Eye anatomy can be pretty intimidating when seeing it for the first time. However, when we break it down into varying sections, we can start learning this anatomy bit-by-bit and working out what the different parts of the eye do.
Human Eye Anatomy
Fig. 1 - Overview of eye anatomy
The human eye is slightly different anatomically compared to other animals and primates. The most significant difference between human eye anatomy and the anatomy of other animals is that our eyes have three distinct colours. We have our:
- sclera - the white area on our eyeball
- iris - the part of your eye that determines your eye colour
- pupil - the black part in the centre
Other animals do not have this much difference in the colours of their eyes.
The reason human eye anatomy is different from the eye anatomy of other primates is fascinating! The whites of our eyes, i.e. the sclera, are the main difference between our eyes and other primates. The whites of our eyes allow us to see which direction the people we are talking to are looking. If someone stands across the room from you, you may not be able to tell what colour their eye is, but you can tell which direction their eyes are looking in because of their white sclera! This helped humans evolutionarily, as it allowed us to hunt in packs more efficiently and avoid predators when in groups.
Eyeball Anatomy
The anatomy of the eyeball can be divided into the external eye anatomy and internal eye anatomy. We will detail the anatomy of these two different sections in the 'external eye anatomy' and the 'internal eye anatomy' sections.
The anatomy of the eye follows the function of the eye; when we understand the function of the eye, we can understand its anatomy. The eye's primary role is as a sensory organ. It sends information to the brain so that we can interact with our environment. For this reason, the eye needs to have photoreceptors. These are the cone cells and rod cells found in the retina, which are broken down by light so that a message can be transmitted to the brain. Now that we understand the role of these photoreceptors, we can understand why the different aspects help to focus and refract light onto the retina (where the photoreceptors are found).
The eye also plays an important immune function. The conjunctiva and sclera protect the eye but also help to produce tears and mucus. These tears contain an enzyme called lysozyme, which helps to break down pathogens such as viruses and bacteria.
Remember that enzymes are proteins that play an essential role in many metabolic reactions. They have active sites which are specific to substrates. Enzymes also have optimum pHs and temperatures at which enzyme activity is at its highest.
We can break down each part of the eye into whether it plays a role in maintaining the eyeball's shape, protecting the body from microbes, or whether it helps in refracting light and sending the message to the brain.
The eye isn't the only place where we can find photoreceptors. Plant cells, such as those that make up chlorophyll, are known as photoreceptors. Their roles are similar but not identical to the eyes' photoreceptors. During photosynthesis, the photoreceptors found in chlorophyll absorb light energy, which can then convert to chemical energy for the plant. The name of one of these photoreceptors is chlorophyll a. This is the primary photoreceptor in plants.
External Eye Anatomy
The external eye refers to the parts of the eye that are visible outside the body. You can see these parts of the eye when looking at someone's eye. We will break down each of the features of the eye and describe their function.
There are many functions carried out by the external eye, including refraction. This is important as it allows our eye to ensure that light hits the correct part of the inner eye (the retina) to send clear messages to the brain via the optic nerve.
Remember, refraction is the process by which the direction that light travels changes.
Table 1. The structures of the external eye
External part | Function |
Sclera | This is the white part of the eye that surrounds the iris and pupil. The role of the sclera is to protect the inner eye. |
Conjunctiva | Found around the sclera. Our conjunctiva plays a role in protecting our inner eye, too, as it allows for lubrication by producing tears and mucus. This helps the eye to fight against pathogens and avoid infection. |
Cornea | A layer at the front of the eye covers the pupil and the iris. The primary function of the iris is to refract the light along with the lens. |
Iris | The primary function of the iris is to control the pupil's diameter. This is important as it helps to protect the inner eye from damage, as too much light entering can be harmful. The iris is the part of the eye we often discuss in day-to-day life. When you ask someone what their eye colour is, you're really asking them what the colour of their iris is! |
Pupil | The area through which light travels to enter the internal eye, specifically the retina. The pupil and iris work together to ensure that the correct amount of light is reaching the retina. |
The sclera and conjunctiva protect the eye, whilst the iris and pupil's primary function revolves around ensuring enough light hits the correct location at the back of the eye. These are the main functions of the external eye.
