The document provides information on the special senses and anatomy of the human eye. It discusses the eye as the dominant sense and its role in vision. The three layers of the eye - fibrous, vascular and sensory layers - are described. In particular, it outlines the structures of the retina such as the rods and cones, and how visual signals are transmitted via the optic nerve. Common vision conditions like myopia, hyperopia and cataracts that result from refractive errors are also summarized.

1. SPECIALSPECIAL SENSES
EXPERT VISION ACADEMY

2. The Senses
 Special senses
 Smell
 Taste
 Sight
 Hearing

3. Vision
 Dominant sense
 70% of all sensory receptors are in the eyes
 Converts the energy of light into electrical nerve
impulses that are interpreted by the brain as sight
 Visual receptor cells (photoreceptors) sense and
encode patterns of light that enter the eye
 Brain uses these signals to give us images

4. sclera
iris
pupil
tear
drainage
canal
cornea
Medial
commisure
lateral
commisure
palpabre
palpabre
Lacrimal
caruncle

5. The 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

6. The Eye
 Eye acts much like a camera
 Lens of eye adjusts to bring object into clear
focus
 Pupil of eye constricts to allow less light to
enter in bright setting or dilates to allow more
light to enter in darker setting
 Through bending of light rays, image reaches
retina

7. Accessory Structures of the Eye
 Eyelids and eyelashes
 Conjunctiva
 Lacrimal apparatus
 Extrinsic eye muscles

8. Accessory Structures of the Eye

9. Accessory Structures of the Eye
Eyelids
 Eyelids (palpebrae)
 Tarsal glands lubricate the eye
 The tarsal glands (modified sebaceous gland) secrete a fluid
to keep the eye lids from adhering to each other.
 The tarsal plate gives form and support to the eyelids.
Eyelashes
 Eyelashes double/triple row of hairs.
 Eyelashes are innervated by nerve ending – trigger blinking
Eyebrows
 Shade
 inhibit sweat
 help protect the eyeballs from foreign objects, perspiration, and the
direct rays of the sun.

11. Accessory Structures of the Eye
Conjunctiva
 Membrane that lines the eyelids
 Connects to the surface of the eye
 Secretes mucus to lubricate the eye
 Functions:
 major
 produce a lubricating mucus
 prevents the eyes from drying out
 other
 protection
 prevents foreign objects from penetrating beyond the confines of its sac
 Conjunctivitis is inflammation, produces a pinkish
discoloration (pinkeye)

12. Accessory Structures of the Eye
Lacrimal apparatus
 Lacrimal gland—produces lacrimal fluid
 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

13. FLOW OF TEARS
Lacrimal gland
Lacrimal ducts
Sup. or inf. lacrimal canal
Lacrimal sac
Nasolacrimal duct
Nasal cavity

14. Accessory Structures of the Eye
 Function of the lacrimal apparatus
 Protects, moistens, wash and lubricates the eye
 Empties into the nasal cavity
 Properties of lacrimal fluid
 Dilute salt solution (tears)
 Contains antibodies and lysozyme

15. Accessory Structures of the Eye
Muscles
 Two types:
 Extrinsic
 Are skeletal muscles
 Move the eye voluntarily
 Intrinsic
 Are smooth involuntary muscles
 Located within the eye
 Iris muscle
 Ciliary muscle

16. Accessory Structures of the Eye
 Extrinsic eye muscles
 Six muscles attach to the outer surface of the eye
 Produce eye movements

17. Accessory Structures of the Eye

18. Structure of the Eye
 Three Layers or coats or tunics forming the wall
of the eyeball
 Fibrous layer
 Outside layer
 Vascular layer
 Middle layer
 Sensory layer
 Inside layer

19. Structure of the Eye

20. Structure of the Eye: The Fibrous Layer
 Sclera
 White connective tissue layer
 Seen anteriorly as the “white of the eye”
 Cornea
 Transparent, central anterior portion
 Allows for light to pass through
 The only human tissue that can be transplanted
without fear of rejection
 Both Composed of dense avascular connective tissue

21. Structure of the Eye: VascularLayer
Choroid
 Is a blood-rich nutritive layer in the posterior of the eye
 Pigment prevents light from scattering
 Modified anteriorly into two structures
 Ciliary body—smooth muscle attached to lens that change the
lense shape
 Iris—regulates amount of light entering eye
• Pigmented layer that gives eye color
• Composed of circular and radial muscles
• Reflex contraction of circular muscle in bright light (small dia of
pupil)
• Reflex contraction of radial muscle in dim light (large dia of pupil)
 Pupil—rounded opening in the iris

22. bright light normal light dim light

23. Layers of Eye

24. Structure of the Eye: Sensory Layer
 Retina contains two
layers
 Outer pigmented
layer
 Inner neural layer
 Contains receptor
cells
(photoreceptors)
 Rods
 Cones

25. Structure of the Eye: Sensory Layer
 Three layers of neurons make up a major portion of the
retina. Named in the order in which the conduct impulses:
 Photoreceptor neurons
 Bipolar neurons
 Ganglion neurons
 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

