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The Human Eye

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    1. The Human Eye

    2. How do your eyes work? The Human eye has the task of focusing light onto the retina. The retina is a thin layer of neural cells that lines the back of the eyeball, it is a part of the central nervous system. The cornea and lens help to converge light rays to focus onto the retina.

    3. 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 the iris. 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.

    4. Anatomy of the Eye Retina nerve 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 outer covering of the eyeball. Vitreous - the clear, jelly-like substance that fills the middle of the eye. The vitreous gives the eyeball its shape.

    5. 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 out extra 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.

    6. 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 the brain, interpret the images to extract form, meaning, memory and context of the images.

    7. 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

    8. Conjunctivitis (Pink Eye) An infection of the conjunctiva (the outer-most layer of the eye that covers the sclera). Conjunctivitis requires medical attention. Easily treated but contagious though normal contact

    9. 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.

    10. 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 more cone shape than its normal gradual curve. can cause substantial distortion of vision, with multiple images, streaking and sensitivity to light. Treatment includes Corneal transplant or wearing corrective lenses

    11. 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.

    12. 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 extraocular muscles 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.

    13. 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 is the 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 a tapetum. 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

    14. 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 causes light 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.

    15. Cow Eye Dissection Step by Step

    16. Cow Eye Dissection Step by Step

    17. Cow Eye Dissection continued

    18. Cow Eye Dissection continued