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The Eye & Vision
The Eye and Vision
 70 percent of all sensory receptors are
in the eyes - only see 1/6th of eye
 Each eye has over a million nerve fibers
 Protection for the eye
 Most of the eye is enclosed in a bony orbit
 A cushion of fat surrounds most of the eye
 Eyelids – aka.
palpebra
Meets at medial
and lateral
canthus
Protection and
Shading
 Eyelashes
Figure 8.1b
 Meibomian glands – tarsal glands –
modified sebacious glands produce an
oily secretion to lubricate the eye
 Chalazion (ka-lay-zee-an)- inflammation
Chalazion
 Ciliary glands –
modified
sweat glands
between the
eyelashes
 Sty inflammation
 Conjunctiva
 Membrane that lines the eyelids
 Connects to the surface of the eye
 Secretes mucus to lubricate the eye
 Conjunctivitis – inflammation of – sometimes
known as “pink eye”
Conjunctivitis
 Lacrimal
apparatus
 Lacrimal gland –
produces lacrimal
fluid
 Lacrimal canals –
drains lacrimal
fluid from eyes
Figure 8.1a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.4b
 Lacrimal sac –
passage of fluid
towards nasal cavity
 Nasolacrimal duct –
empties fluid the
nasal cavity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.4c
 Properties of lacrimal fluid
 Dilute salt solution (tears)
 Contains antibodies and lysozyme
 Protects, moistens, and lubricates the
eye
 Empties into the nasal cavity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.5
 6 eye muscles attach to the outer
surface of the eye
 Produce gross eye movements
 Rectus muscles: lateral, medial,
superior, inferior
 Oblique muscles: inferior, superior
 Need to be able to label on diagram &
identify the motions they are
responsible for
Slide 8.6
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
 The wall is composed of three tunics
 Fibrous tunic – Outer layer
 Vascular tunic (uvea) – middle
layer
 Sensory tunic
 inside layer
Figure 8.3a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.7
 Sclera
 White connective tissue layer
 Seen anteriorly as the “white of the eye”
 Cornea
 Transparent, central anterior portion
 Allows for light to pass through
 Repairs itself easily
 The only human tissue that can be
transplanted without fear of rejection
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.8
Cornea transplant
 Choroid – posterior portion
Blood-rich nutritive tunic
Dark pigment prevents light from
scattering
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.9
 Modified interiorly
 Ciliary body – anterior
 smooth muscle
 Attached to lens
 Iris
 Pigmented layer that gives eye color
 Pupil – rounded opening in the iris for
light passage
 Circular & radial fibers regulate opening
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.9
 Neural layer contains receptor cells
(photoreceptors)
 Rods & Cones
 Signals pass from photoreceptors via a
two-neuron chain
 Photoreceptors to bipolar neurons
 Bipolar neurons to ganglion cells
 Signals leave the retina toward the brain
through the optic nerve
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.10
Neurons of the Retina
Figure 8.4
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.11
 Rods
 Most are found towards the edges of the
retina
 Allow dim light vision and peripheral vision
 Perception is all in gray tones
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
8.12a
 Cones
 Allow for detailed color vision
 Densest in the center of the retina
 Fovea centralis
 area of the retina with only cones
 Area of greatest visual acuity
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
8.12b
 No photoreceptor cells are at the
optic disk, or blind spot
Site of optic nerve leaving eyeball
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
8.12b
 There are three
types of cones
 Different cones
are sensitive to
different
wavelengths
 Color blindness is
the result of lack
of one cone type
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.6
Slide 8.13
 Biconvex crystal-like structure
 Held in place by a suspensory ligament
attached to the ciliary body
Figure 8.3a
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.14
 Activity of ciliary muscle of ciliary body changes
lens thickness to focus light onto retina
 Cataracts: clouding of lens
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.14
Cataract Removal
 Aqueous humor in Anterior Segment
 Watery fluid found in chamber between the
lens and corne
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Reabsorbed into venous blood
 Blocked drainage = glaucoma
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
 Vitreous humor
 Gel-like substance
behind the lens
 Keeps the eye from collapsing
 Lasts a lifetime and is not replaced
 Keeps retina firmly against wall of
eyeball
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide
8.15b
Vision
Light



Light must be refracted (bent) so that it
focuses on the retina to form an image.
The cornea, aqueous humor, & vitreous
humor have a constant refractory power
The lens is the only refractory structure
that can change shape in order to change
refractory power.
Focus

Distance vision is accomplished by:



Ciliary muscles relaxation
Suspensory ligaments tightening
Results in:


Maximum flattening of lens (decreased
convexity)
Decreased refraction
Focus


Close vision is accomplished by:
 Ciliary muscles tightening
 Suspensory ligaments relaxing
Results in:
 Lens rounds up (increased convexity)
 Increased refraction
 Requires continuous muscle contractions
 Prolonged periods of close vision can cause
eyestrain
Lens Accommodation
 The eye is set for
distance vision
(over 20 ft away)
 The lens must
change shape to
focus for closer
objects
 This ability
decreases with age
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Figure 8.9
Slide 8.16
Real Image
Real image is projected on retina:
 reversed from left to right
 upside down & smaller
 Brain adjusts our perception
Figure 8.10
Copyright © 2003 Pearson Education, Inc. publishing as Benjamin Cummings
Slide 8.17
Correcting the Eye


Correct Focus = emmetropia
Nearsightedness = myopia






Distant objects are blurry (near objects clear)
Focus of light in front of retina
Eyeball too long
lens too strong – too convex
Cornea is too curved – too convex
Requires concave lens to correct
Emmetropia
Myopia
Correcting the Eye
Farsightedness = hyperopia






Near objects are blurry – distance objects are
clear
Focus of light beyond the retina
Short eyeball
lazy lens – lens too weak – too concave
Cornea too flat – too concave
Requires convex lens to correct
Hyperopia
Astigmatism



Unequal curvatures in cornea & lens
Blurred vision
Specially ground lenses required to
compensate irregularities
Presbyopia




Decreasing lens elasticity so lens cannot
adjust for close vision
Causes difficulty focusing on near objects
Typically age related – corrected with
reading glasses, etc.
Near point of accommodation – closest
distance at which you can focus on an
object