Understanding Eyes

1.1: Getting to know the visual system

When I first started Optometry school, one of my textbooks was Wolff’s Anatomy of the Eye, a great lump of a book about 300 pages thick, full of diagrams showing the structure of the eye.

It didn’t include anything about the way the eye actually worked — just the structure. I couldn’t believe there could be so much to know about one little part of me.

And yet, over time I began to realise that Wolff’s was just an introduction, and there was so much more. Extending that complexity to the rest of our body… wow.

Luckily for you, I’m going to give you a only a brief overview on this page — just enough to understand what I’m talking about when we get to thinking about how eyes work, and don’t work.

Structure of the Eye

Original: Holly Fischer Vector: Pixelsquid, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons

The Cornea

The cornea is the clear window at the front of the eye. It’s got a very precise curve, because it also acts as a lens, focusing the light on to the back of the eye. It’s really important that the cornea is clear, and that the surface is very smooth. Every time you blink, the lids spread a very thin layer of tears over the cornea, which creates that ultra-smooth optical surface. Anything that disrupts that tear layer (such as dry spots, or bits of mucous in the tears) can decrease your quality of vision.

The Iris

The iris is what we see as the colour of the eye, usually somewhere between blue and brown. Think of it as like a circular curtain — it restricts the amount of light coming into the eye. The gap in the middle is the pupil. We see the pupil as black, because light goes into the eye but generally doesn’t come back out. An exception is when we use a super-bright light such as a camera flash. The bright light goes in and illuminates the inside of the eye, and then we see light coming back out again, which shows up as those eerie red pupils in photos.

The Lens

The lens is a pretty remarkable structure. You can’t see it, because it sits behind the iris, and it’s clear. In young people, it’s very flexible, and lets them adjust their focus from far-away vision to up-close. Kids can focus on objects that are extremely close, but over the years the lens keeps producing new cells, and it gets more and more crowded in there, which makes the lens get tougher and less flexible.

By our mid-forties, most of us are having trouble focusing on fine detail up close, and that’s why we start needing reading glasses. By age sixty our focusing flexibility is pretty much zero, so it levels off there.

The major change later in life is that the lens starts to lose clarity. It’s kind of like those clear plastic bottles that if you leave them outside in the sun they eventually turn milky-white. When a lens loses clarity, it’s called cataract. It’s a normal ageing change, like grey hair or wrinkly skin. Just as there’s no point where we suddenly switch from smooth skin to having wrinkly skin, there’s no exact point where we suddenly say we have cataracts, it creeps up gradually. And like grey hair, some people get it earlier than others, and some medications and other conditions can bring on cataracts earlier. But sooner or later, we all get cataracts, as long as we live long enough.

The Vitreous

The vitreous is a clear gel that fills the inside of the eye. It doesn’t really do much, but really becomes significant if there is a bleed somewhere in the eye that gets into the vitreous.

Retinal photograph. To understand this view, imagine you are looking into someone’s eye, through the Cornea, the Pupil and the clear gel — you’ll see the inside surface of the back of the eye. The whole area is Retina, and the darker patch right in the middle is the Macula. That white circle is the head of the Optic Nerve, just as if you were looking straight down on a basin and seeing the plug hole. There are blood vessels on the surface of the Retina, but actually 90% of the blood vessels are in the Choroid, right behind the Retina.

Image by Mikael Häggström, used with permission.

The Retina and Choroid

The Retina is a really important one for many people with vision impairment. It’s a very complex structure — technically it’s an extension of the brain. It’s a very thin layer that coats the inside of the eye, and acts like the film (or sensor) of a camera. The job of all the bits I’ve described above is to get a clearly-focused image on to the retina. The retina has a layer of light-sensitive cells called Photoreceptors, which pick up the picture and transmit it to nerve fibres in other layers of the retina. Those nerve fibres carry the picture to the Optic Nerve (see below), but the nerves aren’t just simple transmission lines — they have connections to the other nerve cells around them, and have already partly processed the image even before the signal has left the eye.

The whole process of receiving and transmitting the image is very intensive, so it needs a lot of nutrients, which are largely supplied by the layer of blood vessels behind the retina called the Choroid. The process of converting light into nerve signal also produces a lot of waste products and a great deal of heat, both of which are taken away in the blood. The choroid is basically the retina’s personal assistant, so if the choroid malfunctions the retina won’t work properly either.

The Macula and the Fovea

The Macula is a small part of the Retina, right in the middle. It has a much higher density of photoreceptors and nerves, because it is the part that gives us more detailed vision. Having that higher density makes it more energy-intensive — it requires more nutrients, and generates more heat and more metabolic waste, so it’s the part of the retina most prone to breaking down. Problems with the Macula are behind a large proportion of vision impairment conditions.

The Fovea is like the Macula-of-the-Macula. It’s a very tiny area, right in the middle of the Macula, which gives us ultra-high detail. This is the bit we use when we say we are looking straight at something.

The Optic Nerve

All the retinal nerve fibres travel towards the optic nerve, through which they travel out of the eye, the same way water in a basin travels to the plughole and down the pipe. The nerve fibre part of the optic nerve is only about 1.5mm (about 1/16th of an inch) across, but that’s a lot — that makes it the largest sensory nerve in our bodies, carrying something like 75% of the total amount of the sensory information we get about the world.

If something destroys the optic nerve, it’s like someone cutting your phone line. The phone goes dead, not because of a problem with the phone, but because the signal can’t get through. Conditions like glaucoma and optic neuritis affect the optic nerve.

The optic nerves from each eye travel back to a point in your head where they partly cross. Vision from the left side of where each eye was looking travels on from there to the right side of the brain, and vision from the right side of each eye travels to the left side of the brain. This is important to know if something damages one side of the brain or the other (such as in a stroke), because it can affect the vision we see out of both eyes, but just affecting one side.

The Brain

In the end, all seeing is really done in the brain. The part that is considered ‘vision’ is the Occipital Cortex, which is right at the very back of the head. So damage anywhere between the eyes and the back of the head has the potential to affect vision. But the nerve fibres don’t all go just to the vision part of the brain. Some fibres go off to other parts, which can give some interesting results when the vision part of the brain is damaged but other parts of the brain might still respond to information from the eyes.

Beyond the vision part of the brain, the information gets transferred to other parts of the brain to build up the overall experience we call ‘seeing.’ Sometimes people have perfectly good eyes, and perfectly good vision parts of the brain, but still have trouble integrating that vision with the rest of themselves and so experience vision problems.