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Seatwork: The Eye
Question:
Why do newborn babies have blue
eyes?
• Answer: The iris pigment is not yet developed.
• Although irises come in different colors, they
contain only brown pigment. When there is a
lot of pigment, eyes appear brown or black. If
there isn’t much pigment, the shorter
wavelengths of light are scattered from the
non-pigmented parts, and eyes appear blue,
green, or gray.
a. accessory organs-aid eye in its function
(1) eyelids—4 layers (skin, muscle,
connective tissue, conjunctiva)
(a) orbicularis oculi—closes lids
(b) levator palpebrae superioris—
opens (raises upper lid)
(c) conjunctiva—lines inner surface
(not on cornea); protects eye from
foreign objects
(2) lacrimal apparatus-produces tears that lubricate &
cleanse eye
(a) lacrimal gland—secretes tears (moist, &
antibacterial due to lysozyme)
(b) nasolacrimal duct—collects fluid & eventually
enters nose
(3) extrinsic muscles—move the eye in various
directions—3 pairs:
(a) inferior rectus (moves eye down) & superior
rectus (moves eye up)
(b) inferior oblique & superior oblique (rotate eye)
(c) lateral rectus (moves eye outward) and medial
rectus (moves eye inward)
b. structure of the eye—fluid filled hollow
sphere with 3 layers
(1) outer tunic (a.k.a. fibrous tunic)
(a) anterior 1/6; transparent bulge
(cornea)
i. few cells & no blood vessels
ii. arranged in regular patterns
(what gives the cornea its transparency)
(b) posterior 5/6; white portion
(sclera)
i. collagenous & elastic fibers
ii. protection & attachment for
extrinsic muscles
iii. posterior portion pierced by
optic nerve
(2) middle tunic (vascular layer)
(a) choroid coat—posterior 5/6 of the
globe of the eye
i. many blood vessels—nourishment
for other eye tissues
ii melanocytes—absorb excess light
(darkly pigmented)
(b) ciliary body—contains ciliary process &
ciliary muscles; forms ring around eye
(c) lens—transparent & biconvex
i. held in place by strong suspensory
ligaments
ii. surrounded by thin capsule attached
to suspensory ligaments
iii. action of ciliary muscles increases or
decreases tension on ligament; lens
shape changes in turn
iv. accommodation: ability of lens to
adjust shape to facilitate focusing
example: lens becomes more convex
(thickens) to view object closer
(d) iris—colored portion from outside, between
cornea & lens; circular & radial set of muscles that
adjust light entering pupil
i. divides into 2 chambers filled with
aqueous humor—make up anterior
cavity
anterior chamber: between cornea & iris
posterior chamber: occupied by the lens
(between iris & vitreous humor)
ii. aqueous humor—watery fluid in
chambers—circulates through pupil
[a] provides nutrients & helps
maintain shape
[b] glaucoma—buildup of pressure
(3) Inner tunic—retina (back of eye to ciliary body)
thin, transparent & contains photoreceptors
(a) macula lutea—central region, yellowish
spot. Has depression: fovea centralis (point
of sharpest vision in retina)
(b) optic disk—medial to fovea. Where
axons converge to form optic nerve
(vessels & nerve fibers leave from here)
Called “blind spot” (no rods or cones)
(c) posterior cavity—space bound by retina,
lens & ciliary body. Contains vitreous
humor (jellylike)
c. refraction of light (bending of light)
(1) cornea refracts light more than lens
(2) convex surfaces converge light (focus it to a point)
(3) image focused on retina is inverted & reversed
d. visual receptors—modified neurons (rods &
cones) rods: elongated; cones: blunt shaped
(1) closest to choroids layer (inverted)
(2) differences—many more rods than
cones in human eye
(a) rods—lower threshold of light
intensity (more sensitive to light)
(b) rods: vision in dim light
(c) rods: achromatic vision; cones allow
color vision
(d) cones—give visual acuity (cones
in fovea) allow for sharp, color images
e. visual pigments
(1) rods—rhodopsin (a.k.a. visual purple)
light sensitive pigment in rods; stable in
dim light
(2) cones—3 different sets of pigments
(a) higher threshold of light intensity
(chromatic vision)
(b) respond to red, green & blue
wavelengths.
f. stereoscopic vision—depth & distance perception
(1) possible because pupils 6-7cm apart
(requires 2 eyes)
(2) objects 20 ft away or closer produce
slightly different retinal images
(3) interpreted as depth perception by
visual cortex of brain; leads to 3D perception
g. visual nerve pathway—axons of ganglion
cells in retina leave eyes to form optic nerves
(1) optic nerves leave retina
(2) go to optic chiasma
(a) fibers from nasal (medial) half of
retina cross over to opposite side
(b) fibers from lateral (temporal) half
of retina go to same side of brain
(3) optic tracts—impulses travel to thalamus
on optic tracts.
(4) optic radiations—from thalamus’ back,
impulses enter paths called optic radiations
which travel to visual cortex of occipital lobe
h. visual problems
(1) nearsighted—myopia—image focuses
in front of retina; corrected by concave
lens
(2) farsighted—hyperopia—image focuses
behind retina; corrected by convex lens
(3) astigmatism—imperfection of cornea
or lens—can lead to eyestrain, headache
(4) presbyopia—loss of efficiency
(decreased power of accommodation that
occurs with aging)
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