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The Human Eye


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How do your eyes work?

The Human eye has the task

of focusing light onto the

retina.

The retina is a thin layer ofneural cells that lines the

back of the eyeball, it is a

part of the central nervous

system.

The cornea and lens help toconverge light rays to focus

onto the retina.


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Anatomy of the Eye

Choroid- the intermediate layer of the eye

that is located underneath the iris.

Conjunctiva- the mucous membrane that

lines the inner surface of the eyeball. The

conjunctiva continues over the forepart(part we see) of the eye.

Cornea- the clear front window of the eye.

The cornea transmits and focuses light into

the eye.

Iris- the colored part of the eye. The iris

helps regulate the light that enters the eye.

Pupil- the dark center in the middle of theiris. The pupil determines how much light

is let in to the eye. It changes sizes to

accommodate for the amount of light that is

available.

Lens- the transparent structure inside the

eye that focuses light rays on to the retina.


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Anatomy of the Eye

Retinanerve layer that lines the back of the

eye. The retina senses light and creates

impulses that are sent through the optic

nerve to the brain.

Macula- a small area in the retina that

contains special light sensitive cells. The

macula allows us to see fine details clearly.

Optic Nerve- the nerve that connects the

eye to the brain. The optic nerve carries the

impulses formed by the retina to the brain,

which interprets them as images.

Sclera- the thick, tough, white outercovering of the eyeball.

Vitreous- the clear, jelly-like substance that

fills the middle of the eye. The vitreous

gives the eyeball its shape.


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How you see an object

The light rays enter the eye through the

cornea (transparent front portion of eye to

focus the light rays)

Then, light rays move through the pupil,

which is surrounded by Iris to keep outextra light

Then, light rays move through the

crystalline lens (Clear lens to further

focus the light rays )

Then, light rays move through the

vitreous humor (clear jelly like substance) Then, light rays fall on the retina, which

processes and converts incident light to

neuron signals using special pigments in

rod and cone cells.


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How you see an object continued

These neuron signals are transmitted

through the optic nerve,

Then, the neuron signals move through

the visual pathway - Optic nerve > Optic

Chiasm > Optic Tract > Optic Radiations> Cortex

Then, the neuron signals reach the

occipital (visual) cortex and its radiations

for the brain's processing.

The visual cortex interprets the signals as

images and along with other parts of thebrain, interpret the images to extract

form, meaning, memory and context of

the images.


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Cataracts

A cataract is an hardening of the

normally transparent lens that leads

to blurred vision.

Cataracts form for a variety of

reasons, including long-term

ultraviolet exposure, secondary

effects of diseases such as diabetes,

or simply due to advanced age. Surgical Treatment is neccessary


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Conjunctivitis (Pink Eye)

An infection of the conjunctiva

(the outer-most layer of the eye

that covers the sclera).

Conjunctivitis requires medicalattention.

Easily treated but contagious

though normal contact


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Coloboma

A coloboma(also part of the rare Cat eye

syndrome) is a hole in one of the structures of the

eye, such as the eyelid, iris, retina, choroid or optic

disc. The hole is present from birth and can be

caused when a gap called the choroid fissure

between two structures in the eye, which is

present early in development in the uterus, fails to

close up completely before a child is born. The

classical description in medical literature is of a

key-hole shaped defect. A coloboma can occur in

one or both eyes.


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Eye Disorders

Keratoconus is a degenerative non-

inflammatory disorder of the eye in which

structural changes within the cornea

cause it to thin and change to a morecone shape than its normal gradual curve.

can cause substantial distortion of vision,

with multiple images, streaking and

sensitivity to light.

Treatment includes Corneal transplant orwearing corrective lenses


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Facts about the Eye

Basic Structures:

The lens grows layers like an onion. As you get older, the lens becomes less flexible

because the buildup of layers compacts the center of the lens, making it more rigid. When

the lens becomes less flexible, it cant change shape to focus on things nearby. This is

why people need glasses as they get older.

The lens does only about 20 percent of our focusing. The cornea does the other 80

percent. The lens changes shape so that you can focus on things that are near and things

that are far away. The ciliary body controls the shape of the lens.

Cones are one type of photoreceptor cells in the retina. They are responsible for daylight

and color vision.

Rods are the other type of photoreceptor cells. They respond to dim light.


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Facts about the Eye continued

Basic Structures continued:

The fovea is a dimple in the retina where cones are concentrated and vision is most

accurate.

