Macro and Microscopic
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Protected by bony orbits of the skull
Send information to brain directly via optic
nerve.
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From 2 outgrowths of the brain that form the
optic nerves & the optic cup:
 The posterior lining of the eye contains
photoreceptors
 In a mature eye called the retina
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Palpebrae: eyelids!
 Thin flaps of skin
 Controlled by orbicular
muscles
 Close when objects are
placed near the eye: reflex
arc
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Eyelashes
 Edges of eyelid
 Protect from dirt
1. Iris
2. Lacrimal caruncle
5. Lower lid
7. Pupil
8. Sclera
9. Upper lid
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Sclera
Choroid
Retina
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Tough, outermost, white layer
Surrounds and protects the eyeball.
Its front surface, the cornea, is transparent to
let light enter the eye.
 Lacks blood vessels
 Gets nutrition through diffusion
 Ideal for transplants
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The choroid coat is the middle layer of the
eyeball.
Consists of the colored portion of the eye
known as the iris.
 Iris has a hole in its center called the pupil.
 Light enters through the pupil and the size of the
pupil is regulated by the iris.
 Constricting = parasympathetic muscles
 Dilating = Sympathetic muscles
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Too much light “bleaches” the photopigment,
rhodopsin to opsin
 Reduces ability to see
 Opsin must be “reconverted” to rhodopsin
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This is related to “night vision” issues with
sudden darkness…
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Transparent body
Lies directly behind the pupil
Held in place by ciliary muscles (run in
circular, longitudinal, radial orientation;
change lens shape)
Focuses light rays of images on the retina
 Images inverted (both L to R and Up to Down)
 Visual cortex reorients these
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Increases amount of energy reaching
photoreceptive cells
What happens when source moves closer?
 All light isn’t focused on retina
 Makes image “fuzzy” because adjacent cells
stimulated
 An “accommodating” lens clears image
up by refocusing light
Close image = ROUND lens (decreases radius of
curvature)
 Far image = FLAT lens (increases radius of
curvature)
 To round the lens, contract muscles in ciliary
body:
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 Contracting a circular muscle reduces the aperture
 This decreases the tension on the suspensory
ligaments, allowing lens to “round up”
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Innermost layer of the eyeball.
Contains microscopic structures:
 Rods
▪ Low-light
▪ Non-color vision
 Cones
▪ Bright-light
▪ Color vision
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At center of retina, have fovea centralis
 Concentration of cone cells
 1:1 cell/neuron ratio (gives good resolution)
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Farther outward, mix of rods and cones, with
just rods in peripheral vision
 Mostly b & w, low-light, low-resolution peripheral
vision
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Eye directs fovea centralis at objects to
maximize clarity
A “blind spot” occurs where the optic nerve/tract
exits the eye
Optimal vision
Blind spot
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Aqueous humor—in the anterior cavity in
front of the lens
 Provides nutrients to cornea, lens, other
structures
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Vitreous humor—in the posterior cavity
behind the lens
 Gelatinous
 Holds retina to outer wall of choroid
 May contain “floaters”, which must be removed
surgically
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Mucous membrane covering the front surface
of the sclera and lining the eyelid
 Produces tears
 Barrier to microbes
 Susceptible to trauma, infections, chemical
irritation, and allergic reactions
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Conjunctiva: thin, transparent
epithelium covers the surface
 Tiny blood vessels
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Innermost layer of retina contains rods and
cones
Impulse travels from the rods and cones
through the bipolar and ganglionic layers of
retina
Nerve impulse leaves the eye through the
optic nerve; the point of exit is free of
receptors and is therefore called a blind spot
Visual interpretation occurs in the visual
cortex of the cerebrum