Download Ch8 Power Point - Eyes

Chapter 8 – Special Senses
Eye
sphere – 1 inch in diameter –
only see 1/6 of eyeball
External Structure
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Eyelids – protection
 Come together at medial and lateral
canthus
 Eyelashes along border
 Tarsal glands – lubricate eye
External Structrure cont.
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Conjunctiva – lines eyelids and covers
outer surface of eyeball
 Ends at edge of cornea
 Secretes mucous for lubrication
 Conjunctivitis – pink eye
External Structure cont.
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Lacrimal glands – above lateral end of
eye
 Release tears which move across
eye into lacrimal canals → lacrimal
sac → nasolacrimal duct → empties
into nasal cavity
 Tears also contain antibodies and
lysozymes
 Cleanses and protects
External Structure cont.
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Extrinsic (external) muscles – 6
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Lateral rectus – laterally
Medial rectus – medially
Superior rectus – elevates
Inferior rectus – depresses
Superior oblique – elevates and lateral
Inferior oblique – depresses and lateral
Internal Structure
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Tunics = coats; humors = fluids
Sclera – outermost – aka fibrous tunic
 Thick, white
 Central anterior is clear = cornea
 Where light enters
Internal Structure cont.
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Vascular tunic
 Choroids – posteriorly – dark
 Prevents light from scattering
 Ciliary body and ciliary zonule – hold
lens in place
 Iris – has opening called pupil
 Muscles control diameter –
regulates light
Internal Structure cont.
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Sensory tunic
 Retina – stops at ciliary body
 Contains photoreceptors – rods and
cones
 Pass signals through bipolar and
ganglion cells to optic nerve → optic
cortex = vision
 Are all through retina except where optic
nerve leaves – called the optic disk (blind
spot)
Internal Structure cont.
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Rods – more dense at edge
 Allow peripheral vision
 Allow to see at night
Cones – more dense in center
 Allow color vision
 3 types – different wavelengths
 Missing cones = colorblindness
Fovea centralis – pit next to optic disc
 Contains only cones
 Point of sharpest vision
Internal Structure cont.
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Lens – focuses light on retina
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Divides eye into 2 chambers
Anterior (aqueous) segment
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Contains aqueous humor – clear, watery
Provides nutrients to lens and cornea
Reabsorbed through scleral venous sinus
(canal of Schlemm)
Blocked sinus = glaucoma – increase of
intraocular pressure
Internal Structure cont.
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Posterior (vitreous) segment
 Contains vitreous humor – gel
like
 Keeps eye from collapsing in –
maintains intraocular pressure
Cataracts – lens becomes milky
Light Pathway
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Refraction – bending of light – occurs when light
passes through substances with different densities
Occurs as light moves through cornea, conjunctiva,
aqueous humor, lens, pupil, vitreous humor, retina
Lens changes shape – causes more/less bend in
light
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Greater bulge (convexity) – more bending
Flatter – less bending
Eye at rest is set for distant vision
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About 20ft and no change is necessary
Closer and lens must bulge – accommodation
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Ciliary body contracts – lens becomes convex
Vision
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Emmetropia – “harmonious vision”
Myopia – nearsightedness – “short vision”
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Light is focused in front of retina
Eyeball is too long or cornea is too curved
Correction = concave lenses
Hyperopic – farsightedness – “far vision”
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Light focuses behind retina
Eyeball is too short
Correction = convex lenses
Visual fields & pathways to brain
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Nerves leave eye via optic nerve
At optic chiasma some (medial) fibers
cross
Optic tracts – contain fibers from both
eyes
 Joint neurons in thalamus and are sent
to occipital lobe of brain
Allows binocular vision because the
visual fields overlap
Eye reflexes
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Photopupillary reflex – pupil
constriction due to bright light
Accommodation papillary reflex –
constriction for viewing close objects
Development
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Vision is the only sense not fully functional
at birth.
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No tears until 2 weeks
Newborn sees in gray tones
5 months – close focus, following moving
objects
By 5 – color vision well developed, depth
perception
6/7 – emmetropia
About 40 – presbyopia may occur – type of
farsightedness