Eye and vision

What are the main parts of the eye and their function?

The eye is the organ of sight. The eye receives images and sends them to the brain via nerves. There are eight main parts to the eye: cornea, iris, lens, macula, optic nerve, pupil, retina and vitreous.

Cornea: the clear front window of the eye. The cornea transmits and focuses light into the eye.

Iris: the coloured part of the eye. The iris helps regulate the amount of light that enters the eye.

Lens: the transparent structure inside the eye that focuses light rays onto the retina.

Macula: a small area in the retina that contains special cells that enable fine details to be seen clearly.

Optic nerve: the nerve that connects the eye to the brain. The optic nerve carries the impulses formed by the retina to the brain, which interprets them as images.

Pupil: the dark centre in the middle of the iris. The pupil determines how much light is let into the eye. It changes size to accommodate for the amount of available light.

Retina: the nerve layer that lines the back of the eye. The retina senses light and makes impulses that are sent through the optic nerve to the brain.

Vitreous: the clear, jelly-like substance that fills the middle of the eye.

How does vision work?

The eye receives information from reflected light – for example it can only make sense of what is in a room if there is light, not if the room is pitch black.

Light enters the eye through the cornea, the clear bulge at the front of the eyeball. The cornea directs the light into the pupil in the middle of the iris. Depending on the amount of light available to the eye, the iris will contract or expand, making the pupil smaller or larger, to limit the amount of light required. The more light that is available, the smaller the pupil becomes; restricting the amount of light entering the eye. The less light available, the larger the pupil becomes; allowing more light to enter the eye.

Once the light passes through the pupil it goes through the lens, which sits behind the iris. Light then passes through the vitreous (the clear gel that fills the inside of the eye) through to the retina. The macula sits at the centre of the retina and processes fine detail into accurate vision.

The retina contains light receptors known as rods and cones. These light receptors allow the retina to convert light into electrical impulses, which are then transmitted along the optic nerve to the brain and processed into vision. The optic nerves from both eyes join together in the brain in such a way that images from both eyes are merged to give binocular vision (a single image created from both eyes).

The part of the brain which receives these visual messages is called the visual cortex and lies at the very back of the brain.

References and further reading

Mary D. Allen Laboratories (n.d.) Basic eye anatomy. EyeSightResearch.org http://www.eyesightresearch.org/background.htm
Bionic Vision Australia (n.d.) How does healthy vision workhttp://bionicvision.org.au/eye/healthy_vision
Fred Hollows Foundation (2012) Anatomy of the eyehttp://www.hollows.org/en-au/au/eye-health/the-eye

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