Transcript The Human Eye

The Human Eye
Sight most important sense in humans
80% of info from reading, TV, video,
and the world around us
Structure of the Eyeball
Figure 15.4a
EYE
Eyelid: protective cover, wets surface
Extrinsic Eye Muscles
Figure 15.3a, b
Lacrimal Apparatus
Figure 15.2
Structure of the Eyeball
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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
3 layers
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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
Aqueous humor
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Watery fluid between cornea and lens
Begins refracting (bending) light
Pupil
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An opening into the eye
Light enters through here
Size controlled by iris
Dark because the inside of the eye is dark
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“redeye”---light bounces off blood vessels in retina
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Iris
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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
Color of iris determined by amount of
melanin, a pigment
*lots of melanin=brown
*very little or none=blue
Lens
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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
Lens refracts or bends light; focal point
is where rays cross
Image formed on the retina is inverted,
upside down;
the brain “turns” it right side up
Retina
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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
Specialized cells
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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
Sensory Tunic: Retina
Figure 15.6a
Vision
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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
Optic Nerve
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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 cones……blind spot
Vision center of the Brain
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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
Occipital
Lobe
Common Eye Problems
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Each year over 1 million eye injuries
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Most could be prevented
Prevent Eye Injuries
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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
Prevent Eye Injuries in Sports
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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
Other precautions
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Don’t 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
Other eye diseases
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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 contacts…do NOT put them back in
until ok’d by dr
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
Retinal detachment
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Often after a blow to the head but can be
spontaneous
Flashes of light and “floating” spots
Need to see doctor immediately
Vision Problems
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Myopia or nearsightedness: cannot see
far away
Caused by elongated eyeball which causes
image to fall before retina
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
Color blindness
color vision deficiency
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Genetic, carried on X chromosome
Most often in males
Usually red/green deficit