Top 10 Facts About Eyes
Of all our senses sight is the most technically complex and arguably the most useful for our survival and enjoyment of the world in which we live. We can scan our immediate environment in a matter of seconds looking for danger, for food or for a potential mate. We can find high ground or climb a tree and see for miles around us to get a better understanding of what threats or opportunities are nearby.
Eyes also seem to impart a lot about us and our character. We can often read a persons mood through their eyes and when we fall in love, eye contact with the other person is amazing. We can watch a sunrise or paint or interpret the expressions of people talking to us to give the conversation another layer of context. Or we can close our eyes and find rest from the hustle and bustle.
How our eyes developed through the process of evolution would be an interesting topic to delve into, but we wanted to get to know the physical make up of the eye a bit better first. There really are some amazing facts about our eyes and that comes as no surprise given what they do and what they mean to us.
10. The Ciliary Body Is The Unsung Hero Of The Eye
Other parts of the eye get all the limelight and invitations to swish parties. Fair enough to; the lens, retina and cornea are pretty special bits of equipment. But the ciliary body should not be forgotten. In some ways it’s like the conductor of an orchestra – or perhaps the conductors baton.
The ciliary body surrounds the lens. It’s the structure that has the ligaments and muscles that manipulate the lens. When we look at something far away and need the lens to flatten out, the ciliary body does the flattening and visa versa when we are looking at something close up. In fact the ciliary body is constantly adjusting the shape of the lens to suit our focusing needs.
Without it we’d be a bit like owls (who can’t more their eyes). We’d have to move our whole heads to move our eyes.
Wow, this guy really explains the ciliary body well.
9. Having Two Eyes Helps Us See In 3D
Seeing things in 3D is incredibly useful when we are working with tools, building things, or trying to judge distances (spear tonights dinner for example). There are three main attributes of our sight that our brain uses to enable us to see things in 3 dimensions.
Having our two eyes side-by-side and pointing in the same direction enables ‘binocular disparity’, which simply means that each eye sees slightly different images of the same thing. If you hold your finger or an object in front of your face and close one eye then the other, you will notice the object moving. In fact the object is staying still, it’s just that each eye sees the object from a different angle.
And talking about angles, your eyes will be more ‘crossed’ when you are looking at something up close when compared to looking at something far away where your eyes are not so crossed.
Lastly there is an effect called ‘parallax’ that happens when something close to us appears to move across our field of vision faster than something further away.
The capabilities of the eyes and the computing power of the brain combine these things to produce an extremely accurate 3 dimensional model of what we are looking at.
Here’s how 3D works.
8. We All Have A Blind Spot
That’s right, every single human on earth is partially blind. Not that we really notice it because the spot that is blind is very small and having two eyes tends to compensate for this small loss.
Where the optic nerve joins with the eye and connects with the retina there are no light receptor cells so any part of an image that falls on this spot, the Blind Spot, does not get transmitted to the brain.
You can do this Blind Spot Test and check it out for yourself. Very odd.
This video not only explains the blind spot and the blind spot test, but how to see blood vessels in your eyes!
7. The Muscles That Move the Eyes Are The Body’s Most Active
It makes sense once you think about it. Apart from when you are sleeping, or blinking, your eyes are open and looking at stuff. Whether you are sitting, walking, driving, eating, whatever, you will be looking at things and those little eye muscles, the lateral and medial rectus and the muscles in the ciliary body, will be working overtime trying to keep up.
Luckily the eyeball only weighs about 28 grams so there isn’t a lot to move around, but the eye muscles do get tired if you read or stare at a computer screen for a long time.
Did you know that the various ways we move our eyes have names? We sure didn’t. Learn all about eye movement terminology here:
6. The Macula Is The Eyes High Definition Chip
The macula is a small oval shaped group of ganglion cells that sit in the middle of the retina at the back of the eye. This is right in the sweet spot of our vision, right where we are looking, and the concentration of photoreceptor cells in the macula gives our center of vision enormous clarity.
If you look at an object you will see the object very clearly. Crisp and sharp. You will also notice objects away from the center of your vision, on the periphery of your vision. These will be less clear and even a bit blurry. Our field of vision means that we can only really focus on one area at a time and it’s this area at the center of our vision that the macula reads.
Not only that, there is a small spot at the center of the macula called the fovea that has even more light sensitive cones than the macula. When you are reading, trying to fix the cogs in a watch or put a cotton thread through the eye of a needle you will be using the fovea.
The very best high definition is not in your new TV, it’s right there in the front of your head.
Macular degeneration is a serious disease of the eyes. Here’s a video so you know what to watch out for.
5. Having an Iris Stops Us Looking Like An Alien
The iris is the beautiful colored part of the eye. The blue, green, brown or mixed colored ring around the pupil. Technically the iris is a pigmented membrane that sits between the cornea and the lens and can open and close like a diaphragm to let more or less light into the eye, exactly like an aperture does on a camera.
