Anatomy of the eye

The eye

Anatomy of the eye.The eye is the organ of sight.

It is a small paired organ, each eye being a leathery sphere of about 2.5cm diameter.

The function of the eye is to target an object of interest, gather and focus light from the object and transmit a clear image to the light-sensitive tissues which line the back of the eye where the image is received and initially processed.

The image is then transmitted by electrical impulses along the optic nerve, a nerve which connects the back of the eye to the brain. The optic nerves join together in the brain in such a way that images from both eyes are merged to give binocular vision.

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

From the visual cortex, connections reach out to many other parts of the brain. The experience of sight is the result of very high-level processing of the basic electrical impulses which are the raw input from the eyes themselves.

Important tissues of the eye

Eye anterior chamber cross-section with lens, angle, ciliary body, iris and cornea. Cornea: This is the clear front ‘window' of the eye, where its dense leathery wall is specialised to make it perfectly clear and allow clear images to enter the inside of the eye. It also acts as a powerful lens to refract or focus the light from an object.

Iris: Acts as a diaphragm or circular shutter to control the amount of light entering the eye. The iris gives the eye its beautiful colour.

Lens: This is a clear specialised protein structure which helps focus the images and adjusts the eye's focusing power according to whether the object being viewed is close or far away.

Vitreous Gel: This is a clear, firm jelly which forms the main bulk of the eye and helps to support its internal structure.

Ciliary Body: This is a muscle which changes the shape of the lens to allow clear fine focusing. It is also a gland which produces a watery fluid, the aqueous fluid. The balance between the production and the drainage of the aqueous is what determines the pressure to which the eye is ‘pumped up'.

Retina: This is the nerve tissue which lines the inside of the eye. It consists of very finely layered and delicate nerve tissue. The central part of the retina is the part we use when we look at something. This part of the retina is called the macula. It contains the greatest concentration of light-sensitive cells, called photoreceptors.

Optic Nerve: This structure is formed from all the fine nerve fibres which come from all over the retina. Where they all meet, they are gathered into a bundle which exits the eye through a fine grid of tiny holes in the wall of the eye and then extends back, like a fine cable, taking the impulses to the brain.

All of these tissues are finely structured and most of them are quite delicate except for the outer wall of the eye which is quite tough. The other tissues need protection, so the eye sits in a well-protected cavity in the face (called the orbit) where it is surrounded by bone which is rigid in parts and able to crumple in other parts. There are good reflex mechanisms to protect the eye and a good system of washing (with tears) and wiping the clear front surface (the eyelids' blinking action).

Common problems

Macula with normal eyeball anatomy. Cornea: Can be affected by trauma, by infections such as trachoma, and by nutritional problems such as Vitamin A deficiency (called Nutritional Blindness) which leads to gross abnormalities and dryness of the surface cells. Trachoma and nutritional blindness are major sight-affecting problems in developing parts of the world where many people live in poverty. Trauma is an important cause of vision loss in impoverished agricultural communities.

Lens: When the lens loses its clarity light transmission is affected severely because all light entering the eye is focused there. This loss of clarity is called cataract. It is the major cause of blindness in the world today. Cataract is not preventable but it is treatable.

Retina: Can be affected by many diseases. The main ones are an ageing change in the central part, the macula, called macular degeneration, and the effects of diabetes on the retina, called diabetic retinopathy.

Macular degeneration is the main cause of untreatable visual loss in developed countries and is very difficult to treat. Diabetic retinopathy can be treated with laser but it must be identified early before it becomes too advanced to treat. It is becoming a rapidly increasing problem worldwide, as diabetes becomes more common.

Optic Nerve: If the pressure within an eye is too high, it can damage the optic nerve at the point where it leaves the eye. This condition is called glaucoma. There are various common ways for the pressure to become abnormally high. People's susceptibility to this damage is highly variable. Glaucoma is a major cause of blindness and visual disability throughout the world.

All of these conditions can and do cause blindness.

Note: This information is general in nature and is not a substitute for specialist medical advice. Have your eyes checked regularly every two years, even if you have not noticed any symptoms or changes.

About the author

Dr David Moran, Ophthalmologist MB BS (Hons) FRANZCO

Dr David MoranDavid first met Fred Hollows in the mid 1970s, and worked with him on the ground-breaking National Trachoma and Eye Health Program. David has been involved with The Foundation since its earliest days, and was a board member for five years until May 2010. A practising ophthalmologist, David has presented at numerous conferences and authored many papers and publications related to eye health.

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