Don’t worry, there are only a couple of parts that you need to worry about. In fact, probably only one, the cornea, but we’ll explain a few more just for kicks so you can feel really educated.

Now, we’re going to go for a little walk through your eye, from the outside in. This will take us through the cornea, the iris (and the pupil), the lens, and the vitreous, all the way to the retina.

The cornea

(Remember this one, it’s important.)

The cornea is clear tissue all over the top of your eye. At least, it better be clear, otherwise you won’t see very well. Before surgery, there was nothing really all that wrong with your cornea, but despite its innocence the laser has whacked the heck out of it just so that you wouldn’t have to wear glasses.

Now, the cornea’s actually got several layers. First, there’s a very thin sort of outside layer on the top — that’s the part that meets the air. It’s called the epithelium. Those of you with dry eye problems might want to try and remember that one because the epithelium is the part that’s all dried out and hurting like crazy. Also, those of you that had PRK and have questions about the healing process in the early days should try and remember the epithelium because the surgery tackled yours in a particularly unkind way. Second, there’s a super-thin membrane, called Bowman’s layer. We don’t know anything special to say about that, except that sometimes minor things happen to it that the doctors confuse with flap wrinkles. Third, there’s the really thick part, the stroma. Most of your cornea is stroma, and that’s the part of the cornea that laser surgery has most to do with. Many of the Bad Things that happen during surgery have to do what the laser did, and didn’t do, to the stroma. Fourth, there’s another super-thin membrane we’re not very interested in called Descemet’s layer, and fifth and last, there’s the counterpart to the epithelium but on the inside, called the endothelium. For the most part you will only care about the fourth and fifth layers if you’re going to get a corneal transplant.

Now, that’s what it’s made of, but what does it do?

The cornea provides what we call focusing power. You don’t need to know a lot about that but as you’ve probably guessed it’s a good thing to have. OK, if you really want to know, the whole point is to try to focus light onto the retina at the back of your eye. Now, the cornea provides about 2/3 of your total focusing power. The rest is from the lens.

The iris... and the all-important pupil

After the cornea comes the iris. The iris is the coloured part of your eye. It regulates the entrance of light into the eye. How? (drumroll) The pupil. In fact we really don’t care all that much about the iris except for the fact that the pupil is the round black thing right smack in the middle of it. The pupil is always changing size. It gets really small if you’re stoned, or you’re looking straight at the sun (don’t do that) and gets really big if your eye doctor put some nasty drops into it or you’re sitting in the put late at night. The pupil is basically just a hole in the iris — an opening that gets bigger or smaller depending on things like how much light is hitting it, whether you’re trying to focus on your toenails or on the south downs, and even how you feel. If we wanted to get fancy we’d call these changes pupillary reflex, but that’s not really important.

If your night vision is trashed, you definitely want to know what your pupil is and it would be a good idea if you found out how big it is. It would also have been a good idea if your surgeon had found out how big it really is and even better if he had actually known what it meant or cared, but we won’t get into that just now.

The lens

The lens is an easy part because you can think of it like the auto-focus part of your camera, but in fact you don’t really need to know all that much about it. Remember what we said about the focusing power in the cornea? Of course not. Anyway, to summarise, (a) focusing power is a good thing, and (b) 2/3 of it comes from the cornea and 1/3 of it comes from the lens.

Now, the 2/3 of focusing power in the cornea is just sort of there. It is what it is and doesn’t move or flex. But the lens is different. It flexes, so that you actually can change focus from near to far and back again like your camera, only faster and better. Changing focus all the time like that is called accommodation. Now, the younger you are, the more flexible the lens is, and the better job it does at helping you focus on things near and far. But as you get older, it gets stiffer and stiffer (much like some of your joints, in fact... sorry, didn’t mean to remind you about that). It can no longer flex as much and so you lose your ability to accommodate. That’s called presbyopia and what it really means in practice is that the nearest point at which you can focus gets further and further away from your eyes. That’s why you see all your friends hitting their forties and starting to wear reading glasses, or bifocals, or (gasp) trifocals.

The only other really important thing about the lens that you might want to know is that there are a bunch of muscles and fibres attached to it that help it change shape so that you can change what you’re focusing on all the time. These muscles are the ciliary muscles. The reason these muscles might matter to you is if they’re spasming. If you’ve had surgery and one eye ends up noticeably different from the other, or there’s some other really difficult focusing problems going on that your poor eyes can’t quite cope with, your ciliary muscles might start spasming. That hurts and it doesn’t do your vision a lot of good either.

The vitreous humour, and those pesky floaters

Don’t worry, we’re almost done. Just two parts to go.

The vitreous humour is some sort of jelly-like stuff that just hangs between your lens and the very back of your eye (the retina — more on that below). The reason you care about the vitreous is that there are little translucent specks or particles floating around in it which, if you’re staring at a cloudy sky, you might actually see moving around. These are called floaters. We’ve all got them and in normal people they’re really not that big of a deal. But we know many people who, after surgery, have found they’ve got lots more of them, or that they’ve got some really big ones that are kind of distracting.

The only other reason you might need to know about the vitreous is that a piece of it might pull away from whatever it’s attached to and start floating around. That’s not exactly dangerous, but the symptoms you get (strange light flashes, and/or sudden new floaters) might mean there’s Something Bad happening like a retinal tear or a vitreous haemorrhage. If you start seeing strange flashes of light or have sudden new floaters, you should run, not walk, to your eye doctor and make sure he has a good long look at you.

The retina

Last, there’s a thing at the back of the eye called the retina. That’s the part that sends pretty pictures to your brain (well, as long as the pictures it is getting are pretty). That means that the retina is Very Important. The main things you need to know about the retina are that if it’s not working right, you’re not seeing right, and that for it to work right, light needs to hit it Just Right, and that if light is hitting it Wrong, it isn’t going to work right and you won’t see right.

I really don’t know anything else about the retina except that there are Bad Things that can happen to the retina like retinal tears. If you want to know more about that, ask your eye doctor.

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