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The Eye and Vision 70% of all sensory receptors are in the
eyes 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
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Human Skull, Anterior View
Figure 5.11
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Human Skull, Inferior View
Figure 5.9
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Adipose tissue provides protection to the eyeball within the orbit.
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Accessory Structures of the Eye (details follow) Eyelids and eyelashes Conjunctiva Lacrimal apparatus Extrinsic eye muscles
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Accessory Structures of the Eye
Figure 8.1
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External Anatomy of the eye and accessory structures
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Accessory Structures of the Eye
Figure 8.2a
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Accessory Structures of the Eye Eyelids and eyelashes
Tarsal glands lubricate the eye (oil) Ciliary glands are located between the
eyelashes
A sty is an inflammation of cilary glands (modified sweat glands) that lie between the eyelashes
and help lubricate the eyeball.
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Accessory Structures of the Eye Conjunctiva
Membrane that lines the eyelids Connects to the surface of the eye Secretes mucus to lubricate the eye
Conjunctivitis: Inflammation of the conjunctiva, which is the mucus membrane that lines the internal surface of the eyelids and eyeball.
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Accessory Structures of the Eye Lacrimal apparatus
Lacrimal gland—produces lacrimal fluid Dilute salt solution (tears) Contains antibodies and lysozyme
Lacrimal canals—drain lacrimal fluid from eyes Lacrimal sac—provides passage of lacrimal
fluid towards nasal cavity Nasolacrimal duct—empties lacrimal fluid into
the nasal cavity Function of the lacrimal apparatus
Protects, moistens, and lubricates the eye Empties into the nasal cavity
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Accessory Structures of the Eye
Figure 8.2b
Tears released by the lacrimal glands flush across the eyeball into the lacrimal canals to the lacrimal sacs to the nasolacrimal duct into the nasal cavity……so when you cry your nose runs!!
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Accessory Structures of the Eye Extrinsic eye muscles
Six muscles attach to the outer surface of the eye
Produce eye movements
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Accessory Structures of the Eye
Figure 8.3a–b
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Accessory Structures of the Eye
Figure 8.3c
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both eyes look or move together in the same direction, simultaneously:
dextroversion (looking right)— right lateral rectus left medial rectus
levoversion (looking left)— left lateral rectus right medial rectus
supraversion or sursumversion (looking straight up)— right & left superior recti right & left inferior obliques
infraversion or deorsumversion (looking straight down)— right & left inferior recti right & left superior obliques
dextroelevation (looking right and up)— right superior rectus left inferior oblique
dextrodepression (looking right and down)— right inferior rectus left superior oblique
levoelevation (looking left and up)— right inferior oblique left superior rectus
levodepression (looking left and down)— right superior oblique left inferior rectus
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Structure of the Eye Layers forming the wall of the eyeball
Fibrous layer Outside layer
Vascular layer Middle layer
Sensory layer Inside layer
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Structure of the Eye
Figure 8.4a
b.
c.
d. l.
a. q. i.
k.
p.
r. h.
o.
n.
j. (suspensory ligament)
m.
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Structure of the Eye
Figure 8.4b
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Structure of the Eye: The Fibrous Layer Sclera
Tough, white, opaque fibrous connective tissue layer
Seen anteriorly as the “white of the eye” Cornea
Anterior transparent, part of fibrous layer Allows for light to pass through Repairs itself easily The only human tissue that can be
transplanted without fear of rejection
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Structure of the Eye: Vascular Layer Choroid is a blood-rich nutritive layer in the
posterior of the eye..forms most of the pigmented vascular tunic Pigment prevents light from scattering
Modified anteriorly into two structures Ciliary body—smooth muscle attached to lens Iris—regulates amount of light entering eye;
contains muscles that controls the size of the pupil. Pigmented layer that gives eye color Pupil—rounded opening in the irisIntrinsic muscles of the eye are under the control of
the autonomic nervous system
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Structure of the Eye: Sensory Layer Retina “sensory tunic” contains two
layers Outer pigmented layer Inner neural layer
Contains receptor cells (photoreceptors) Rods Cones
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Structure of the Eye: Sensory Layer Signals pass from photoreceptors via a two-
neuron chain Bipolar neurons Ganglion cells
Signals leave the retina toward the brain through the optic nerve
Optic disc (blind spot) is where the optic nerve leaves the eyeball Cannot see images focused on the optic disc
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Structure of the Eye: Sensory Layer
Figure 8.5a
f.
