets you cry when you're sad and makes tears to protect itself? Which part has muscles that adjust to let you focus on things that are close up or far away?
If you guessed the eye, you're right! Your eyes are at work from the moment you wake up to the moment you close them to go to sleep. They take in tons of information about the world around you — shapes, colors, movements, and more. Then they send the information to your brain for processing so the brain knows what's going on outside of your body. You can check out different parts of the eye by looking at your own eye in the mirror or by looking at but not touching a friend's eye.
Some of the eye's parts are easy to see, so most friends will say OK. Most friends won't say OK if you ask to see their liver! The eye is about as big as a ping-pong ball and sits in a little hollow area the eye socket in the skull. The eyelid protects the front part of the eye. The lid helps keep the eye clean and moist by opening and shutting several times a minute. This is called blinking , and it's both a voluntary and involuntary action, meaning you can blink whenever you want to, but it also happens without you even thinking about it.
The eyelid also has great reflexes , which are automatic body responses, that protect the eye. When you step into bright light, for example, the eyelids squeeze together tightly to protect your eyes until they can adjust to the light. And if you flutter your fingers close but not too close!
Your friend's eyelids shut automatically to protect the eye from possible danger. And speaking of fluttering, don't forget eyelashes. They work with the eyelids to keep dirt and other unwanted stuff out of your eyes.
The sclera is made of a tough material and has the important job of covering most of the eyeball. Think of the sclera as your eyeball's outer coat. Look very closely at the white of the eye, and you'll see lines that look like tiny pink threads.
These are blood vessels, the tiny tubes that deliver blood, to the sclera. The cornea say: KOR-nee-uh , a transparent dome, sits in front of the colored part of the eye.
The cornea helps the eye focus as light makes its way through. It is a very important part of the eye, but you can hardly see it because it's made of clear tissue. Like clear glass, the cornea gives your eye a clear window to view the world through.
The axons of all those ganglion cells weave together to create the thick, ropey optic nerve—your second cranial nerve—which leaves the back of your eyeball and carries those impulses up to the thalamus and then on to the brain's visual cortex. So that's the basic anatomy and event sequencing of human vision, but what I really want to talk about are those two types of photoreceptors—your rods and your cones. Cones sit near the retina's centre, and detect fine detail and colour. They can be divided into red, green and blue sensitive types, based on how they respond to different types of light.
But they're not very sensitive, and they really only hit their activation thresholds in bright conditions. Rods, on the other hand, are more numerous, and more light-sensitive.
But they can't pick up real colour. Instead, they only register a greyscale of black and white. They hang out around the edges of your retinas and rule your peripheral vision. Since these receptors function so differently, you might not be surprised to learn that your rods and your cones are also wired to your retinas in different ways.
As many as different rods may connect to a single ganglion cell—but because they all send their information to the ganglion at once, the brain can't tell which individual rods have been activated, and that's why they're not very good at providing detailed images.
All they can really do is give you information about objects' general shape, or whether it's light or dark. Each cone, by contrast, gets its own personal ganglion cell to hook up with, which allows for very detailed colour vision, at least if conditions are bright enough.
And all this brings us back to that weird flag. Why does staring at this flag and then looking at an empty white space make us see a phantom flag of different colours? Well, it begins with the fact that our photoreceptors can make us see afterimages.
Some stimuli, like really brilliant colours or really bright lights, are so strong that our photoreceptors will continue firing action potentials even after we close our eyes or look away. The other part of the illusion has to do with another bug in our visual programming If you stare long enough at a brightly coloured image, your cones will receive the same stimulus for too long, and basically stop responding.
In the case of the flag, you looked at an image with bright turquoise stripes. Because your retinas contain green, red and blue-sensitive cones, the blue and green ones got tired after a while, leaving only the red ones left to fire. Then, you looked at a white screen. That white light included all of the colours and wavelengths of visible light.
Within the retina is the macula the size of a pinpoint, vitreous body and blood vessels. The optic nerve protrudes from the back of the eyeball and consist of blood vessels. When the light enters the eye, it is focused to a pinpoint on the macula, a small area in the centre of the retina at the back of the eye. The macula is responsible for central detailed vision, allowing you to see fine detail and colour, read and recognise faces.
When light stimulates the nerve cells in the retina, messages are sent along the optic nerve to the brain. The optic nerves from the two eyes join inside the brain. Behind the lens, light meets the vitreous body of your eye—a transparent, squishy gel that gives your eye its round shape. This gel is clear and enables light to easily pass through it, reaching the retina at the back of your eye. The retina contains millions of nerve cells that are experts at sensing light.
You might have heard these cells referred to as rods and cones before—they come in two different shapes. Cones give us the sharp images and vivid colors we see in bright light, while rods provide us with peripheral and night vision. This part of the brain knows how to turn those electrical impulses from the retina into an image that makes sense to you.
As you can see, this process of decoding light into sight is fairly complicated, involving many steps and delicate parts that have to work in sync. However, it all happens so fast that, to us, it seems instantaneous.
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