People used to believe that our eyes emited rays of light and that is how we saw things around us. But that was until an Arab scholar named Ibn al-Haytham came and corrected that mistake by arguing that light is reflected from objects around us and then bounces off into our eyes and that’s indeed how we see things. Our eyes are a very complex and invaluable Structure that we all need to know more about. So in this article, we are going to dive deep into our eyes, how they work, what are they composed of, and finally some visual defects that might happen in each part of our eyes.

But before we get into how we see things and how vision works, we need to know first what are the components of our eyes.

The Components of the Eye

If we take a vertical cross-section of the human eye, we will find the structures listed below from the front backward.

The Cornea

It is the clear outermost layer of your eye. It looks exactly like a transparent contact lens. It helps you focus the light so that you can see clearly.

The Sclera

That is the white outer layer of your eye. It is the backward continuation of your cornea. In fact, they both form the outer protective layer of our eyes. That part is very important as it may turn yellow if you got jaundice (a problem in your liver).

The Iris

That is the part that gives your eyes their colour. It has two sets of muscles that control the diameter of the hole in the middle of the iris. These muscles are:

• The dilator pupilae muscle: which as its name implies causes dilatation of the iris so as to let more light in which happens in dark places where you can barely see things to help you navigate your way without getting stuck.
• The constrictor pupilae muscle: when it contracts, it limits the amount of light passing through it. That is what happens when you spend some time out on a sunny day so as to protect your eyes from excess light that might be harmful to your eye. You might have noticed that when you stand for some time in the sun and then move to a darker place you can’t see clearly what is inside that place for some moments and then you start to see things clearly. Well, that is because your pupil was constricted and limiting the amount of light that goes into your eyes when you were in a bright place. And then when you moved to a darker place, your pupil takes a few seconds to adapt and change its diameter to allow more light in to be able to see things clearly in that dark place.

Want to test your pupillary muscles?

Let’s do that experiment together.

• First, bring a torch or use your mobile phone torch.
• Then stand in front of a mirror in a well-lit room and look into your eyes.
• Try to observe that hole in your iris.
• focus the light of the torch on your eyes.
• Observe how that hole becomes smaller and smaller as you bring the torch light closer. And how it gets bigger (dilates) as you move the light away from your eyes.
• And here you go, you have just done a very famous test called “The light reflex”. Your doctor might do that test to make sure your eye and its nerve supply are okay.

The Pupil

That is the hole that you can see in the middle of your iris. Its diameter is under the control of the above-mentioned muscles.

The Lens

It is a clear lentil-shaped structure inside your eye that works with your cornea to focus light rays coming out of an object on the retina.

The Ciliary Body

That part contains a tiny muscle called the ciliary muscle that connects to your lens by some thin filaments called suspensory ligaments of the lens. This muscle controls the curvature of your lens and therefore its power. When the ciliary muscle contracts, the suspensory ligaments become loose. That in turn causes your lens to get more convex which means increasing the power of your lens and vice versa.

The Choroid

That layer provides blood supply to the inner and outer layers of your eye.

The Vitreous

That is a transparent jelly-like structure that keeps your eyes rounded and prevents them from collapsing.

The Retina

That is the innermost layer of your eye that lies at the back of the eye. It is also a light-sensitive structure that contains special cells called photoreceptors that can turn the light into electrical signals. There are two main types of cells inside your retina: the cones and the rods. Cones are responsible for colour vision and rods are involved in detecting movements and they only give the Brain white and back images.

The Optic Nerve

That is a cord-like structure that transmits electrical signals from your retina to your brain, especially to the occipital cortex.

How do we see things?

So in order to see an object, the light coming out of that object needs to pass through specific structures in your eye until it gets transformed into an electrical signal that your brain recognizes and translates into a meaningful image. And although that might sound a little bit complex, but in real life, that process takes only a fraction of a second to happen.

So here is how it happens:

• First light passes through your transparent cornea with minimal refraction.
• Then it passes through your pupil.
• Then it hits your lens where it gets refracted and focused even more.
• Then it moves into your eye until it hits your retina.
• Then the photoreceptors in the retina transform that light energy into an electrical signal that passes to your brain through your optic nerve. 
• Finally, in the cerebral cortex, that signal gets translated by your brain into a meaningful image.

Why do we need a brain to see?

First of all, the image that is formed after light hits your lens is flipped upside down and also revered from right to left which means your brain can’t make a meaningful image out of this thing. So your brain corrects the position of this image to make it meaningful to you. This process takes a fraction of a second and none of us is aware of it.

Second of all, your brain does not only identify objects, but it also gives you more information that is associated with that object. For example, when you see a pen, your brain not only tells you that this is a pen, but it also associates that pen with certain things like writing. That is a very interesting thing to know as in some diseases, where a specific area of the occipital cortex gets damaged, the patient can only see things but they can’t tell what are they or how we use them or what are they named despite having healthy and normal eyes.

The problems of the visual cortex are really interesting to know since the eyes are completely normal and healthy. The problem here is in the visual cortex (The Occipital lobe of the brain).

Visual Agnosia

Visual agnosia happens if there’s a lesion in the occipital lobe of the brain due to trauma, haemorrhage, thrombosis, or even a tumour. In this disorder, The patient can see everything around them but they can’t recognize what they are looking at. To know more about this strange, yet interesting disorder, read “The man who mistook his wife for a hat” book.

Visual Defects

Have you ever seen someone wearing glasses and wondered why they wear them? That is because those people have a problem with their eyes. In this section, we are going to discover some problems that might occur in our visual system. We are going to focus only on refractive errors of they eye.

Refractive Errors

Since both the cornea and the lens work together to focus light rays on your retina, any change in their curvature would lead to a defect in their refractive power. This in turn will cause a refractive error in the eye that would require glasses to be corrected.

refractive errors include two major problems:

Conclusion

Now we have discussed the structure of our eyes in detail and talked about the importance of your brain in the process of vision. It’s time to test yourself out. Ready?

Q2. Which structure of these keeps your eye rounded and prevents it from collapsing?

1. Lens.
2. Cornea.
3. Vitreous body.
4. Sclera.

Q3. Our eyes emit light rays and that is how we see things around us.

1. Yes.
2. No.

Q4. When you are in a very dark place your pupils dilate.

1. Yes.
2. No.

The correct answers:

The post How do we see? The stunning structure of our eyes first appeared on Learning Mole.