e.
g. (photoreceptors)
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Structure of the Eye: Sensory Layer
Figure 8.5b
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Structure of the Eye: Sensory Layer Neurons of the retina and vision
Rods Most are found towards the edges of the
retina Allow dim light vision and peripheral vision All perception is in gray tones
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Structure of the Eye: Sensory Layer
Neurons of the retina and vision Cones
Allow for detailed color vision Densest in the center of the retina Fovea centralis— tiny pit inth macula lutea;
contains only cones No photoreceptor cells are at the optic disc,
or blind spot
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Structure of the Eye: Sensory Layer Cone sensitivity
Three types of cones Different cones are sensitive to different
wavelengths Color blindness is the result of the lack of
one cone type
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Sensitivities of Cones to Different Wavelengths
Figure 8.6
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Lens Important light bending structure of the
eye; shape can be modified Biconvex crystal-like structure Suspensory ligament attaches the lens
to the ciliary body
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Lens
Figure 8.4a
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Lens Cataracts result when the lens becomes
hard and opaque with age Vision becomes hazy and distorted Eventually causes blindness in affected
eye
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Lens
Figure 8.7
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Two Segments, or Chambers, of the Eye Anterior (aqueous) segment
Anterior to the lens Contains aqueous humor
(fluid filling the anterior segment of the eye)
Posterior (vitreous) segment Posterior to the lens Contains vitreous humor
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Anterior Segment Aqueous humor
Watery fluid found between lens and cornea (anterior segment of eye)
Similar to blood plasma Helps maintain intraocular pressure Provides nutrients for the lens and cornea Reabsorbed into venous blood through the
scleral venous sinus, or canal of Schlemma (drains the aqueous humor of
the eye)
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Posterior Segment Vitreous humor
Gel-like substance occupying the posterior segment of the eyeball
Prevents the eye from collapsing Helps maintain intraocular pressure
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Ophthalmoscope Instrument used to illuminate the
interior of the eyeball Can detect diabetes, arteriosclerosis,
degeneration of the optic nerve and retina
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Posterior Wall of Retina as Seen with Ophthalmoscope
Figure 8.8
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Pathway of Light Through the Eye Light must be focused to a point on the
retina for optimal vision The eye is set for distance vision
(over 20 feet away) Accommodation—the lens must change
shape to focus on closer objects (less than 20 feet away)
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Pathway of Light Through the Eye
Figure 8.9
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Pathway of Light Through the Eye Image formed on the retina is a real
image Real images are
Reversed from left to right Upside down Smaller than the object
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Images Formed on the Retina
Figure 8.10
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Figure 8.11
Visual Fields and Visual Pathways
Optic chiasma Location where the optic
nerves cross Fibers from the medial side
of each eye cross over to the opposite side of the brain
Optic tracts Contain fibers from the
lateral side of the eye on the same side and the medial side of the opposite eye
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Eye Reflexes Internal muscles are controlled by the
autonomic nervous system Bright light causes pupils to constrict through
action of radial, circular, and ciliary muscles Viewing close objects causes accommodation
External muscles control eye movement to follow objects
Viewing close objects causes convergence (eyes moving medially)
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A Closer Look Emmetropia—eye focuses images
correctly on the retina Myopia (nearsighted)
Distant objects appear blurry Light from those objects fails to reach the
retina and are focused in front of it Results from an eyeball that is too long
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A Closer Look Hyperopia (farsighted)
Near objects are blurry while distant objects are clear
Distant objects are focused behind the retina
Results from an eyeball that is too short or from a “lazy lens”
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A Closer Look Astigmatism
Images are blurry Results from light focusing as lines, not
points, on the retina due to unequal curvatures of the cornea or lens
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Homeostatic Imbalances of the Eyes Night blindness—inhibited rod function
that hinders the ability to see at night Color blindness—genetic conditions that
result in the inability to see certain colors Due to the lack of one type of cone
(partial color blindness) Cataracts—when lens becomes hard
and opaque, our vision becomes hazy and distorted
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How a colorblind person sees.
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Homeostatic Imbalances of the Eyes Glaucoma—can cause blindness due to
increasing pressure within the eye Hemianopia—loss of the same side of
the visual field of both eyes; results from damage to the visual cortex on one side only