Conjunctivitis is the most common infection of the eye. This is when the blood vessels next to the conjunctiva - the layer lining the sclera on our external eye - become infected by either a viral or bacterial pathogen. You may have more commonly heard of this condition as pink eye, as it sometimes causes the sclera of the affected person to turn from its usual white colour to a pink/red colour.
Accommodation
Accommodation is the process by which the lens of the eye becomes thicker or thinner to either focus the eye on a distant object or focus the eye on a nearby object.
Accommodation is vital as it helps us to adjust our focus to objects at different distances.
If you're looking at your phone and someone from across the street calls your name, your eyes need to adjust their focus from looking at the nearby phone to the person standing a distance away.
To achieve this, two components of the eye are required. These are the ciliary muscle and the suspensory ligaments of the eye. The ciliary muscle is a circular ring of muscle that surrounds the lens; suspensory ligaments attach the lens and ciliary muscle.
Remember that the Ciliary muscle is Circular because they both start with the letter C!
The ciliary muscles and suspensory ligaments work in an antagonistic way. This means that when one contracts, the other relaxes. When our ciliary muscles contract, the suspensory ligaments slacken (relax) and vice versa. Let's have a look at what happens when the ciliary muscles relax and contract;
When the ciliary muscle relaxes, it becomes wider. This causes the suspensory ligaments to tighten, which pulls the lens thinner. The ciliary muscle relaxes, and suspensory ligaments become taught when we focus on a distant object.
When the ciliary muscle contracts, it becomes smaller. This causes the suspensory ligaments to slacken, which causes the lens to return to a thicker shape. This sequence of events happens when we focus on a nearby object.
The Ciliary muscles Contract when focusing on an object Close to us!
Fig. 2 - Pupil dilation and constriction
Internal Eye Anatomy
We can split the aspects of the internal eye into different categories depending on their function. The aqueous and vitreous humour plays a role in maintaining the eyeball's shape. The retina, lens, and optic nerve are essential in vision, as they help refract light and send a message to your brain.
Fig. 3 - Refraction of light to the retina
Table 2. Structures of the internal eye
Internal part | Function |
| We discussed above how the cornea plays an essential role in refracting light. The lens also carries out this function, ensuring light hits the correct area within the internal eye. |
Retina | The retina is arguably the most important aspect of the eye. Here, we find the photoreceptor cells. These are cells that can break down once light hits them, allowing for a message to be sent to the brain via the optic nerve. This is essentially what allows us to see. |
Optic nerve | It is located near the very back of the eye. As a sensory nerve, it only carries sensory information to the brain. It takes the messages about sight from the retina to the brain. |
Aqueous Humour | This is a jelly-like substance found near the front of the eye. Its role is to keep the eye hydrated and to ensure the eye maintains its characteristic 'ball' shape. |
Vitreous Humour | This plays the same role as the aqueous humour. However, it is found more posteriorly (near the back) of the eye than the aqueous humour. |
The optic nerve is the only nerve in the body that technically is not a nerve. Instead, it is a projection from the brain.
Eye Anatomy Diagram
Now that we have discussed the internal eye anatomy and the external eye anatomy, make sure you can label these different areas of the eye on a blank diagram.
Fig. 4 - Overview of eye anatomy
Eye Anatomy - Key takeaways
- The eye can be divided into the internal and external eyes, separated by the lens
- Behind the lens, we find the vitreous humour, the retina, and other substances.
- In front of the lens, we find the aqueous humour and the cornea, iris, and pupil.
- Each aspect of the eye plays a role in vision, protection or maintaining shape.
- The retina contains rod and cone cells that allow for vision.
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