The Eyes and Vision

Structure of the eye

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.

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.

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.

Some disorders of the eyes and vision

Details of the following disorders can be found at www.patient.co.uk/display/16777233/

• 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

Further more detailed information available free online

The Organ of Sight

From Gray's Anatomy Online
Web: www.bartleby.com/107/224.html

Special Senses And Disorders

From Textbook in Medical Physiology And Pathophysiology
Web: www.mfi.ku.dk/ppaulev/chapter5/cap5_bog2.htm

Human Physiology/Senses

From Wikibooks, the open-content textbooks collection
Web: http://en.wikibooks.org/wiki/Human_Physiology/Senses