Anatomy of the human eye

Parts of the human eye 

The human eye is a sensory organ, sitting in the socket called the orbit, surrounded by skin, muscle and a strong layer of tissue. There are many parts of the eye that work together to make it work and, if looked after properly, protect it from some sight-threatening diseases and injuries. An adult eyeball is only about 24 mm across (about the size of a large marble), yet it holds everything you need to see.

Here's what each part does, from the front of the eye to the nerve that reaches your brain:

Sclera 

The sclera is the white part of the eye, a fibrous layer consisting of collagen that is continuous with the cornea. The sclera surrounds most of the eye, front and back, and offers protection to the eye whilst conserving its structure and protecting it from injury.

Cornea

The cornea is the transparent dome shape that is the outer cover in front of the iris and pupil, behind a thin tear film. The main function of the cornea is vision. As it is one of the first areas the light reaches when it enters the eyes, it is responsible for most of our eyesight. If the cornea is too flat, too curved or misshapen, it causes refractive errors, such as short or long-sightedness and astigmatism. Astigmatism happens when the cornea is shaped more like a rugby ball than a football. Toric contact lenses (shaped to match an uneven cornea) sit stably on your eye and bring your vision back into focus.

Iris

The iris is the brown, blue or green coloured part of the eye. It sits behind the cornea, in front of the lens, in a ring around the pupil. The iris adjusts to different light settings, allowing the right amount of light through the pupil. It dilates in darker light conditions and constricts in brightly lit environments or when looking at close objects. In bright light your pupil can shrink to about 2 mm across. In the dark it can open to around 8 mm to let in more light.

Function of the iris 

The iris adjusts to different light settings, allowing the right amount of light through the pupil. It dilates in darker light conditions and constricts in brightly lit environments or when looking at close objects.  

Lens 

The crystalline lens is the transparent tissue that sits directly behind the pupil. When we are born, the lens is flexible and reshapes to allow the eyes to focus on varying distances. As we age, the lens loses its elasticity, which leads to the inevitable ageing condition presbyopia. If focusing up close starts to feel harder (the everyday sign of presbyopia), multifocal lenses correct near, middle, and far vision in a single lens.

Vitreous humour

The vitreous humour is a gel that sits behind the lens, at the back of the eye. It contains a combination of water, collagen, and proteins. It helps the eye hold its round shape, which protects it from knocks and injury. It takes up two thirds of the eye's structure and attaches to the retina.

Retina

The retina is a light-sensitive tissue, lining the back of the eyes. This tissue works as a receptor for light. Like a film camera, the retina processes light through photoreceptor cells to detect colour and different lighting, which helps the optic nerve to send impulses to the brain, turning into images.  

Function of the retina 

This tissue works as a receptor for light. Like a film camera, the retina processes light through photoreceptor cells to detect colour and different lighting, which helps the optic nerve to send impulses to the brain, turning into images.  

Optic nerve 

The optic nerve is the biggest nerve of the eye. It sits at the very back of the eye and connects to the brain. The optic nerve sends signals and impulses to the visual cortex of the brain, which is responsible for our vision. These impulses are transmitted through millions of fibres in the optical nerve.

How do the eyes work? 

Most of us never stop to think about it. But turning light into a clear picture is a fast, clever process. Here's how your eye does it, step by step. The various structures mentioned are all necessary to work together for us to perceive sight. If any of these parts are not working properly, due to eye conditions, diseases or injuries, our vision is affected.

Vision

The first step to eyesight is when light passes the thin tear film and through the cornea, where the eye first focuses. After the cornea, the light goes through the pupil (the opening of the iris), controlling how much light is let through.

The lens then works with the cornea to focus the light correctly, as it passes through the vitreous humour to the retina.

The retina forwards the received image through the optic nerve to the brain. Your brain reads that information and shows you the picture, which is your eyesight.

Colour perception

Most people see the world in full colour, and they have their retina to thank for it.

The photoreceptors in the retina are equipped with two types of sensory cells, rods, and cones. The rods are responsible for processing changes in brightness and vision in low light environments, while variants of cones oversee processing colour vision.

The different variations of cones are each responsible for perceiving light that reacts to colour with varying wavelengths. In rough terms, the shortest waves we see are around 450 nanometres (nm) and look blue. Waves around 530 nm look green. The longest, close to 700 nm, look red. The cones recognise each colour's wavelengths and pass this information to the brain.

Looking after the parts of your eye

Your eyes cope with a lot every day, from screens and sunshine to the odd stray eyelash. Every part has its own job. Some sit more in harm's way than others, so a few simple habits go a long way.

Cornea

The cornea is the most exposed part of your eye. At only about half a millimetre (0.5 mm) thick, it is more delicate than it looks. A knock or a scratch can irritate the surface. Allergies to pollen, pets, or dust can leave your eyes watering, itchy, or red. To keep the surface comfortable, wash your hands before you touch a lens, clean your lenses with a fresh contact lens solution, and give tired eyes a rest.

Retina

The retina sits right at the back of the eye. It is well protected from knocks and scratches, but it is still sensitive. You can't see or feel changes here the way you would a sore, gritty cornea. So a regular eye test is the best way to keep an eye on it. An optician can spot changes in your retina long before you would notice them yourself.

Keeping your eyes comfortable

Good eye care is mostly simple. Wear clean, well-fitting lenses, take breaks from your screen, and rest your eyes when they feel tired. Whatever your eyes need, our eye care guides can help you wear your lenses happily. And if anything ever feels off, speak to your optician.