The aqueous humor is the clear fluid that helps the cornea keep its rounded shape. The fat that surrounds the eye is there for a reason. It helps cushion the eye and protect

it from the hard bone of the eye socket.

The human eye is a slightly asymmetrical (uneven) sphere with an approximate diameter

of 24-25 millimeters. It has a volume of about 6.5 cubic centimeters.

Each eyeball is held in position in the orbital cavity (the area of your skull where your eyes

fit) by various ligaments, muscles and facial expansions that surround it. The extraocularmuscles move the eyeball in the orbits. When you move our eyes from side to side, your

are using your extraocular muscles.

The blind spot is the area where the optic nerve leaves the retina. Each eye has a blind

spot where there are no photoreceptor cells.

The eye has tiny blood vessels that carry blood to the retina.


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Cow Eyes vs. Human Eyes

A cows cornea has about seven or eight layers of material. We

have three to five layers. The cow needs these extra layers of

protection because it spends so much time grazing close to the

ground, where its eyes could be damaged by sticks or other objects.

Another difference between the cows eye and the human eye isthe shape of the pupil. The cows pupil is oval. Our eyes have

round pupils.

Cows cannot see color, only shapes. Humans can see color

because they have cones in their retinas. Cows do not have these

cones.

The shiny blue-green tapetum helps the cow see at night. Many other animals have atapetum. You may have seen their eyes glow when your cars headlights flash on them.

The tapetum helps animals see at night by reflecting the light entering the eye back at the

retina a second time.

The iris is the part of the eye that gives us brown, blue or green eyes. Human eyes can be

different colors, but all cows have brown eyes.

Vision Problems


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Cow Eyes vs. Human Eyes continued

Nearsightedness is called myopia. Myopia is a refractive error and

means that a person sees objects that are close by more clearly

that those that are far away. Myopia is inherited and is often

discovered in children between the ages of 8 and 12. Eyeglasses

or contacts can correct the problem.

Farsightedness is called hyperopia. Hyperopia causes near

objects to be blurred and distant objects to appear clear. Like

myopia, hyperopia is usually inherited and can be corrected with

eyeglasses or contact lenses.

Astigmatism occurs when the corneal curve is steeper in one

direction than in the other, like the back of a spoon. This causeslight rays to focus at multiple points on the retina, distorting both

near and far vision. Almost everyone has some degree of

astigmatism, but vision is not noticeably affected unless the

uneven curvature is severe. Many people have astigmatism in

combination with myopia or hyperopia.


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Step 1:

Examine the outside of the eye. See how many parts of the

eye you can identify. You should be able to find the white

(sclera) and the clear covering over the front of the eye

(cornea). You should also be able to identify the fat and

muscle surrounding the eye.

Cow Eye Dissection Step by Step

Step 2:Make the first incision where the sclera meets the cornea. Cut

until the aqueous humor is released. The aqueous humoris a

clear fluid that helps the cornea keep its rounded shape.

Step 3:Rotate the eye and cut around the cornea. Be careful not to

cut too deep or you may cut the lens. As the cornea starts to

cut free, hold the cornea in the center and make the last cuts

around it.


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Cow Eye Dissection Step by Step

Step4:Once you have removed the cornea, place it on the board (or

cutting surface) and cut it with your scalpel or razor.

Step 5:With the cornea removed, the next step is to pull out the iris.

Place one finger in the center of the eye. Find the iris and

pull it back. It should come out in one piece.

Step 6:It can be a bit tricky to remove the lens with the vitreous

humor attached. It works best if you cut slits in the sclera.

Be careful not to cut the lens.


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Cow Eye Dissection continued

Step 7:After enough incisions have been made in the sclera,

you should be able to remove the lens. Sometimes the

vitreous humor will be removed along with the lens.

Step 8:Hold up the lens and look through it. If the lens is too

slippery, pat it dry and try again.

Step 9:With the vitreous humor now removed, you should be

able to turn the eye inside out.


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Step 10:The thin tissue on the back of the eye is the retina. Find the spot where

the retina is attached. The shiny blue-green material is the tapetum.

Cow Eye Dissection continued

Step 11:Find the spot where all the retinas nerves collect. It is called the blind

spot. This is where all the nerves go out the back of the eye, forming the

optic nerve.

Step 12:

Return your attention to the outside of the eye. Locate the optic nerve. Tosee the separate fibers that make up the optic nerve, pinch the nerve with

a pair of scissors or with your fingers.

Step 13:Once the dissection is complete, properly dispose of the remains and

wash your hands.

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