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Friday, June 3, 2011

The Eyes and Vision

This article gives a brief overview of the eyes, eye function, and how we see.

Structure of the eye

Cross-section diagram of the eye (185.gif)Diagram detailing the macula (190.gif)

The coloured part of your eye is called the iris. The iris is made up of muscle fibres which help to control the size of the pupil. The pupil is not an actual structure but the circular opening in the middle of the iris. The pupil appears as the dark central part of the eye. The pupil can change size depending on the amount of light going through it. In darkness your pupils will get bigger to allow more light in.

The retina is a layer of the eyeball, found on its back wall. It contains highly specialised nerve cells which convert the light received into electrical signals to be passed to the brain. Near the centre of the retina is the macula. The macula is a small highly sensitive part of the retina. It is responsible for detailed central vision. It contains the fovea, the area of your eye which produces the sharpest images.

The white of your eye is called the sclera. This is a protective layer which covers all of the eyeball except the cornea. The cornea is a transparent layer which allows light to enter the eye. Beneath the sclera is the choroid. The choroid is another layer of the eye, which lies between the retina and the sclera. Its function is to provide oxygen and nutrients to the retina below. In order to do this the choroid has many blood vessels within it. At the front of the eyeball the choroid connects with the ciliary body.

In order for an object to be ‘seen’ the light coming from the object must hit the retina. Structures in the eye are needed to make sure that light entering the eye reaches the retina and is focussed. The cornea and lens help to do this by bending light entering the eye. The cornea gives an initial bend to the light. But the lens is the 'fine tuner'. The lens can change shape with the help of the ciliary body which contains fine muscle fibres. Depending on the angle of the light coming into the eye, the lens becomes more or less convex (curved) to focus the light correctly onto the macular on the back of the eye. This is very similar to a lens in a camera which focusses the light onto the film. The optic nerve carries the electrical impulses created in the retina to the brain.

The eye needs to keep its shape so that light rays are focussed accurately onto the retina. So, most of the eye is filled with a substance like jelly called the 'vitreous humour' (humour meaning fluid). The front of the eye is filled with a clear fluid called 'aqueous humour', which is more watery.

The aqueous humour is made continuously by cells that line the ciliary body. The fluid circulates through the front part of the eye, and then drains away through an area called the 'trabecular meshwork', which is located near the base of the iris. So, there is constant production and drainage of fluid.

Eye movements

The movement of each eye is controlled by six muscles that pull the eye in various directions. For example, to look left, the lateral rectus muscle of the left eye pulls the left eye outward and the medical rectus of the right eye pulls the right eye inward towards the nose. Levator palpebrae superioris opens the upper eyelid.
eye muscles side view (315.gif)

The eyelids

The upper and lower eyelids help to protect the eye, and keep its surface moist. The upper eyelid is more movable and is attached to a special muscle, called levator palpebrae superioris. This muscle allows you to control the movements of the upper eyelid. Eyelids help to spread the tear film across the eye. They also contain a special oil which slows down the evaporation of the tear film.

The eyelids are made up of several different layers including the conjunctiva. The conjunctiva is the thin layer you see on the inside of your eyelid, which makes contact with the eyeball itself. The surface of the eyeball also has its own conjunctiva. When the blood vessels in this conjunctiva become enlarged they can be seen, giving a ‘bloodshot’ appearance. Eyelashes help to stop debris and direct sunlight from entering the eyes.

Tear formation
To avoid damage to the sensitive surface of the eye it needs to be kept moist. The eyes are in constant contact with your eyelids. Without some form of lubrication, the friction created between the two layers of conjunctiva would cause rubbing. To prevent this, and to help remove debris, the eye produces a tear film. The tear film is made up from three layers - the main middle watery layer, the thin outer lipid (oily) layer, and the thin inner mucus layer.
eye with eyelid detail (263.gif)tear film (264.gif)

Diagram of the eye and tear production (180.gif)
The main middle watery layer is what we may think of as 'tears'. The watery fluid comes from the lacrimal glands. There is a lacrimal gland just above, and to the outer side, of each eye. The lacrimal glands constantly make a small amount of watery fluid which drains onto the upper part of the eyes. When you blink the eyelid spreads the tears over the front of the eye.

Tiny glands in the eyelids (meibomian glands) make a small amount of lipid (oily) liquid which covers the outer layer of the tear film. This layer helps to keep the tear surface smooth and to reduce evaporation of the watery tears.

Cells of the conjunctiva at the front of the eye and inner part of the eyelids also make a small amount of mucus-like fluid. This allows the watery tears to spread evenly over the surface of the eye.

The tears then drain down small channels (canaliculi) on the inner side of the eye into a tear 'sac'. From here they flow down a channel called the tear duct (also called the naso-lacrimal duct) into the nose.

Tear formation in people can also occur in response to emotion. When this happens the lacrimal glands simply produce more lacrimal fluid which spills over the eyelids.

How does the visual system work?

When you look at an object you see it because light reflects off the object and converges (to focus or come together) on the layer of the eye called the retina. The eyes receive light from many different directions and distances. To be seen, all light must focus on the comparatively tiny area of the retina. This means the eyes have to bend light from different angles and directions so that it comes together on that very small part of the eye.

Firstly light passes through the transparent cornea. Most bending of light occurs here. Light then travels through the pupil and hits the lens. The lens also bends light, increasing the amount focussed on the highly specialised cells of the retina. In myopia (short-sightedness) rays of light focus on a point before the retina. This means that distant objects can’t be seen clearly. In hypermetropia (long-sightedness) light converges on a point behind the retina. This means nearby objects can’t be seen well.
eye focusing (314.gif)

The retina is made up of millions of light sensitive nerve cells called photoreceptors. These cells help to turn the light into electrical signals which are sent to the brain to be interpreted. Photoreceptors contain special chemicals which are changed when light hits them. This change causes an electrical signal which is sent to the brain via the optic nerve. Different types of photoreceptor allow us to see in a huge range of different conditions, from dark to light, and all the colours of the rainbow. Photoreceptors called rods help us to see in dark light, whereas cones help with colour vision and in bright light.

The electrical signals travel to a part of the brain called the thalamus via the optic nerve. The thalamus acts as a relay station, sending on the information received from the optic nerve to an area of brain called the visual cortex. The visual cortex is a specialised part of the brain which processes visual information. Located at the back of the head, it interprets the electrical signals to get information about the object's colour, shape and depth. Other parts of the brain put this information together to create the whole picture.

Some disorders of the eyes and vision

  • Acute Glaucoma
  • Amblyopia
  • Blepharitis
  • Cataracts
  • Chalazion
  • Conjunctivitis
  • Corneal Injury
  • Dry Eyes
  • Ectropion
  • Entropion
  • Glaucoma (Primary Open Angle)
  • Herpes Simplex Eye Infection
  • Macular Degeneration
  • Retinal Detachment
  • Short Sight - Myopia
  • Sjögren's Syndrome
  • Squint (Childhood)
  • Stye
  • Subconjunctival Haemorrhage
  • Tear Duct Blockage of Babies
  • Uveitis and Iritis
  • Watering Eyes



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Disclaimer:

Disclaimer: These articles is for information only and should not be used for the diagnosis or treatment of medical conditions. we used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions.