Vrajesh U. Patel
Ms. Griswell
LNG 341
21 November 2003
The Human Eye
The human
eye is a vital organ of the body. It collects light and turns it into electron messages that are sent to the brain. The brain
then turns those signals into a picture, which can be seen. The functions of the eye are possible through the main parts of
an eye; with help of all parts working of eye is possible. The organs of the eye are the cornea, the aqueous humor, the iris,
the pupil, the lens, the vitreous humor, and the retina. If one organ of the eye does not perform its work properly, refractive
errors will occur. Five major refractive errors include myopia, hyperopia, astigmatism, presbyopia and diabetic retinopathy.
Vision problems can be corrected, however, with eyeglasses, contact lenses, laser surgery, and artificial eyes.
The eye is a hollow sphere with clear gel-like material; it functions much like a camera.
It functions to change the lighting environment and to focus on distant objects, making it possible to see what that object
looks like. Light rays which are entering the eye are converted into impulses and sent to the brain which then produces a
picture. The cornea, a transparent layer of tissue, helps to bring objects into focus. With the help of the pupil, the cornea
bends the rays of light which are entering the eye. According to Hall, who is an ophthalmologist and also a good professor,
the pupil is the dark, round opening in the center of the colored iris. There is a lens behind the pupil which makes adjustments
in the path of the light ray so that it focuses upon the retina. Hall indicates that the retina is the membrane containing
the photoreceptor nerve cell that lines the inside back wall of the eye. The retina can be compared to the film of camera. This photoreceptor nerve cell make changes in the light rays, sends it to the brain
in the form of electrical impulses, and the brain predicts an image. When the optical system of the eye does not focus properly,
this error is known as a refractive error. A refractive error depends upon the curvature of the cornea and its ability to
bend the entering light ray! Humans can see only a small part of electromagnetic
radiation between thirty-eight and seven hundred and sixty nanometers (Hall).
The eye is divided into several parts. All of these parts together form the eye. If
there is a deficiency of even a single part, the result will be a vision problem or the loss of the entire eye. The different
parts of the eye are the cornea, the aqueous humor, the iris, the pupil, the lens, the vitreous humor and the retina.
The cornea is outermost layer of the eye. The
cornea is a transparent layer made up of tissues. The cornea is surrounded by
white, non transparent tissue called the sclera. Light can easily pass through it because of its transparency. There are no
blood vessels as the cornea is made up of several layers. The optical property is measured from the whole cornea rather than
a single tissue. The refractive index of the cornea is 1.3771. The cornea is .55 mm away along the axis from the corneal pole,
and its radius of curvature of posterior is six and half mm and anterior is 7.8mm (Hahn, Cornea).
The space between the cornea and the lens is filled with the aqueous humor. It is a clear
liquid like a gel. It is fairly homogeneous and, as a result, the optical properties are easily measured. This liquid fills
up the space between the lens and the cornea, and is known as the anterior chamber. The refractive index of the Aqueous Humor
is 1.336 (Hahn, Aqueous Humor).
The iris is directly behind the aqueous humor.
The iris gives color to the eye. The hole in middle of iris is called the pupil. The contraction of the iris makes the pupil
smaller, and relaxation makes the pupil larger (Hahn, Iris).
The pupil is in the center of the iris. The hole in the iris, which allows light to enter
the eye, is called the pupil. The pupil changes its size with respect to the amount of light and where an object is placed.
Alpern and Campbell assert that The pupil impulse appears to be triggered by a mixed cone and rod stimulation. The diameter
of the pupil along horizontal axis is two mm to eight mm (Hahn, Pupil).
