The eye is a complicated organ. It has highly specialized cells, muscles, tissues, fluids, etc. Every part of the eye plays a crucial role in producing the vision we all rely on so much in our daily lives.
Luckily for you, unless you’re an eye doctor, you don’t have to know about the vast majority of it to understand how vision works.
In this lesson, I will introduce you to the main parts of the eye that are responsible for producing vision. It is important to have some basic understanding of the anatomy discussed in this lesson in order to understand why some people need glasses, and what the glasses are actually doing to fix the problem.
Basic Eye Anatomy
All of the parts of the eye necessary to understand vision are contained in the following diagram.
Take a moment to examine the picture and try to make sense of what you’re looking at.
The ‘Front View’ should be easily recognizable. It is the view of the eyes that we see when we are face to face with someone and looking at their eyes.
The cross-section view, on the other hand, could be a little confusing at first. We are not used to viewing the eyes from this perspective, so let’s break it down a little.
A cross-section view of an object is very helpful when wanting to see what’s inside an object. We can obtain a cross-section view of things by cutting them right down the middle, taking one of the halves, and then turning it sideways to see what’s inside.
You are in fact already a low-key expert at doing cross-sections if you’ve ever cut up some fruits or vegetables. Let’s look at some food cross-sections that you’ve undoubtedly already created at some point in time.
|Cross-section of a melon||Cross-section of a kiwi||Cross-section of a red pepper|
Can you see how looking at a cross-section of the fruit/vegetable gives us an understand of what’s inside? We can do the exact same thing – schematically – with body parts to get a better understand of what is inside them. Here are some examples:
|Cross-section of a tooth||Cross-section of a heart||Cross-section of a brain|
Now you should have a pretty good understanding of the cross-section view of the eye I showed above. Let’s look at it again with a few parts labeled.
Let’s go through each part one at a time.
Iris & Pupil
The iris is a colorful, ring-like circular structure that sits in the front of the eye. The circular gap in the center of the iris is called the pupil.
Through contraction and relaxation of the muscles of the iris, it can become bigger or smaller. In doing so, it affects the size of the pupil.
As the iris gets smaller, the pupil grows bigger. As the iris gets bigger, the pupil gets smaller. Here is an illustration of this.
The iris is opaque, meaning that light does not through it. For light to reach the inside of the eyes, it has to pass through the pupil.
When the pupil is large (the eye on the left) a lot more light will be able to enter the eye. When the pupil is small (the eye on the right) a lot less light will be able to get into the eye.
It is by constantly adjusting the size of the pupil (by constantly adjusting the muscle tone of the iris) that the eye is able to control the amount of light reaching its light-sensitive interior.
Too much light getting into our eyes would be very uncomfortable for us so the pupils will get really small to block out a lot of that light. Not enough light getting into our eyes would make it very difficult for us to see, so the pupils will get very big in order to allow as much light to enter the eyes as possible.
Gives You Color!
Aside from controlling the light levels entering the eyes, the iris also gives us our eye color. Most people worldwide have brown eyes (~79%), but they can also be blue (~16%), grey (~3%), or green (~2%), depending on what kind of pigment, or lack of pigment our eyes produce.
The cornea is a little more difficult to see on a front view of the eye because it is transparent.
It is a very thin, curved structure that ‘vaults’ or ‘domes’ over the iris and pupil.
Recall from Lesson 2 that objects that are transparent and curved meet the definition of a lens. Indeed, the cornea of the eye acts very much like a lens! We’ll learn more about this in Lesson 5.
The cornea is much easier to see in a cross-sectional view of the eye. It is the part at the very front of the eye that covers the entire iris, pupil, as well as all the fluid (called the aqueous humor) found in the front of the eye.
In order to visualize what the cornea looks like on its own, think of a contact lens. Contact lenses are designed to act as extensions of the cornea. They completely drape over the cornea and just add little more thickness in certain places in order to alter the overall power of the cornea.
The retina is a part of the eye that most people will never see. It is tucked away in the very back of the inside of the eye.
Think of the retina as a very thin (even thinner than the cornea) lining of the inside of the eye. It is physically attached to the walls of the eye. Keep that in mind as it will be extremely relevant in our future discussions of nearsightedness and farsightedness.
The function of the retina is to detect and absorb light. It is made of specialized cells that are very sensitive to light called rods and cones. As a result, the retina is said to be light-sensitive.
Here we will see the front view of a real retina, and schematic representation of the cross section of a retina.
It would not be wrong to think of the retina as the place where ‘vision’ begins. As light enters the eyes and eventually touches down on the retina, the retina generates signals that are sent to the brain, which are then interpreted as what we see.
An analogy that is often made for the retina is the film, or nowadays, the sensor in cameras. These are the parts of a camera which capture the image, just like the retina is the part of the eyes that ‘captures’ our vision.
- The iris and pupil are interconnected. The larger the iris is the smaller the pupil is, and the smaller the iris is the larger the pupil is. Together they control the amount of light that reaches the inside of the eyes.
- The cornea is located at the very front of the eye. It is curved and transparent, thus it acts as a lens and focuses light.
- The retina is a thin layer that lines the inside of the eye. It is light-sensitive and when exposed to light it sends vision-signals to the brain.
Here is a differently stylized graphic of a cross-section of an eye. See if you can find and identified the parts of the eye discussed in this lesson.
And here are the answers:
That’s it for now! With the eye anatomy that you now know, you are set to understand how vision works as well as how the different refractive conditions (conditions that cause you to need glasses) work.
Of course, there are more parts to the eyes than what was covered in this lesson. In later lessons when we start covering more complicated eye conditions, I will introduce more eye anatomy to you.
In the next lesson, we will use the knowledge we gained from Lessons 1-4 to understand how vision works.