The structure of the eye
Sight is a series of events triggered by light, which involves the eyes and the brain. Light reflected from an object passes through the clear cornea at the front of the eye and is focused by the lens onto the retina at the back of the eyeball. The retina is a layer of light-sensitive cells called rods and cones. Rod cells are concentrated around the edge of the retina. They help us to see things that aren't directly in front of us (peripheral vision).
They also help us to see in dim light and to recognise movement. Cone cells are found in the centre of the retina, where light is focused by the cornea and lens. This area is called the macula. Cone cells give us detailed vision, which we use when reading or watching television. They are also responsible for most of our colour vision. Rod and cone cells collect light waves and send them as electrical signals to the optic nerve at the back of the eye.
This is made up of thousands of nerve fibres that pass electrical signals to the brain. It is the brain that processes this information and interprets what we see.
Other structures in the eye include:
- Iris €“ the coloured, circular part of the eye in front of the lens, which controls pupil size
- Pupil €“ the circular opening at the centre of the iris, which controls the amount of light that enters the eyes
- Conjunctiva €“ the mucous membrane that lines the eyelids and covers the front of the eye
- Sclera €“ the white part of the eye, which acts as a tough coating, protecting the outer eye
- Aqueous humor €“ the clear, watery fluid that fills the front part of the eye between the cornea, lens and iris
- Vitreous body €“ a colourless mass of soft, gel-like material that fills the eyeball behind the lens.
Reflective exercise
Helen, 29, comes to the pharmacy complaining that her eyes feel dry and gritty. She wears contact lenses and takes an oral contraceptive. Her job involves working at a computer in an office that is often air conditioned during the summer. She asks for some drops. Her eyes are not red or sore.
What would you recommend?
This sounds like a mild case of dry eye, which occurs when the eyes do not make enough tears, or the tears evaporate too quickly, causing the eyes to become dry and irritated. Explain to Helen that taking oral contraceptives, staring at a screen in a dry environment and wearing contact lenses are all risk factors for dry eye. Suggest that Helen tries lubricating eye drops. These supplement natural tears and make the eyes feel more comfortable. Ask Helen which type of contact lenses she uses and check compatibility with your range of eye drops. Although Helen can use drops frequently, she should consider a different product (perhaps an ointment for night-time use) if she needs to apply them more than four to six times a day. Working at a computer screen often means that people blink less frequently, so Helen may benefit from making a deliberate effort to blink more. She could also use a humidifer in her office to moisten the air.
What if:
Helen returns to the pharmacy with the same gritty sensation in her eye and has recently started developing redness and itching. Helen has had hayfever before, but usually suffers from nasal symptoms.
This sounds like allergic conjunctivitis. Ask Helen if she has been exposed to any allergens recently, such as pet dander, dust mites or pollen. Smoke or ingredients in personal products like shampoo can also irritate the conjunctiva. A bacterial or viral infection could also be the cause. Refer to the pharmacist if you're unsure.
What if:
Helen mentions that she's been spending a lot of time outdoors recently and suspects her symptoms may be due to hayfever. Recommend she tries eye drops containing sodium cromoglicate.
What if:
Helen comes back to the pharmacy two weeks later. Her symptoms haven't improved, but also haven't worsened. Refer Helen to the pharmacist. Her symptoms may not have been brought on by an allergy and could be the result of wearing contact lenses that aren't suitable for her.
What if:
Helen also asks for an OTC treatment for her 39-year-old boyfriend. She mentions that he has developed crusty eyelashes, itchy eyelids and sore, stinging eyes as a result of his dandruff. She thinks a shampoo for severe dandruff will do the trick. Helen's boyfriend will need to come in to the pharmacy and be seen by the pharmacist. His symptoms sound similar to blepharitis, which can be caused by seborrhoeic dermatitis or a bacterial infection. In the meantime, he should practise good, daily eye hygiene to help manage his symptoms.