Next to the pupil is the lens. The lens is the
focusing element of the eye that helps to focus the light ray which is entering to a particular point. The lens is curved
from both sides. The ciliary muscle changes the shape of the lens as necessary depending on the point where the total object
is placed. The lens is curved rather than flat for focusing on objects, but zonular fibers pull the lens and flatten its surface
and the ciliary muscle relaxes tension and makes the curved lens again. Accommodation is made as needed by the lens to let
the light ray enter. If a point is near, accommodation will be approx. four inches or ten cm. If a point is far, accommodation
will be infinity. If there is darkness, accommodation will be 26.4 inches of sixty seven cm. Myopia, hyperopia, and other
vision disorders are caused by unfavorable changes in the lens. The lens readily absorbs short wavelengths as blue rather
than other colors of the spectrum. The refractive index of the lens is 1.42 accommodate. The distance between the lens and
the cornea is 3.6mm -unaccommodated and
3.2 accommodated. Radius of curvature of the lens is 10.2 unaccommodated and 6.0 accommodated (Hahn, Lens).
Behind the lens, there is the vitreous humor
which takes empty space between the lens and the retina. Hahn explains that the vitreous humor is the clear liquid between
lens and the retina. The vitreous humor fills the space between the retina and the lens. Refractive index of the vitreous
humor is 1.336 (Hahn, Vitreous).
Finally, after the vitreous humor there comes
the main part of eye: the retina, the main part of eye that helps chiefly in seeing an object. The retina is a part of the
eye that responds to light. There are two types of photoreceptors or the cells that responds to light: the rods and the cones.
The retina is about 200-250 micrometer thick. The rod cells are more light- sensitive than the cone cells. The rods help accommodate
for changes from light to dark; they contain one type of pigment. In dark or dim rooms, human vision uses the rods primarily;
the rods are not helpful for color vision. There are many more rods than cones. There are approximately hundred and twenty
million rods. The cones are not as sensitive as the rods are to light, but they are sensitive to three main colors which are
known as primary colors. The primary colors are red, blue and green. The work of the cones is to send messages to the brain
which then translate messages as to the color and shape of the object. Only the rods are effective in dim light; the cones
are effective in producing bright color vision. There are about six million cones in human retina. Because of a defect of
the cones and the rods, some people can not determine colors. This is known as color blindness. The fovea is the part of the
retina that helps in providing the clearest vision. The cones and the fovea are closely located, compared to the other parts
of the eye. The fovea is created by blood vessels and nerve cells so that light has direct path to the photoreceptors. Some
people have a blind spot; this is because one part of the retina does not contain the photoreceptors. Thus the image of an
object that falls on this part of the retina cannot be seen by that person (Lippincott 24-27).
In order to predict image, the retina sends messages
to brain. The brain creates a signal that shows the shape and the color of the image. The process of image prediction begins
with the light entering the eye. The light is bent or refracted by passing through the cornea with the help of the pupil.
Then light rays pass through the lens to the vitreous and finally strike the light sensitive rod and cones cells in the retina.
The light rays fall on the retina, but the image form is inverted, so to have vertical and real image, it has to be sent to
the brain. Now the visual process starts in the retina. The rod and the cone cells turn the light rays into an electrical
activity called impulses. These electric impulses are carried through the optic nerve to the vision center of the brain. The
brain now interprets the image and, thus, the image: is predicted (Hubel 7-11).
In addition to functions of the brain and other
part of the eye, if there is deficiency in work of any one part lead to vision problems will result. There are five major
refractive errors that lead to vision problem, including myopia, hyperopia, astigmatism, presbyopia and diabetic vision.
The scientific name of nearsightedness is myopia.
This condition is common refractive error of the eye. St. Luke says, It occurs when light entering the eye focuses in front
of the retina rather than on the retina. In the nearsightedness, the cornea is steeper than that of the normal eye. Nearsighted
people typically see near things but have difficulty in seeing far away objects. This leads to blurred distant images on the
retina. This problem is often found in younger children between the ages of eight and twelve who have problems in reading
notes on a chalkboard. Nearsightedness is a result of using dim light, reading too much or deficiency of nutrition. Nearsightedness
is also sometimes related to heredity. Myopia can be corrected by eyeglasses or by contact lenses. The treatment also depends
upon the age, activities and occupation, and it can also be corrected by the eyeglasses, contact lenses and refractive surgery.
The refractive surgery can be done only at a certain age and under certain conditions, but even through the surgery cures
ninety percent of the problems, ophthalmologists prefer to avoid surgery as eyeglasses and contact lenses are better than
the surgery (St. Luke, The Nearsightedness).
