All about the structure of the human eye
13 February 2020
Anatomy of the eye
The human eye is an incredible organ. Made up of many intricate parts working together, the end result is something that almost everyone relies on heavily every single day – our vision. Even though we value our vision the most of all our senses (accounting for 80% of all impressions), a lot of people know surprisingly little about how our eyes work. We can break the eye down into parts that we see externally when looking in the mirror, and parts which aren’t visible because they’re inside the eye, or further towards the back.
Visible parts of the eye
Eyelid: Your eyelid covers your eye to protect it from dust, grit, and perspiration that could cause damage. It opens and closes both voluntarily and involuntarily, and facilitates blinking to help keep the eye hydrated and well-lubricated.
Pupil: The pupil is the part of the eye which we see through, and changes size depending on light levels. If you are in a particularly bright environment, the pupil contracts to let less light in, while if you’re in a darker setting, it will expand to let more light in. This helps us to see well in different light levels, making sure that the correct amount of light reaches the retina at the back of the eye.
Sclera: The sclera is the white part of your eye, providing a protective outer layer. It covers the optic nerve and its can also be a good indication of your eye health. For example, a red sclera might suggest that your eyes and tired or dry, while a yellow-tinted sclera could indicate liver problems.
Iris: The iris is the coloured part of your eye and is what actually controls the size of the pupil. This means that it regulates how much light gets into the eye. This iris is made from connective tissue and muscle surrounding the pupil, and its structure, pattern and colour are just as unique as your fingerprint!
Internal parts of the eye
Cornea: The cornea is the clear surface at the front of your eye, allowing light to enter the eye. It directly covers your iris and pupil, providing a layer of protection. The cornea is what we operate on for laser eye surgery procedures, as it is imperfections in the curve of your cornea that create an eye prescription, requiring you to need glasses. The smoother the surface of your cornea is, the better your vision will be.
Lens: The lens is located behind your iris and is the part of the eye which provides focus. The lens can change shape to alter the focal distance of the eye, focusing light rays that pass through it to hit the retina at the right angle. As you get older, a build-up of protein in the eye can mean that the lens becomes cloudy. This is called a cataract. Thankfully, your lens is easily removable and can be replaced with an artificial clear lens to provide good vision again.
Aqueous humour: The aqueous humour is a watery fluid that your eyes constantly produce in order to maintain good eye pressure and nourish your cornea. This keeps your eyes healthy and, in turn, contributes to good vision. It is drained from the eye at the same rate that it is produced (when this rate isn’t constant, it leads to glaucoma) and its presence is vital for good vision.
Ciliary muscle: The ciliary muscle is the part of the eye that actually changes the shape of the lens, allowing it to focus on different distances. It also holds the lens in the correct position in the eye’s middle layer and regulates the flow of the aqueous humour within the eye.
Medial rectus muscle: There are six extraocular movement muscles in your eye (medial rectus, lateral rectus, superior oblique, superior rectus, inferior rectus, and the inferior oblique) and the medical rectus is the largest of them. It moves the pupil closer to the midline of your body (towards your nose) and makes sure that the eye is aligned correctly. If there are problems with the medial rectus, it can lead to strabismus.
Lateral rectus muscle: This is the muscle which is responsible for lateral – or sideways – movement of the eye, particularly movements away from the midline. Again, if there are issues with the lateral rectus muscle, you may experience esotropia. This is a form of strabismus where the eye turns inwards because the muscle is either too weak, or isn’t working properly to move it away from the midline.
Retina: The retina is a layer of tissue at the back of the eye. The primary purpose of the retina is to receive light from the lens and send signals to the brain to process it into a visual image. The retina contains two types of photoreceptor cells: rods and cones. Rods are responsible for picking up on motion, dark and light, while cones detect colour vision. Problems with the retina can lead to loss of vision, so preserving your retinal health is crucial.
Choroid: This is a major blood vessel which sits between the retina and the sclera at the back of the eye. It nourishes the outer layers of the retina and keeps the eye at the right temperature. It also provides the right amount of oxygen and blood flow to the retina, helping the eye to function well.
Macula: The macula is the central part of your retina and is around 5mm in diameter. A healthy macula means we will have clear vision and be able to see fine details. When the macula becomes diseased, such as with macular degeneration, your central vision is affected. This obviously has a huge impact on your day to day life, and can keep worsening until all vision is lost.
Optic nerve: The optic nerve is the part of your eye which transmits visual signals from the retina to the brain, to be processed into images. It contains over a million nerve fibres and is actually considered to be part of the central nervous system. One of the most common ways the optic nerve can be damaged is by glaucoma. Eye pressure builds up, compressing the optic nerve, meaning visual signals can’t be transmitted effectively anymore.
Vitreous humour: The vitreous humour is a liquid in your eye with the consistency of gel, and sits behind your lens but in front of your retina. If any substances enter the vitreous humour, they are referred to as floaters. They can be small flecks of blood or clusters of cells and, while they can be annoying to see in your line of vision, they are typically harmless. With age, your vitreous thins and can separate from the retina, causing “posterior vitreous detachment”. This causes even more floaters but isn’t sight-threatening.
If you have any questions about your eye health, please don’t hesitate to get in touch with us on 0800 093 110 or by emailing firstname.lastname@example.org. We’ll be able to assess your visual needs at a consultation and recommend a treatment for you, based on your eye health and prescription.
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