Senses: Vision Ch. 17-2. Accessory Structures of Eye Eyelids Eyelashes Eyebrows Lacrimal apparatus...
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Transcript of Senses: Vision Ch. 17-2. Accessory Structures of Eye Eyelids Eyelashes Eyebrows Lacrimal apparatus...
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Senses: Vision
Ch. 17-2
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Accessory Structures of Eye
• Eyelids• Eyelashes• Eyebrows• Lacrimal apparatus• Extrinsic eye muscles
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Accessory Structures of Eye
• Eyelids – palpebrae– Functions:
• Shade eyes during sleep• Protect eyes from light• Protect eyes from foreign objects• Spread lubrication over eyeballs
– upper eyelid is more moveable – levator palpebrae superioris muscle
– Where eyelids meet is called commissure – lateral and medial
– Lacrimal caruncle – reddish bump in medial commissure that secretes oil and sweat – causes “sleep” in your eyes
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Infections
• Sty – infection in oil gland of eyelash follicle• Chalazion – infection in tarsal glands of eyelid
– Tarsal glands secrete fluid onto eye for lubrication
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Accessory Structures of Eye
• Lacrimal apparatus– Produces and drains
lacrimal fluid or tears• Eye muscles
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Anatomy of the Eyeball
• 2.5 cm in diameter• 3 anatomical layers
– Fibrous tunic– Vascular tunic– Retina
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Tunic Layers
• Outer layer – fibrous– Cornea – transparent, focuses light rays– Sclera – white of the eye– Optic nerve – transmits info to brain
• Middle layer – vascular– Choroid coat – contains blood vessels– Ciliary body – holds the lens in place– Lens – focusing– Aqueous humor – fluid surrounding lens– Pupil – opening for light to enter
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Tunic Layers
• Inner layer– Retina – visual receptor
cells– Vitreous humor – fluid
supports internal structures
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Eye
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Structures of Eyeball
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Retina
• Made of cells that are light receptors – photoreceptors
• Rods and cones– Rods – black and white– Cones - color
• Color blindness– Lack of cones
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ColorblindnessA genetic trait that affects boys more than girls. The location of the gene is on the X chromosome
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Image Formation
• Seeing is like taking a picture• The object must be focused on a “film” –
retina• The correct amount of light must be present –
pupil
Light bends - refraction
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Refraction
• When light bends as it moves between two mediums – air and water
• Images on the retina are upside-down and have right-to-left reversal
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Image Formation Process
• Light hits the cornea and is bent• Light leaves the cornea and is bent again• Light enters the lens where it is focused on the
retina• So why don’t we see everything upside-down?
– Very early on the brain “learns” how to coordinate the images and make them correct
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Accommodation
• Most of the focusing is done by the cornea
• 25% must be done by the lens• Our lens is convex on both
sides in order to produce clear images
• The lens will increase its curvature in order to focus all images exactly
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Animations
• http://www.bbc.co.uk/science/humanbody/body/factfiles/sight/sight_animation.shtml
• http://www.biologymad.com/resources/eye.swf
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Abnormalities and Changes
• Presbyopia – lens loses elasticity with age– Around age 40 the lens can’t focus near images and people
need glasses• Normal eye – emmetropic – can reflect images perfectly
of objects 20 ft away• Myopia - near-sightedness
– Eyeball is too long for the focusing power of the lens or lens is thicker than normal
• Hyperopia – far-sightedness– Eyeball is too short for the focusing power of the lens or lens
is thinner than normal
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Abnormalities