But what we really like about the iris is that it’s a core component of what makes us individuals. That little splash of color makes us look human and adds to our character. Blue-eyed boy, brown-eyed girl, the iris is even part of music folklore.
Apart from these important features, the real reason the iris is so important is because without it we would definitely look like an alien. Just ask the albino monk in The Da Vinci Code.
Watching eyes move in slow motion can be pretty weird. Very weird in fact.
4. The Conjunctiva Protects and Cleans The Eye
In fact the eyelids form the first layer of defense for the eyes, but it’s the conjunctiva that performs the more complicated protective roles.
The conjunctiva is a clear mucous membrane that covers the entire front visible area of the eye as well as the inside surface of the eyelids. It remains moist at all times so the eyelids can open and close easily and the cornea doesn’t become scratched and damaged. Any dust, grit or germs that get into the eye are caught by the moist conjunctiva and washed to the corners of the eye.
Without the conjunctiva a number of things would happen. The surface of the eye would be dry, it would get scratched and it would be incredible painful to close our eyelids. Any dirt, even tiny bits of dust that got into the eye would have to be washed away manually. The same with germs. We would have to manually keep washing the eyes to avoid infections.
Conjunctivitis, inflammation of the conjunctiva, is common and a real pain in the… eye of course.
3. The Cornea Is The Eyes Lens Adaptor
While the lens rightly gets applause for its very cool capabilities, the cornea is actually more important for refracting (bending) light into the eye in the first place. In fact the cornea is responsible for most of the eyes refractive and fixed focusing powers. It’s like a lens adaptor that extends the capabilities of a camera’s main lens.
Like the eyes lens, the cornea does not have blood vessels as they would scatter the light entering the eye. The cornea receives oxygen directly from the air and other nutrients from the aqueous fluid surrounding the lens and filling the posterior chamber.
Unlike the lens, the cornea is full of unmyelinated nerve endings, meaning the nerves are not sheathed and are therefore extremely sensitive to everything – touch, chemicals, hot or cold. In fact it is estimated that the cornea experiences pain at roughly 500 times the intensity of the pain felt on the skin.
This is a protective design that makes us shut our eyelids pretty quickly if we don’t like what’s going on near our eyes.
A corneal transplant. Sorry, don’t watch if you don’t like to see eyes being operated on. Yuk!
2. The Retina Is Like a 500 Megapixel Large Format Video Camera
Remember when digital cameras first came out and we thought having one with 5 megapixels was pretty cool. Well the eye is equivalent to a digital video camera that’s 100 times more powerful than that was, well over 500 megapixels.
The retina, essentially the back wall of the eye, converts the image formed by the light shining through the lens into electrical impulses that are sent to the brain along the optical nerve. The retina is a membrane made of light sensitive tissue that performs the same function as film did in the old analog cameras.
In a slight twist on the construction of the eye, the retina is actually brain tissue. It, and the optical nerve, are part of the bodies central nervous system so although the retina lines the inside back wall of the eye, it’s technically part of the brain.
A very simple, if slightly dry look at the eye and the macula.
1. The Eye’s Lens was the World’s First Autofocus Camera
The lens of the eye is a clear, round, biconvex, crystalline sphere (yeah!) that refracts the light entering the eye and focuses it on the retina. It sits behind the cornea and anterior chamber and is partially covered by the iris. (We see it from the outside as the pupil, the black circle in the centre of the eye. It only looks black because we are looking through the clear lens into the dark, black body of the eye.)
Sounds reasonably simple doesn’t it; a clear round thing that light goes through. But of course it’s not as straight forward as it seems.
The lens is living tissue so it has to receive nutrients just like any other part of the body, but to remain clear so that light can pass through it unimpeded it can’t have blood vessels to deliver the nutrients. Instead the lens sits in aqueous fluid that carries the oxygen, glucose and other elements to it, and waste products away from it.
The lens itself can’t be made of normal cells as normal cells have nuclei and other bits and pieces that would break up the light passing through it. Instead, the lens is made of protein fibers that lie parallel to one another, horizontally front to back, looking from the front like the concentric layers of an onion.
Most importantly, the shape of the lens can be changed to provide us with the incredible gift of focus. When we look at an object close to us the lens becomes more curved and when we look at something far away it becomes flatter. More curved, flatter, more curved, flatter, book, tree, book, tree. Automatic focus. Amazing.
Why we wear sunglasses:
When we die, the cornea absorbs the aqueous humor (the liquid that fills up the front chamber of the eye around the cornea and lens), thickens up and goes hazy. From the outside we will look a bit like a zombie.
Luckily a bit warmth, 88F (31C), will see the fluid loosen up and move out of the cornea, restoring the transparency for a period of time – long enough for our friends and loved ones to say goodbye without having to worry about us rising from the dead because we look so damn weird.