1. Chapter 14
The Human Eye

2. Objectives Students should be able to:
State the relationship between receptors, the central nervous system and the effectors
Describe the gross structure of the eye as seen in front view and in horizontal section
State the principal functions of component parts of the eye in producing a focussed image of near and distant objects on the retina
Describe the pupil reflex in response to bright and dim light

3. Sense organs
Sense organs are receptors that receive stimuli from the environment and inform your body of any changes in the environment

4. 14.2 The Human Eye Upper eyelid eyelashes Tear gland pupil
Conjunctiva (covers sclera)
cornea
Lower eyelid
iris

5. Cornea
Transparent layer continuous with sclera (white portion of the eye)
Specialised form of conjunctiva
Refracts or bend light rays into the eye
Conjunctiva
Transparent membrane
Secretes mucus, thus helping to keep the front of the eyeball moist

6. Iris
A circular sheet of muscles
Controls the amount of light entering the eye
Pupil
A hole in the centre of the iris
Allows light to enter the eye

7. Eyelids
Protect the cornea from mechanical damage
Eyelashes
Help to shield the eye from dust particles
Tear gland
Secretes tears to wash away dust particles
Secretes tears to keep the cornea moist for atmospheric oxygen to dissolve
Secretes tears to lubricate conjunctiva, helping to reduce friction when the eyelids move

8. Chapter 14 The Human Eye
Structure and Function of the Eye (Concept Map)
19 May 2019
Concept Map page 281

9. Vertical Section of The Human Eye
Concept Map page 281
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10. Vertical Section of The Human Eye
suspensory
ligament
Attaches lens
to ciliary body
Concept Map page 281
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11. Vertical Section of The Human Eye
suspensory
ligament
iris
Controls amount
of light entering
the eye
Concept Map page 281
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12. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
Refracts light rays;
keeps eyeball firm
Concept Map page 281
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13. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
pupil
Allows light to
enter the eye
Concept Map page 281
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14. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
pupil
cornea
Refracts light rays
into pupil
Concept Map page 281
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15. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
pupil
cornea
lens
Concept Map page 281
Focuses light rays
onto retina
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16. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
pupil
cornea
lens
Concept Map page 281
ciliary
body
Contains ciliary
muscles which
control curvature
or thickness of the
lens
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17. Vertical Section of The Human Eye
suspensory
ligament
iris
aqueous
humour
pupil
cornea
lens
Concept Map page 281
ciliary
body
viterous
humour
Refracts light rays;
keeps eyeball firm
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18. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
Controls movement
of eyeball
iris
aqueous
humour
pupil
cornea
lens
Concept Map page 281
ciliary
body
viterous
humour
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19. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
Protects against
mechanical injury
pupil
cornea
lens
Concept Map page 281
ciliary
body
viterous
humour
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20. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
choroid
Pigmented black to
prevent internal reflection
of light; blood vessels
bring oxygen and
nutrients to the eyeball
pupil
cornea
lens
Concept Map page 281
ciliary
body
viterous
humour
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21. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
choroid
pupil
fovea
(yellow spot)
cornea
Region where images are sharply focused
lens
Concept Map page 281
ciliary
body
viterous
humour
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22. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
choroid
pupil
fovea
(yellow spot)
cornea
lens
optic nerve
Concept Map page 281
Transmit nerve impulses
from eye to brain
ciliary
body
viterous
humour
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23. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
choroid
pupil
fovea
(yellow spot)
cornea
lens
optic nerve
Concept Map page 281
ciliary
body
blind spot
Region over optic nerve; no photoreceptors,
so insensitive to light
viterous
humour
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24. Vertical Section of The Human Eye
suspensory
ligament
rectus
muscles
iris
sclera
aqueous
humour
choroid
pupil
fovea
(yellow spot)
cornea
lens
optic nerve
Concept Map page 281
ciliary
body
blind spot
viterous
humour
retina
Light-sensitive layer containing
cones responsible for colour
vision in bright light and rods
responsible for vision in dim light
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25. Controlling the entry of light into the eye
The pupil of the eye controls the amount of light that enters the eye
Size of the eye is controlled by 2 sets of involuntary muscles in the iris
One set arranged radially
One set arranged in a circle around the pupil

