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
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.
Myopia: Myopia also known as nearsightedness, is when people can see things that are close to them, but not
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
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
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
Some related web sites are
Contact lens and
Eyes and Vision Topics.