Next to nearsightedness is farsightedness. The
scientific name for farsightedness is hyperopia. Farsightedness is a common refractive error of the eye. St. Luke says, It
occurs when light entering the eye focuses behind of the retina. In farsightedness, the cornea is flatter than that of the
normal eye. Farsighted people typically see things that are far away but have difficulty in seeing by near objects. This leads
to a blurred nearer image on the retina. This problem is often found in older people age 40 or above. The symptoms of the
farsightedness are headaches, pulling sensations, burning, and tiredness when reading. Farsightedness is also sometimes related
to heredity. Hyperopia can be corrected by eyeglasses or by contact lenses. Refractive surgery is done sometimes in older
patients (St. Luke, The Farsightedness).
After farsightedness their come another error
called astigmatism. Astigmatism is a refractive error. The cornea of an eye with astigmatism is oval like a football instead
of spherical like a basketball, and it is smooth. This kind of the cornea has both steeper curve and a flatter curve, and
this cause light to focus on more than one point. With astigmatism, one often sees blurred images. Astigmatism occurs along
with nearsightedness or farsightedness. Astigmatism can be corrected by the eye glasses or contact lenses. Astigmatism can
also be corrected by the surgery called astigmatic keratotomy which reshapes the cornea to the original shape. Most ophthalmologists
prefer using eye glasses and the contact lens rather than the surgery to correct the astigmatism, because there can be problems
while correcting the eye (Kellogg, The Astigmatism).
Lastly and after astigmatism there comes another
error know as presbyopia. Presbyopia is a refractive error. A person who is affected by presbyopia often has difficulty in
focusing the eyes for reading or close work. It usually affects people aged forty years or above. With presbyopia, one loses
the ability and capacity of changing the lens and the cornea as required focusing on an object. Presbyopia occurs suddenly
as the eye loses flexibility. Some symptoms of presbyopia include the tendency
to hold books at arms length, blurred vision when reading, eye fatigue, and headaches when doing close work. Bright light
or more direct light are needed for normal reading. The presbyopia is corrected with eyeglasses as necessary. Bifocal and
the trifocal lenses may correct presbyopia, but these lenses should be used only if the person had hyperopia or myopia or
the astigmatism before presbyopia. With this correction, a person will able to read and can see properly (Kellogg, The Presbyopia).
Next to all of above error this is error which
is due to diabetes. Diabetes is a disease that causes many problems, but it is especially hard on the eyes and can lead to
blindness among adults. Diabetes is a condition in which there is improper regulation and use of glucose for energy. This
problem occurs due to a failure in the production of insulin. Poorly regulated and high levels of sugar in the blood change
the optics of the eye, resulting in blurred vision. These problems can be seen after approximately twenty years of diabetes.
This type of effect on the eye caused by diabetes is called diabetes retinopathy. There are two stages in diabetes retinopathy.
The first stage is nonproliferative retinopathy. In this type, the small blood vessels become weak and leak fluid, resulting
in distortion of the retina. At this stage, a person has blurred vision. The next stage is proliferative diabetic retinopathy
which occurs when new abnormal blood vessels grow to replace old ones. This growth can result in severe visual loss or blindness.
Some common symptoms of diabetic retinopathy are blurred vision, sudden loss of vision, and floaters and flashes. Diabetic
retinopathy can mainly be cured by laser surgery. Laser surgery is one of the best ways to correct this problem. The vision
of a person who has diabetes can be restored by keeping diabetes in control. Thus this problem can be corrected by the laser
surgery at a high level and by controlling the diabetes at a low level or at the beginning of the problem. It can also be
cured by glasses and lenses (Watkins, Diabetes Retinopathy).
However there are three methods to cure vision
problems. They are laser method, glasses and contact lenses and the artificial eye.