26. Action of Iris in Bright Light
Chapter 14 The Human Eye
Action of Iris in Bright Light
19 May 2019
Figure 14.5 (top) page 276

27. Action of Iris in Bright Light
Circular muscles of the iris contract 1
Figure 14.5 (top) page 276
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28. Action of Iris in Bright Light
Circular muscles of the iris contract 1
Radial muscles of the iris relax 2
Figure 14.5 (top) page 276
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29. Action of Iris in Bright Light
Circular muscles of the iris contract 1
Radial muscles of the iris relax 2
Figure 14.5 (top) page 276
Pupil constricts and this reduces the amount of light entering the eye 3
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30. Action of Iris in Dim Light
Chapter 14 The Human Eye
Action of Iris in Dim Light
19 May 2019
Figure 14.5 (bottom) page 276

31. Action of Iris in Dim Light
Radial muscles of the iris contract
Radial muscles of iris will contract 1
Figure 14.5 (bottom) page 276
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32. Action of Iris in Dim Light
Radial muscles of the iris contract 1
Circular muscles of the iris relax 2
Figure 14.5 (bottom) page 276
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33. Action of Iris in Dim Light
Radial muscles of the iris contract 1
Circular muscles of the iris relax 2
Figure 14.5 (bottom) page 276
Pupil dilates and this increases the amount of light entering the eye 3
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34. Low light intensity, Circular muscles relax, radial muscles contract
High light intensity, Circular muscles contract, radial muscles relax
Pupil becomes smaller
Low light intensity, Circular muscles relax, radial muscles contract
Pupil becomes bigger

35. What is the pupil reflex?
It is a reflex action (recall ‘Nervous System)
Pupil changes in size as a result of changes in light intensity
Pupil usually becomes larger when the surrounding light intensity is low, and smaller when the light intensity is high

36. Stimulus (change in light intensity)
Receptor (retina)
Sensory neurone in optic nerve
Brain
Motor neurone
Effector (iris)

37. 14.3 How do we see?
Light falling on object is reflected. Reflected rays fall on the eye.
These rays are refracted through the cornea and the aqueous humour (watery fluid) onto the lens.
The lens causes further refraction and the rays converge to a focus on the retina

38. Continue….
The image on the retina stimulates either the rods or the cones depending on the intensity of the light.
Image formed can be:
Upside down (inverted)
Back to front (reversed)
Smaller in size than the object being observed (diminished)

39. Continue….
Impulses produced by light falling on the rods and cones are transmitted via the optic nerve to the rear of the cerebrum.
The brain interprets these impulses

40. Chapter 14 The Human Eye How Do We See? 19 May 2019
Figure 14.7 page 278
Figure 14.7 page 278

41. How Do We See?
Light rays are refracted through the cornea and the aqueous humour onto the lens 1
lens
cornea
object
aqueous
humour
Figure 14.7 page 278
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19 May 2019
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42. How Do We See?
Light rays are refracted through the cornea and the aqueous humour onto the lens
The lens causes further refraction 1
lens
cornea
object 2
aqueous
humour
Figure 14.7 page 278
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Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
19 May 2019
19 May 2019 42 42

43. How Do We See?
Light rays are refracted through the cornea and the aqueous humour onto the lens
The lens causes further refraction 1
lens
Rays converge to focus on retina
cornea
object 2
retina 3
aqueous
humour
Figure 14.7 page 278
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Copyright © Marshall Cavendish International (Singapore) Pte. Ltd.
19 May 2019
19 May 2019 43 43

44. How Do We See?
Light rays are refracted through the cornea and the aqueous humour onto the lens
The lens causes further refraction 1
lens
Rays converge to focus on retina
cornea
object 2
retina 3
aqueous
humour
image
Figure 14.7 page 278 4
The image on the retina stimulates either the rods or the cones, depending on the intensity of the light
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19 May 2019
19 May 2019 44 44

