How the eye works 

The human eye is a wonderful thing. Although small in size, the eye arguably provides us with the most important of our five senses – our vision. It is one of the most complex organisms in the human body – second only to the brain.

Despite being just over two centimetres in diameter, human eyes have over 2 million moving parts. They can distinguish over 500 shades of grey, and over 2.7 million colours. Eyes are amazing.
 

How a healthy eye works

Your eye works in a similar way to a camera. By focusing images through a series of lenses, our eyes allow us to see the world around us. 

[Also check out eye: symptoms, conditions and treatments
 

 

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Basically, sight is the result of light passing through the lens and being 'encoded’ on the back of your eye by a light sensitive membrane called the retina. The retina sends on that image as electrical impulses to your brain. At the front of the eye is a transparent structure called the cornea. The corneas job is to help focus the incoming light. Just Behind the cornea is a colored, ring-shaped membrane called the iris. The iris has an adjustable circular opening called the pupil that can expand or contract to control the amount of light coming in. Ciliary muscles surround the eyes lens. These muscles hold the lens in place but they also play an important role in how we see. When the ciliary muscles relax, they pull on and flatten the lens, allowing your eye to see objects that are far away. To see closer objects clearly, the ciliary muscles have to contract in order to thicken the lens.

The interior chamber of your eyeball is filled with a jelly-like tissue called the vitreous humour. After passing through your lens, light has to travel through this humour before striking the sensitive layer of cells called the retina.

The retina is made up by millions of two types of specialised cells known as rods and cones. Rods are used for monochrome vision in poor light, while cones are used for color and for the detection of fine detail. Cones are packed into a part of the retina directly behind the retina called the fovea, which is responsible for sharp central vision. The retina transforms the image it gets into electrical messages to the brain in the form of electrical impulses. These impulses are sent to the optic disk on the retina where they get transferred by a further set of electrical impulses along the optic nerve, to be processed by your brain.
 

Check out each part of the eye and its function in more detail below.
 

The four most common problems with vision:

Astigmatism: A defect in the eye caused by non-spherical curvature 
Myopia: Short Sightedness
Hyperopia: Long Sightedness
Presbyopia: Age related Long Sightedness

Most people will develop presbyopia in their 40s or 50s, and start needing reading glasses. The reason is that with age, the lens of the eye gets denser, making it harder for the ciliary muscles to bend it. 

The leading causes of actual blindness include cataracts (clouding of the lens), age-related macular degeneration (deterioration of the central retina), glaucoma (damage to the optic nerve), and diabetic retinopathy (damage to retinal blood vessels). 

If you have an eye disease or condition, such as cataracts or glaucoma, there’s no need to worry. Most eye conditions can now be treated or controlled, especially if caught early.
 

If you want to talk to us about the health of your eyes make an appointment today.

Call us on 0800 99 2020

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The different parts of your eye and what they do

Aqueous humour 
Aqueous means water, and humour means fluid.  This watery stuff fills the front of the eyeball around the lens.

Blind spot
This is a bit of your retina that is not sensitive to light because there are no rods or cones there. It is the spot where the optic nerve is joined on to the retina.

Choroid
The choroid is the middle layer of the eye between the retina and the sclera. It also contains a pigment that absorbs excess light to prevent blurring of vision.

Ciliary Body
The ciliary body is the part of the eye that connects the choroid to the iris.

Ciliary muscles 
These are a circle of tiny muscles around the lens. They change the shape of the lens by squeezing and relaxing. They squeeze (making the lens fat) to look at nearby objects, and relax (making the lens thinner) for far away objects. 

Cone cells
One of the two types of light sensitive cells in the retina. The human retina contains 6-7 million cones; they function best in bright light and are essential for acute vision (receiving a sharp accurate image). The area of the retina called the fovea contains the greatest concentration of cones. It is thought that there are three types of cones, each sensitive to the wavelength of a different primary colour - red green or blue. Other colours are seen as combinations of these primary colours.

Conjunctiva 
This is the lining on the inside of your eyelid and the outside of the front of your eye (except for the special skin of the cornea). You can see some tiny blood vessels on the conjunctiva over your eye. If your eyes get sore, these blood vessels get bigger and your eye looks red.

Cornea 
This is the see-through skin that covers the front of your eye. It is clear like glass and it has no blood vessels in it.

Fovea
A small indentation at the centre of the macula and is described as the area with the greatest concentration of cone cells, the
messages encoded at the centre of the fovea will be interpreted by the brain in the form of a visual image.

Iris
The iris controls the amount of light that enters the eye. The iris is the coloured part of your eye. It forms a coloured muscular
diaphragm across the front of your lens.

Lens
The lens focuses light onto the retina. It changes shape to make sure that the 'picture' on the retina is as clear as possible.

Macula
The yellow spot on the retina at the back of the eye, which surrounds the fovea, the area with the greatest concentration of cone
cells, and is therefore the area of greatest acuity of vision. When the eye is directed at an object, the part of the image that is
focused on the fovea is the image most accurately registered by the brain.

Optic Disk
Is the visible portion of the optic nerve also found on the retina of the eye. The optic disk identifies the start of the optic nerve
where messages from cone and rod cells leave the eye via nerve fibres to the optic centre of the brain. This area is also known
as the 'blind spot'.

Optic nerve
This nerve is a continuation of the retina, leaving the eye at the optic disk, and it transfers all the visual information it gets to the
brain, via millions of nerve fibres branching from the rods and cones. It's a bit like the cable that carries all the TV pictures from
your aerial to your TV so that you can see the programs.

Pupil
This is the hole in the coloured iris. It lets light into your eye. It gets very small in bright light, and bigger in dull light.

Retina
The retina works much in the same way as film in a camera. It is a light sensitive layer that lines the interior of your eye. It is made
up of light sensitive cells known as rods and cones. The human eye contains about 125 million rods, which are necessary for
seeing in dim light. Cones on the other hand function best in bright light - there are between 6 and 7 million in the eye - they are
essential for receiving a sharp accurate image; cones can also distinguish colours. They turn the picture they receive into an
electrical message for the brain, which in turn translates that image to you.

Rod Cells
One of the two types of light-sensitive cells in the retina. There are about 125 million rods, which are necessary for seeing in dim
light. They contain a pigment 'rhodopsin' (or visual purple) which is broken down in the light and regenerated in the dark.
Breakdown of visual purple gives rise to nerve impulses when all the pigment is bleached (i.e. in bright light) and the rods no
longer function; this is when the cones are activated.

Sclera
This is the tough skin that covers the outside of the eyeball (except for the see-through cornea). A protective coat that we call
the 'white' of the eye.

Tear glands
These are small glands inside your upper eyelid. Their job is to make tears to keep the surface of your eyeball clean and moist,
and help protect your eye from damage. When you blink, your eyelids spread the tears over the surface of the eye. Small things
that are on your eye (like specks of dust) wash into the corner of your eye next to your nose. Sometimes tears flow over your
lower eyelid (when you cry, or you have hay fever), but mostly the tears flow down a tiny tube at the edge of your lower eyelid,
next to your nose. (If you look very carefully you can see a tiny dot that is the beginning of that tube). This tube carries the tears
to the back of your nose (this is why your nose 'runs' when you cry)

Vitreous humour
This is a thicker jelly-like liquid that fills the larger part of the eyeball and keeps it in shape. (Vitreous means glassy, because the
vitreous humour is very clear, so that light can pass through it).
 

We told you the eye was amazing.

If you want to talk to us about the health of your eyes make an appointment today.

Call us on 0800 99 2020

Book now