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The Human Eye
Transcript of The Human Eye
An Amazing Organ What are the characteristics of the human eye, and how does it function? Question 1: By Troy T The Parts of the Eye... The Cornea: The cornea is a transparent dome-shaped window on the surface of our eyes, that protects the eye and the light going into our eye refracts off it. It provides two-thirds of our focusing power. The Iris: The iris is the colored part of the eye. It controls light levels that pass through the eye. The pupil is a hole inside the iris which contracts or widens based on the light level. The iris' color comes from the pigment melanin. The Lens: The lens' purpose is to focus the light going into the eye on the retina. The lens refracts light a second time after it is refracted off the cornea. It constantly changes shape, and is held by zonules from the ciliary body. lens cornea iris pupil zonules The Vitreous Humor: The vitreous humor is a thick substance that fills up two-thirds of the eye's volume and is made of mostly water. It gives the eye shape and allows the eye to go back to its regular shape if compressed. Children have vitreous humors with consistencies of egg whites, and they become more liquid with aging. vitreous
humor The Retina: The retina is a multi-layered tissue that is at the back of the eye. Electrical impulses come from the retina and send them through the optic nerve. It contains millions of rods and cones, the two types of photoreceptors in the eye. Rods help us with night vision, and cones help us see colors. The vitreous humor is attached to some parts of the retina. retina The Macula and Fovea: The macula is at the center of the retina, and the fovea is at the center of the macula. It allows us to focus and appreciate detail like reading. The macula and fovea contain most of the cones (color photoreceptors) in the retina. macula fovea The Optic Nerve: The optic nerve transmits electrical impulses to the brain. The primary visual cortex in the occipital lobe of the brain receive the impulses from the retina. There is a spot in the eye called the optic disc, or the blind spot, which is on the optic nerve, and there is no parts of the retina there. optic nerve occipital lobe The Sclera and Conjunctiva: The sclera is a white protective outer coat of the eye, and is thick and opaque. It goes all the way to the back of the eye and the optic nerve is attached to it. You can see part of it around the iris. The extraocular muscles are attached to the sclera. When we get older, the sclera becomes more yellow. The conjunctiva is a transparent tissue that lines from the outer edge of our cornea to inside our eyelids, covering up the visible parts of the sclera. Tiny blood vessels run through the conjunctiva. sclera conjunctiva The Extraocular Muscles: The six extraocular muscles are the superior rectus, inferior rectus, medial rectus, lateral rectus, superior oblique, and inferior oblique. The superior rectus rotates the eye upwards, while the inferior rectus rotates the eye downward. The medial rectus rotates the eye toward the nose, and the lateral rectus rotates the eye toward the ear. The superior oblique and inferior oblique aid the superior rectus and inferior rectus respectively. Question 2: What are some possible eye diseases or conditions, and what are the consequences of having them? Eye Diseases and Conditions Explained… Common Conditions: Possible Diseases: Myopia: Myopia is sometimes referred to as nearsightedness, and occurs when the eye is too wide, and light can't reach the retina. It causes blurry vision when looking at distant objects, and can be corrected with concave lens glasses or contacts. Myopia can also be fixed with refractive surgery. Hyperopia: Hyperopia is commonly referred to as farsightedness, and occurs when the eye is too short, and light goes past the retina. It causes blurry vision when looking at close objects, and can be corrected with convex lens glasses or contacts. Hyperopia can also be fixed with refractive surgery. Astigmatism: Astigmatism is caused when the cornea is irregularly-shaped like a football, and light hits multiple points on the retina. It causes blurry vision at all distances, and can be corrected with toric lens glasses or contacts. Astigmatism can also be fixed with refractive surgery. Floaters: Floaters are sometimes caused by vitreous detachment, when the vitreous humor becomes more liquid. The vitreous humor pulls away from the retina, and tiny bits of the vitreous cast shadows on the retina. Sometimes, there are more serious causes of floaters like retinal detachment, and can cause headaches or even vision loss. Amblyopia Amblyopia is sometimes referred to as lazy eye, and is caused when the brain favors one eye over the other. Strabismus is one type of amblyopia. Strabismus is caused when the eyes aren't aligned properly, and can sometimes result to crossed eye, double vision, or even permanent vision loss. Conjunctivitis: Conjunctivitis is known as pink eye, and is usually contagious. It happens when there is an infection in the conjunctiva. The three types of conjunctivitis are viral, allergic, and bacterial. The allergic type is the only one that is not contagious. Conjunctivitis can lead to swelling conjunctiva, or irritation in the eye. Cataract: A cataract is caused when the old cells of the lens clog up and get trapped in a capsule. They accumulate and cloud the lens. Cataracts can lead to blindness, and are the most common cause of vision loss for people ages 55 and older. Glaucoma: Glaucoma is caused because the aqueous, or the liquid the eye produces, and it normally goes through a drainage system. But, the drainage system gets clogged and builds up pressure. Eventually, there is enough pressure build up that it damages the retina and optic nerve. Glaucoma results in sudden loss of vision or eye pain. Question 3: What is the significance of the ciliary body, and how does it change over time? The Ciliary Body: The ciliary body is right behind the iris, and the zonules that suspend the lens in mid-air are attached to it. The blood vessels supplying the iris also nourish the ciliary body.
One thing that the ciliary body is responsible for is producing the aqueous, or the liquid that creates tears. The ciliary body also controls accommodation by changing the shape of the lens. As the ciliary contract, the zonules make the lens thicken so the eye can easily focus up close. As the ciliary relax, the zonules make the lens thinner for distant objects.
As you get older, around 40 or 50 years old, you develop a condition called presbyopia. The ciliary body and lens lose adjustability which causes difficulty to focus on objects up close. Interesting Facts: • Because of the double refraction, images appear reversed on the retina, but the brain turns them right side up.
• 7-8% of men are color blind, compared to 0.4-0.5% of women.
• It takes twice as much energy to focus with astigmatism.
• It is impossible to sneeze without blinking.
• When babies are born, they cry without tearing.
• An average blink takes 0.1 seconds, and you blink about 12 times a minute.
• The six extraocular muscles are the most active muscles in the body.
• From the time you're born to the time you die, your eyes never grow bigger. The End Thank you for watching. ciliary body zonules optic disc (blind spot)