45. How Do We See?
Light rays are refracted through the cornea and the aqueous humour onto the lens
The lens causes further refraction 1
lens
Rays converge to focus on retina
cornea
object 2
retina 3
aqueous
humour
image
Figure 14.7 page 278 4
The image formed is
inverted
laterally inverted
diminished in size
The image on the retina stimulates either the rods or the cones, depending on the intensity of the light
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46. Focusing (Accommodation)
This is the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina

47. How Eyes Focus on Distant Objects
Chapter 14 The Human Eye
How Eyes Focus on Distant Objects
19 May 2019
Figure 14.8 page 279

48. How Eyes Focus on Distant Objects
Parallel light rays from distant object enters eyes 1
Figure 14.8 page 279
Vertical section of the eye
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49. How Eyes Focus on Distant Objects
Ciliary muscles relax 2 2 1
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view)
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50. How Eyes Focus on Distant Objects
Suspensory ligaments become taut 2 2 3 3 1
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view)
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51. How Eyes Focus on Distant Objects
Lens becomes thinner 2 2 3 3 4 1 4
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view)
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52. How Eyes Focus on Distant Objects2 2 3 3 4 1 4 5
Figure 14.8 page 279
Focal length increases
Vertical section of the eye
Enlarged portion of eye
(front view)
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53. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 4 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view)
Light rays from distant object focused on retina
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54. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 4 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view) 7
Photoreceptors stimulated
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55. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 4 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view) 8 7
Nerve impulses produced by optic nerves
Photoreceptors stimulated
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56. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 4 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view) 8 9 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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57. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 4 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view) 10 8 9
Brain interprets impulses 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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58. How Eyes Focus on Distant Objects2 2 3 3 4 1 6 11 4
Person sees the distant object 5
Figure 14.8 page 279
Vertical section of the eye
Enlarged portion of eye
(front view) 10 8 9
Brain interprets impulses 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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59. Far object, circular muscles of the ciliary body relax
Suspensory ligaments are pulled tight, causing the lens to become flatter and less convex
Image sharply focused onto retina

60. How Eyes Focus on Near Objects
Chapter 14 The Human Eye
How Eyes Focus on Near Objects
19 May 2019
Figure 14.9 page 280

61. How Eyes Focus on Near Objects
Diverging light rays from near object enters eyes 1
Figure 14.9 page 280
Vertical section of the eye
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62. How Eyes Focus on Near Objects
Ciliary muscles contract 2 2 1
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view)
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63. How Eyes Focus on Near Objects
Suspensory ligaments become slacken 2 2 3 3 1
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view)
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64. How Eyes Focus on Near Objects
Lens becomes thicker 2 2 3 3 1 4 4
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view)
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65. How Eyes Focus on Near Objects2 2 3 3 1 4 4 5
Figure 14.9 page 280
Focal length decreases
Vertical section of the eye
Enlarged portion of eye
(front view)
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66. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view)
Light rays from near object focused on retina
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67. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view) 7
Photoreceptors stimulated
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68. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view) 8 7
Nerve impulses produced by optic nerves
Photoreceptors stimulated
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69. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view) 8 9 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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70. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view) 10 8
Brain interprets impulses 9 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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71. How Eyes Focus on Near Objects2 2 3 3 6 1 4 4 11
Person sees the near object 5
Figure 14.9 page 280
Vertical section of the eye
Enlarged portion of eye
(front view) 10 8
Brain interprets impulses 9 7
Nerve impulses produced by optic nerves
Transmission of nerves to brain
Photoreceptors stimulated
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72. Near object, circular muscles of the ciliary body contracts
Suspensory ligaments releases the pull on the lens
Elastic nature of the lens causes it to become thicker and more convex
Image focused sharply on the retina
After a certain distant called near point, the lens cannot adjust anymore. The image formed will then be blur

73. Vertical Section of The Human Eye
Pupil Reflex
A reflex action in which the pupil changes size as a result of changes in light intensity.
Concept Map page 281
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74. Vertical Section of The Human Eye
Pupil Reflex
In bright light
Concept Map page 281
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75. Vertical Section of The Human Eye
Pupil Reflex
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
Concept Map page 281
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76. Vertical Section of The Human Eye
Pupil Reflex
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Concept Map page 281
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77. Vertical Section of The Human Eye
Pupil Reflex
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Concept Map page 281
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78. