Home > Health > Contacts - An Introduction Part I

Contacts - An Introduction Part I: Eyes and Vision

Contact lenses are made of one or another type of polymer. They are placed directly on the eye to correct vision. In this article I will first discuss a few of the basic properties of the eyes and vision and then give a little bit of history of contacts.

Parts and Functions of the Human Eye

Human eyes have a diameter of about one inch and weigh only one fourth of an ounce.

Cornea: The cornea is the clear, outside front part of the eye. It is a lens that refracts (bends) light as it passes through it and it responsible for most of the eye's ability to focus.

Sclera: The sclera is the 'white part' of the eye, it helps to protect the surface of the eye.

Iris: The iris is the colored part of the eye, its function is to open and close to control the amount of light that is let into the eye.

Pupil: The pupil is the center, black part of the eye. It is actually a hole that light goes through, and it changes size, depending upon the movement of the iris. When the pupil gets large, more light is allowed in the eye. When the light is really bright, the pupil gets smaller to keep from too much light getting in and damaging the eye.

Crystalline Lens: The crystalline lens is a transparent tissue behind the pupil that works like a magnifying glass and flexes so that we can see something close-up. As people get older this lens gets harder and cannot flex as well. This is why most people over 45 years old need glasses or contacts to be able to read.

Conjunctive: The conjunctive is a membrane that lines the inside part of the eyelids and the sclera. It allows us to up and down and from side to side as well give us some protection from foreign particles.

Retina: The retina is a layer of nerve cells which are sensitive to light. It contains the cells known as rods and cones which convert light into electrochemical impulses that get sent to the brain. The cone cells are involved with seeing in color and the rods are involved with seeing in low light.

Macula: The macula is an area in the middle of the retina and in the middle of the macula is the fovea. The fovea is a small pit that is the main focal point of the eye.

Optic Nerve: The optic nerve sends signals from the retina to the brain.

Vision Problems

Myopia: Myopia also known as nearsightedness, is when people can see things that are close to them, but not things that are far away. This can be caused by either of both of two things. One, having an eyeball that is longer - from the front of the eye to the back - than normal. This results in the image of the object being blurry. The other cause is when the cornea's curvature is 'over bulged' - too steep. Over 25% of people have myopia. It usually starts when people are young - between 8 and 12 and is hereditary.

Hyperopia: Hyperopia is also known as farsightedness and can be caused by the eyeball being too short. It can also be caused by the cornea not being curved enough - being too flat. It is common for very children to have a little bit of hyperopia, but they grow out of it.

Presbyopia: Presbyopia - aging eyes - it caused when the crystalline lens loses its flexibility and focusing on close-up things becomes difficult. It usually start between 40 and 50 years old.

Astigmatism: Astigmatism - is when the cornea tends to have uneven curvatures. Most people have at least a little bit of astigmatism, but not always enough to cause a problem. Glasses, hard contacts or toric soft contacts can be used to correct it.

Other articles in this series are: Contacts - An Introduction Part II: Contact Types and History,
Soft Contacts - Daily Wear and Disposable,
Rigid gas permeable (RGP) Contacts,
Daily Wear Contacts,
Extended Wear Contacts and
Orthokeratology.

Some related web sites are Contact lens and Eyes and Vision Topics.