Eye Anatomy

Magnificent, complex and hardworking, our eyes are amazing organs. Our sight helps us navigate our way through the world, but sadly, it’s something many of us take for granted.

The anatomy of your eye

Before you can truly understand eye diseases, it helps to learn a little about the eye itself. Despite being small (on average, each eye is around 2.5cm in diameter), it’s one of our most important organs. Our eyes have a lot of working parts and each part has a complex role in helping us see.

How do our eyes work?

Our eyes are like tiny cameras that process the light reflected off surfaces to create images we see. The iris (a muscle that acts like a lens) controls the size of the pupil (similar to aperture). If the light entering our eye is too bright, our iris reduces the size of the pupil. When it’s dark, the iris in the pupil is enlarged or dilated, to maximise the amount of light entering the eye.
After receiving the correctly focused light, the retina’s job is to analyse colour, intensity and form to transmit these as electric impulses to the brain. The optic nerve connects various parts of our brain so that our emotions, experiences and visual impulses are combined together as an image that we not only see, but actually perceive. The image from the retina is actually upside down: our brains flips the image around so we don’t get confused.

Depending on how near or far away the object is that we’re looking at, the cornea and lens work together to bring the light into focus on the retina (the back of the eye). The lens is the more dynamic of the two, especially in young people, changing in its shape (thickness and curvature) to help us shift focus. When the light from an image isn’t perfectly focused, this can often be remedied with the assistance of appropriate glasses or contact lenses.
Cornea: this is the clear front ‘window' of the eye. It continues backwards to form the tough outer fibrous coat of the eyeball. Visible at the white part of the eye, this coat is called the sclera.
Iris: this acts as a gatekeeper to control the amount of light entering the eye. The iris also gives the eye its beautiful colour.
Lens: a clear specialised protein structure which helps focus images by adjusting the eye's focusing power according to how near or far away an object is.
Vitreous gel: this is a clear, firm jelly which forms the main bulk of the eye between the lens and the retina, and helps to support its internal structure.
Ciliary body: this muscle changes the shape of the lens to allow for focusing. It’s also a gland which produces a watery fluid called the aqueous humor. The balance between the production and drainage of this fluid is one of the key factors that creates the pressure to which the eye is ‘pumped up'.
Retina: delicate and finely layered nerve cells which line the inside of the eye. The retina receives light (like the film in analog cameras) and converts it into electrical signals for transmission to the brain.
Macula: this contains the greatest concentration of light-sensitive cells, called photoreceptors, allowing us to see things in great detail and receive colour signals.
Optic nerve: this nerve is formed from all the fine nerve fibres that originate from the photoreceptors in the retina. These fibres gather into a cable-like bundle and exit the back of the eye through small openings in its tough outer coat. The head of this optic nerve is viewed through the pupil as a structure called the optic disc.


How disease affects the eye

Various eye diseases affect the structure and/or the functioning of different parts of the eye. The end stage of many eye diseases is vision loss or blindness. However, many eye diseases are treatable, and the vision loss that can result is preventable. This requires early diagnosis and management before too much damage is done.

The cornea can be affected by infections such as trachoma, and by nutritional problems such as Vitamin A deficiency. Both conditions can severely scar the cornea. Once scarred, the cornea loses its transparency and light cannot enter the eye, resulting in vision loss.  
Cells within the lens are constantly being generated and dying. Being captive within the confines of the lens capsule, they get compacted, and the build-up of protein in the lens that occurs with age can make it cloudy. This cloudiness is called cataract and can affect one or both eyes. Besides ageing, injury, genetic disorders and certain medications can also result in cataract formation. Whatever the underlying cause, transmission of light into the eye is impaired as the lens becomes less transparent, and this leads to visual symptoms such as glare, decreased contrast sensitivity and even vision loss.
The retina can be affected by many diseases. Common ones are diabetes and age-related macular degeneration.
If the pressure within the eye is too high for the delicate retina to withstand, nerve fibres are lost. This condition is called glaucoma and it is recognisable only through specialised tests, including of the field of vision. Damage occurs in the periphery, and gradually closes in towards the centre, which is why this condition is often called the silent thief of sight.

Our work with eye disease

The Fred Hollows Foundation works with partners in developing countries to screen eyes for any sign of disease and then provide treatment accordingly.
We mainly work to reduce the impact of diseases such as cataract, trachoma and diabetic retinopathy, for which safe and cost-effective ways of preventing vision loss or providing treatment to restore sight are available. However, our work isn’t exclusively on these diseases. In 2014, through our comprehensive eye care programs, we also identified and supported the provision of care for over half a million people who were suffering from other eye conditions.

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