26. Layers of Retina

27. Structure of the Eye: Sensory Layer
 Rods
 Most are found towards the edges of the retina
 Allow dim light vision and peripheral vision
 Objects are seen in shades of gray not color
 Contains pigment Rhodopsin
 Rhodopsin is highly sensitive to light, light causes a
rapid breakdown of the pigment into its scotopsin and
retinal components
 Cones
 Allow for detailed color vision
 Densest in the center of the retina
 Fovea centralis—area of the retina with only cones
 No photoreceptor cells are at the optic disc, or blind spot

28.  Cone sensitivity
 Three types of
cones red, green,
and blue
 Different cones are
sensitive to different
wavelengths
 Color blindness is
the result of the lack
of one cone type

29. Lens
 Biconvex crystal-like structure, flexible
 Held in place by a suspensory ligament attached to the
ciliary body
 Focuses image onto retina
 Changes lens thickness to allow light to be properly
focused onto retina
 Cataracts result when the lens becomes hard and
opaque with age
 Vision becomes hazy and distorted
 Eventually causes blindness in affected eye

30. Segments, orChambers, of the Eye
 Anterior chamber
 Posterior chamber
 Vitreous segment

31.  Watery fluid found between lens and cornea
 Similar to blood plasma
 Helps maintain intraocular pressure
 Provides nutrients for the lens and cornea
 Helps to maintain the shape of the eye
 Prevent the eyeball from collapsing
Aqueous humor

32. Vitreous humor
 Posterior to the lens
 Gel-like substance with fine fibrils
 Prevents the eye from collapsing
 Helps maintain intraocular pressure
 Transmits light

33. How do we see?
 For vision to occur:
 An image must be formed on the retina to stimulate
the rods and cones
 Nerve impulses must be conducted to the visual
areas of the cerebral cortex
 Interpretation occurs

34. Formation of a retinal image
 Four processes focus light rays so that they form
clear images on the retina
 Refraction of light rays
 Accommodation of the lens
 Constriction of the pupils
 Convergence of the eyes

35. Process of image formation on the retina
1) Refraction:
bending of light as medium changes to focus light
2) Accommodation of lens for near/distance vision:
shape of lens changed by ciliary muscle to make light focus on retina
3) Dilation /Constriction of pupil:
ANS reflex to prevent scattering of light through edges of lens
4) Convergence of eyes:
to focus both eyes on same object and provide vision
Images are focused on the retina upside‑down and mirror‑image, and the
brain then translates this information.

36. Function of the Lens: Light Refraction
 Bending of light
 Occurs when light passes
at an angle from a
medium of one optical
density into a medium of
a different density
 Convex surfaces cause
light to converge
 Concave surfaces cause
light to diverge

37. • Convex lenses cause
light waves to converge
• Concave lenses cause
light waves to diverge
Air
Glass
(a) (b)
Diverging
light waves
Convex
surface
Light
wave
Converging
light waves
Concave
surface
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38. 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
 Accommodation—the lens must change shape
to focus on closer objects

39. Pathway of Light Through the Eye

40. Focusing on a Near Object

41. Accomodation
Focusing on a Far Object

42. Pathway of Light Through the Eye
 Image formed on the retina is
 Reversed from left to right
 Upside down
 Smaller than the object

43. Images Formed on the Retina

44. Visual Nerve Pathways
 Ganglion cell axons leave the eye to form the
optic sensory nerve
 The nerve fibers cross anterior to pituitary gland,
the optic chiasma.
 Nerve fibers travel to the thalamus and to reflex
centers.
 Optic radiations are nerve pathways that lead to
the visual association area in the occipital lobe.

45. Principal visual pathways from the eyes to the visual cortex.

46. Visual Fields and Visual Pathways

48.  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
Errors of Refraction

49. Errors of Refraction
 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”

51. Errors of Refraction
 Astigmatism
 Images are blurry
 Results from light focusing as lines, not points, on
the retina due to unequal curvatures of the cornea
or lens
 Correction
 special lenses with uneven curvature
 compensates for eye’s asymmetry
 allows the image to focus evenly on retina
 laser

54. Errors of Refraction
Cataracts
 When lens becomes hard
and opaque, our vision
becomes hazy and distorted
 Clouding of lens
 Inadequate nutrient delivery
to deeper lens fibers
 Causes: diabetes mellitus,
smoking, UV damage,
congenital, age-related
hardening, thickening of lens

55. Night blindness
 Inhibited rod function that hinders the ability to see at
night
 Hampers one’s ability to drive safely at night
 most common cause-----prolonged vitamin A deficiency
 leads to rod degeneration
 treatment
 vitamin A supplements
 restore function
 if administered before degenerative changes occur

56. Colorblindness
 Genetic conditions that result in the inability to see certain
colors
 Due to the lack of one type of cone (partial color blindness)
 sex-linked
 more common in males
 most common type
 red-green
 deficit or absence of red or green cones
 seen as same color
 either red or green

57. Glaucoma
 Can cause blindness due to increasing pressure
within the eye
 Normally 15 (12-20) mm Hg
 Above 25mm Hg an individual has glaucoma which
if untreated can lead to retinal damage and
blindness
 Pressure can increase due to decreased drainage
or excess drainage of the aqueous humor which is
normally re-circulated via vessels into vitreous
humor