One of the best ways to cure vision disorders
is through laser surgery. Laser surgery is known as Lasik. Lasik is a surgical procedure which helps in reducing a persons
dependency on glasses or lenses. This surgery changes the shape of the cornea permanently. To have a clear image, the cornea
and the lens of eye must bend as necessary as a result the image falling on the retina. If an image falls in front of or behind
the retina, one will observe blurry images. An imperfectly-shaped cornea or lens or eyeball creates blurriness which is known
as refractive error. The laser surgery uses an intensely hot, precisely-focused beam of light to remove tissue and control
bleeding to correct the shape for better focusing of the retina. This surgery is most often performed on people who have myopia.
Lasik surgery is also known as Laser-Assisted In Situ Keratomileusis and laser vision correction. Moreover in laser surgery,
the doctor will check the health of the eye. The eye must be in perfect condition to have the laser surgery; if the eye is
not, and then surgery cannot be proceed. This surgery takes ten to fifteen minutes and is done when person is awake. Anesthetic
eye drops are applied to completely numb the eye. An eyelid holder is placed to prevent blinking. A special type of knife
is used to cut a hinged flap of a corneal tissue. After entire surgery is performed, the surgeon replaces the flap by folding
it back into its place. It adheres naturally and securely without a need for stitches. The person should be at least eighteen
years old to perform laser surgery or, perhaps, even twenty one if recommended by the surgeon. One should not be pregnant
or nursing as those factors affect laser surgery and may create serious problems. Laser surgery has many risks which the surgeon
has to avoid. After surgery is completed, one may still need to wear glasses or contact lenses for better vision. The surgery
may create problems with night driving, a fuzzy image even if the object is at 20/20, permanent vision loss, flap complications,
light sensitivity or dryness. It also decreases the distance vision at high altitude (Feinberg).
Another way of correcting refractive error is
through glasses and contact lens both correct vision problems, but there are a huge differences between them. The distance
between the eye and glasses is about one cm. and this sometimes creates distortion and poor side vision. If the glasses are
heavy, their overweight may make spots on the face and on the ear. As temperature changes, glasses may have fog on them, which
leads to unclear images. Glasses are annoying to wear in rain and snow. One needs be careful while playing outdoor games.
There are problems, sometime when the frames do not suit a persons face. Glasses are an abnormal, distracting wall between
your eyes and the world. However, contact lenses are worn right on the eye so that one can see natural vision and there is
no problem with side vision as the eye moves. Contact lenses have negligible weight and, thus, there are no such problems
such as spots on the face because of overweight. There is no effect of temperature on lens so there is no way to create fog
on the lens. There is no distraction in lens, so there is no need to take care of it while playing outdoor games. There is
no question of matching contact lens, as they are transparent. Contact lenses dont detract from ones natural appearance. Both
glasses and contact lens need special care. The glasses need to be wiped or sprayed several times in a day, and handled carefully
when one is playing. The contact lenses need to be washed with a special solution. Wearing and removing lenses can be difficult
if one doesnt know how to perform that act (Clam).
Although we have some methods to correct refractive
error, today doctors and engineers are finding ways to develop artificial eyes. Presently, there exists only one type of artificial
eye called a prosthetic eye. An artificial eye is used for only cosmetic reason. The doctors and engineers are still in the
process of making artificial eyes better in a number of ways. Artificial eyes could allow blind people to enjoy life by making
it possible to see. There is hope that the artificial eyes will be available at low cost to every person in the year 2010.
Doctors make guidelines regarding how artificial eyes should be made. Following the guidelines, the eye must not affect the
eyelids or the muscles, and it should be lightweight. It must allow the eye to support the implant and to look it in place. The movements caused by the rectus muscles must be controlled by an artificial eye.
To prevent any infections, pseudo-capsule formation or rejection of the implant, artificial eye must be biocompatible, which
means it must be made of the material that accept surrounding tissues. According to Duever, A material was found in specific
species of coral that is similar to the material human bones are made from. This material can be converted into hydroxyapatite
which can be used in implants. By using this materials the risk of rejecting the eye is reduce and the specific guidelines
are met (Duever).
Thus
the eye is outstanding and one of major organ out of whole human body. Even though human being creates different methods to
cures vision and other problems of the eye, they can not make it as human beings got by god. Human beings are trying their
best to correct vision problems as best they can. At last, the eye is an extra ordinary organ of body human beings have to
prevent it and make its use in helpful way.
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