1. 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

2. Human Skull, Anterior View
Figure 5.11

3. Human Skull, Inferior View
Figure 5.9

4. Adipose tissue provides protection to the eyeball within the orbit.

5. Accessory Structures of the Eye (details follow)
Eyelids and eyelashes
Conjunctiva
Lacrimal apparatus
Extrinsic eye muscles

6. Accessory Structures of the Eye
Figure 8.1

7. External Anatomy of the eye and accessory structures

8. Accessory Structures of the Eye
Figure 8.2a

9. 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.

10. 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.

11. 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

12. Accessory Structures of the Eye
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!!
Figure 8.2b

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

14. Accessory Structures of the Eye
Figure 8.3a–b

15. Accessory Structures of the Eye
Figure 8.3c

16. 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

18. Structure of the Eye Layers forming the wall of the eyeball
Fibrous layer
Outside layer
Vascular layer
Middle layer
Sensory layer
Inside layer

19. Structure of the Eye n. l. d. j. o. a. q. i. h. r. b. k. c. m. p.
(suspensory ligament) a. q. i. h. r. b. k. c. m. p.
Figure 8.4a

20. Structure of the Eye
Figure 8.4b

21. 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

22. 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 iris
Intrinsic muscles of the eye are under the control of the autonomic nervous system

23. Structure of the Eye: Sensory Layer
Retina “sensory tunic” contains two layers
Outer pigmented layer
Inner neural layer
Contains receptor cells (photoreceptors)
Rods
Cones

24. 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

25. Structure of the Eye: Sensory Layerg.
(photoreceptors) f. e.
Figure 8.5a

26. Structure of the Eye: Sensory Layer
Figure 8.5b

27. 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

28. 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

29. 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

30. Sensitivities of Cones to Different Wavelengths
Figure 8.6

31. 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

32. Lens
Figure 8.4a

33. Lens Cataracts result when the lens becomes hard and opaque with age
Vision becomes hazy and distorted
Eventually causes blindness in affected eye

34. Lens
Figure 8.7

35. 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

36. 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)

37. Posterior Segment Vitreous humor
Gel-like substance occupying the posterior segment of the eyeball
Prevents the eye from collapsing
Helps maintain intraocular pressure

38. Ophthalmoscope
Instrument used to illuminate the interior of the eyeball
Can detect diabetes, arteriosclerosis, degeneration of the optic nerve and retina

39. Posterior Wall of Retina as Seen with Ophthalmoscope
Figure 8.8

40. 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)

41. Pathway of Light Through the Eye
Figure 8.9

42. 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

43. Images Formed on the Retina
Figure 8.10

44. 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
Figure 8.11

45. 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)

46. 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

47. 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”

48. 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

49. 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

50. How a colorblind person sees.

52. 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