Off the Contrary: Science Fact

I was reminiscing about elementary school with a friend today, and it conjured up a memory of mine form grade 4. I was in an almost empty classroom for some unknown reason, meaning that I was alone except for quiet Cindy sitting by the wall. And in order to pass the time I developed a challenging and thought-provoking game whereby I would throw my sharpened pencil into the ceiling tiles to see if I could get it to stick. After a few failed attempts the pencil lodged into one of the foamy squares (victory!) but after a brief second it freed itself and fell to the ground. Right in front of Cindy.

I asked Cindy a couple of times to throw/pass the pencil to me, but to no avail. She was conscuois of me, but completely unresponsive. As this was normal for Cindy, I went to go pick it up myself. But just as I reached her feet and bent down to get it, she swooped down faster than I have ever seen her move before and grabbed the pencil. And then brought it up just as quickly. Into my eye.

I had to wear a patch on my eye for a while, but apart from the ridicule of classmates I escaped any real damage. Lucky for me. And best of all, I could pretend to be a pirate on the playground!

This memory was appropriate for our current medical topic: ophthalmology (aka eye doctor stuff). So I thought I would write down some info on our eyes in order to make you appreciate my story even more.

Let me first talk about each part of the eye and what it does:

Cornea - The cornea is a tough, clear layer of cells in front of the eye. It can heal itself quickly, but it hurts so much if you scratch it.

Lens - The lens helps to focus light onto the back of the eye. It can change shape according to how far away you are looking.

Iris - This is the colourful part of your eye. It can contract or expand to let different amounts of light into your eye. The hole it makes is the pupil.

Pupil - As I said, this is not a structure. It's the hole in the middle of the iris that lets light into your eye.

Retina - This is the photosensitive area in the back of your eye. It converts light into nerve impulses and sends them to your brain. This is the part that contains those cells called rods and cones (cones are used for precise vision and for colour, while rods are good for seeing the “big picture” and for night vision).

Now that we know the basics, let's talk about a few weird things about our eyes. First of all, the retina has an area in the middle that is designed to work super well. All of the cells that may interfere with light are pushed to the sides so that the phororeceptors get the most direct light possible. This spot on the retina is called the macula, and the centre of it is called the fovea. When you look directly at something, you are lining up your fovea perfectly with that object. This allows you to see it very clearly.

The fovea contains mostly cones (the precise vision cells), which makes sense because when you focus on something you would like to see it well. But remember that the cones are crappy at night. The rods are the cells that are best at night, and these cells are found in the rest of the retina. So if you've ever looked at a starry sky, you may have seen a faint star out of the corner of your eye, but when you turned to look at it the star disappeared. The only way to see it was to look 5-10 degrees away. This method uses your rods more than your cones, enabling you to see things that don't produce much light.

Now for the second cool thing about eyes. The retina has to send all of the visual info to your brain, and it does this via the optic nerve. But at this area of the retina there are no photoreceptors. So if light shines on the optic disc (where the nerve meets the retina) you will not see anything at all. This is called the blindspot of the eye. Any object that is 30 degrees away from centre (away from your nose) will magically disappear! You don't notice this very often because one eye will compensate for the other one. But you can test this with the picture below. Cover your right eye and focus on the shape on the right with your left eye. Then move your face closer and farther away from the screen slowly until the shape on the left disappears. This is your blindspot. You can do the same by switching eyes and focusing on the other shape.