The Eye and Vision
• Most of eye protected by cushion of fat and bony
orbit
• Accessory Structures protect eye and aid eye
function
– Eyebrows, Eyelids, Conjunctiva, Lacrimal
apparatus
– Extrinsic eye muscles
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Conjunctiva: Transparent mucous membrane
– Produces lubricating mucous secretion
– Conjunctiva covers white of eyes but not
cornea
Lacrimal Apparatus
• Lacrimal glands secrete tears and drain them via
ducts into nasolacrimal duct
• Lacrimal gland in orbit above lateral end of eye
• Tears are dilute saline solution containing
mucus, antibodies, and lysozyme
– Blinking spreads tears toward medial
commissure
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Extrinsic Eye Muscles
• Six extrinsic eye muscles attach to scleral layer
and move eyeball
– Originate from bony orbit; insert on eyeball
– Enable eye to follow moving objects; maintain shape
of eyeball; hold in orbit
– Four rectus muscles originate from common
tendinous ring; names indicate movements
• Superior, inferior, lateral, medial rectus
muscles
– Two oblique muscles move eye laterally and
rotate eyeball
• Superior and inferior oblique muscles
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Structure of Eye:
• Outermost layer; dense avascular connective tissue
– Two areas: sclera and cornea
1. Sclera--Opaque region
• Protects, shapes eyeball; anchors extrinsic eye
muscles
• Continuous with dura mater of brain posteriorly
2. Cornea--Transparent anterior area
• Bends light as it enters eye
• Pain receptors contribute to blinking and tearing
reflexes
• Corneal transplants typically are not rejected by
immune system because cornea has no blood
vessels
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Choroid---Middle pigmented layer
• Contains: choroid, ciliary body, and iris
1. Choroid
• Supplies blood to all layers of eyeball
• Black pigment absorbs light to prevent it scattering
within eye which makes focusing impossible
2. Ciliary body
• Ring of smooth muscle surrounding lens
• Ciliary muscles relax and contract to control lens
shape to allow lens to focus light
• Capillaries of ciliary processes secrete fluid
• Suspensory ligament holds lens in position
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3. Iris-anterior to ciliary body
• Colored part of eye
• Pupil—central opening that regulates light entering
eye
– Close vision and bright light —pupils constrict
– Distant vision and dim light —pupils dilate
– Changes in emotional state—pupils dilate when
subject matter is appealing or requires
problem-solving skills
• Intrinsic eye muscles = ciliary muscle and iris
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Lens
• Biconvex, transparent, flexible, and avascular
• Changes shape to precisely focus light on retina
– filled with transparent protein crystallin
– becomes more dense, convex, less elastic with age
• Cataracts (clouding of lens) consequence of aging, diabetes
mellitus, heavy smoking, frequent exposure to intense
sunlight
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Innermost Layer: Retina
• Delicate 2-layered membrane
– Outer pigmented layer
• Absorbs light and prevents its scattering
• Phagocytize photoreceptor cell fragments
• Stores vitamin A
– Inner Neural layer
• Transparent
• Composed of 3 types of neurons
– Photoreceptors, bipolar cells, ganglion cells
• Signals spread from photoreceptors  bipolar
cells  ganglion cells
• Ganglion cell axons exit eye as optic nerve
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The Retina
• Optic disc (blind spot)
– Lacks photoreceptors
– Site where optic nerve leaves eye
• Photoreceptors (quarter-billion) of two types
– Rods-black and white vision
– Cones-color vision
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• Rods
– Dim light, peripheral vision receptors
– More numerous, more sensitive to light than cones
– Used for peripheral vision because numbers greatest
at periphery
• Cones
– Receptors for color vision which function better in
bright light (daytime vision)
– Macula lutea
• Mostly cones
• Fovea centralis
– Tiny pit in center of macula with all cones; best
vision
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Internal Chambers and Fluids
• Anterior area contains Aqueous humor
• Plasma like fluid continuously formed
• Drains via canal of Schlemm (at sclera-cornea
junction)
• Supplies nutrients and oxygen mainly to lens
and cornea but also to retina, and removes
wastes
• Helps maintain shape and pressure of eye
• Glaucoma: blocked drainage of aqueous humor
increases pressure and causes compression of
retina and optic nerve  blindness
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• Posterior area contains vitreous humor that
– Transmits light
– Supports posterior surface of lens
– Holds neural layer of retina firmly against pigmented
layer
– Contributes to intraocular pressure and maintains
shape of eye
– As you get older, may get floaters—
• caused by age-related changes and/or changes in vitreous
consistency  becomes more liquid  causes shrinking and pulling
away from interior surface of eye. This can cast shadows on your
retina, which you interpret as eye floaters.
• If you notice a sudden increase in number of floaters, contact an eye
doctor immediately — especially if you see flashes of light or lose
your peripheral vision; may be caused by retinal detachment.
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LABWORK
1. Dissect sheep eye
2. Using eye models and charts, locate the
following: sclera, choroid, cornea, retina, iris,
pupil, ciliary body, lens, aqueous humor,
vitreous body, optic disk, optic nerve.
3. Perform eye tests for visual acuity, astigmatism,
and other tests in your lab book, time
permitting.
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The Ear: Hearing and Equilibrium (Balance)
• Three major areas of ear
1. External (outer) ear – hearing only
2. Middle ear (tympanic cavity) – hearing only
3. Internal (inner) ear – hearing and
equilibrium
•
Receptors for hearing and balance respond to
separate stimuli
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External Ear
• Auricle (pinna)
– Funnels sound waves into auditory canal
– Sound is form of energy transmitted in waves;
different sounds have different wavelengths
• External acoustic meatus (auditory canal)
– Short tube lined with skin ceruminous glands
– Transmits sound waves to eardrum
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• Tympanic membrane (eardrum)
– Boundary between external and middle ears
– Connective tissue membrane that vibrates in
response to sound
– Transfers sound energy to bones of middle
ear
Middle Ear
– Small, air-filled, mucosa-lined cavity in
temporal bone
– Eardrum at lateral border and medially is bony
wall containing oval (vestibular) and round
(cochlear) windows
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• Eustachian or Pharyngotympanic (auditory)
tube—connects middle ear to nasopharynx
– Equalizes pressure in middle ear cavity with
external air pressure
Ear Ossicles
• Three small bones in tympanic cavity: the
malleus (hammer), incus (anvil), and stapes
(stirrup)
– Suspended by ligaments and joined by
synovial joints
– Transmit vibratory motion of eardrum to oval
window
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Inner Ear (has sound and equilibrium receptors)
• Bony labyrinth --filled with perilymph
– Three regions: vestibule, semicircular
canals, and cochlea
• Membranous labyrinth -filled with endolymph
– Series of membranous sacs and ducts
• Vestibular apparatus
– Equilibrium receptors in semicircular canals
and vestibule
– Vestibular receptors monitor static equilibrium
– Semicircular canal receptors monitor dynamic
equilibrium
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Vestibule
• Central cavity of bony labyrinth
• Contains two membranous sacs
1. Saccule: continuous with cochlear duct
2. Utricle: continuous with semicircular canals
• Sacs responsible for static or linear equilibrium
– Maculae contain equilibrium receptor
regions
– Responds to gravity and changes in position
of head
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Semicircular Canals
• Three canals (anterior, lateral, and posterior) lie
in three planes of space
–Membranous ducts line canals and
communicate with utricle
• Ampulla of each canal houses equilibrium
receptor region called crista ampullaris
– Receptors respond to angular (rotational)
movements of head
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The Cochlea--hearing
• spiral, conical, bony chamber; size of split pea
– Extends from vestibule
– Coils around bony pillar
– Contains cochlear duct, which contains
Organ of Corti (spiral organ)
• "roof" of cochlear duct is vestibular membrane
•
•
External wall secretes endolymph
"Floor" of cochlear duct composed of basilar membrane which supports organ
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Auditory Processing
• Pitch perceived by impulses from specific hair
cells in different positions along basilar
membrane
• Loudness detected by increased numbers of
action potentials that result when hair cells
experience larger deflections
• Localization of sound depends on relative
intensity and relative timing of sound waves
reaching both ears
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Maculae
• Sensory receptors for static equilibrium
•
One in each saccule wall and one in each utricle wall
• Monitor position of head in space, necessary for
control of posture
• Respond to linear acceleration forces, but not rotation
• Contain supporting cells and hair cells
• Stereocilia and kinocilia are embedded in otolith
membrane studded with otoliths (tiny CaCO3
stones)
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The Ear:
1. Find the following structures in your lab book and
the human ear models:
Pinna, pharyngotympanic (auditory) tube, Tympanic
membrane (eardrum), malleus, Incus, Stapes,
Cochlea, Vestibule, Semicircular canals, Auditory
canal, Round/oval windows, Vestibulocochlear
nerve.
2. View slide of the cochlea and note the three fluidfilled chambers and the organ of Corti.
3. Perform hearing/balance tests in lab book.
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