The Human Eye

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The Human Eye

• Sight most important sense in humans
• 80 of info from reading, TV, video, and the
  world around us

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Structure of the Eyeball
Figure 15.4a
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EYE

• Eyelid protective cover, wets surface

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Extrinsic Eye Muscles
Figure 15.3a, b
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Lacrimal Apparatus
Figure 15.2
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Structure of the Eyeball

• A slightly irregular hollow sphere with anterior
  and posterior poles
• The wall is composed of three tunics fibrous,
  vascular, and sensory
• The internal cavity is filled with fluids called
  humors
• The lens separates the internal cavity into
  anterior and posterior segments

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3 layers

• Sclera tough outer layer, white of the eye
  shape protection
• Cornea clear layer at front of eye protection
  light enters through
• Choroid coat just inside sclera dark brown to
  stop reflection contains many blood vessels

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Aqueous humor

• Watery fluid between cornea and lens
• Begins refracting (bending) light

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Pupil

• An opening into the eye
• Light enters through here
• Size controlled by iris
• Dark because the inside of the eye is dark
• redeye---light bounces off blood vessels in
  retina

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Iris

• Front of choroid layer
• muscular changes to control amount of light
  entering eye
• In bright light-iris is large pupil is small
  less light comes in
• In low light- iris small pupil large more light
  enters eye

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Color of iris determined by amount of melanin, a
pigment lots of melaninbrown very little or
noneblue
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Lens

• Clear, flexible structure which refracts (bends)
  light onto the retina
• Special muscles (ciliary) attach to lens
• hold lens in place
• pull on lens to slightly change its shape to
  help focus

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Lens refracts or bends light focal point is
where rays cross
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Image formed on the retina is inverted, upside
down the brain turns it right side up
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Retina

• Layer of specialized cells
• Where image from lens is formed
• Cells convert image into electrical impulses
  which travel through the optic nerve to the brain

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Specialized cells

• Rods sensitive to weak light but not to color
  allows vision in dim light but not in color
• Cones sensitive to color but only in adequate
  light
• cones for red, green, and blue
• 125 million rods
• 6.5 million cones

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Sensory Tunic Retina
Figure 15.6a
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Vision

• Light sensitive pigment retinal made from
  vitamin A
• Not enough vitamin A night blindness
• Combines with proteins in rods and cones (protein
  are different in each type)
• Light breaks the bond and sends an electrical
  impulse down nerve

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Optic Nerve

• Nerve fibers from rods and cones join together to
  make up the optic nerve
• Carries the impulse to the brain
• Point where the optic nerve leaves the eye has no
  rods or conesblind spot

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Vision center of the Brain

• Back of the brain..occipital lobe
• Turns the image we see right side up and into 3-D
• Enables us to interpret what we see
• Can be fooled.optical illusions, eye witness
  accounts

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Occipital Lobe
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Common Eye Problems

• Each year over 1 million eye injuries
• Most could be prevented

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Prevent Eye Injuries

• Wear goggles when
• Jump starting a car
• Mowing, edging, other lawn power tools
• Using power tools
• Using chemicals such as chlorine, ammonia, bleach
  and paint solvent
• Regular glasses are NOT enough

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Prevent Eye Injuries in Sports

• 40,000 injuries a year
• Use batting helmets with face shields
• Polycarbonate safety goggles for racquet ball,
  basketball
• Helmets and face shields approved by US Amateur
  Hockey Association when playing hockey

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Other precautions

• Dont play with fireworks
• Wear sunglasses that block at least 90 of UVA
  and UVB rays
• Welding masks when using such equipment
• Never let young kids have toys like darts and BB
  guns
• Workplace wear whatever safety goggle or eye
  protection is recommended at all times

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Other eye diseases

• Diabetic retinopathy causes retina to degrade,
  causes blindness if diabetes is not treated
• Cataract cloudy lens due to protein deposits
  very common as we age correctable with surgery,
  replace old lens with a plastic lens

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• Conjunctivitis or pink eye irritation of the
  covering of eye caused by virus, bacteria, should
  see a doctor because only certain medicines ok
  for eyes..if caused by contactsdo NOT put them
  back in until okd by dr

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• Glaucoma pressure in eye damages optic nerve
  easy to detect with eye check up, painless
• Macular degeneration spot on retina that sees
  clear, crisp images in center of field of vision
  most common cause of blindness in the world

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Retinal detachment

• Often after a blow to the head but can be
  spontaneous
• Flashes of light and floating spots
• Need to see doctor immediately

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Vision Problems

• Myopia or nearsightedness cannot see far away
• Caused by elongated eyeball which causes image to
  fall before retina

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Problems of Refraction
Figure 15.14a, b
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• Presbyopia or farsightedness cannot see near,
  usually comes with age

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• Astigmatism irregular curvature of cornea or
  lens

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Color blindnesscolor vision deficiency

• Genetic, carried on X chromosome
• Most often in males
• Usually red/green deficit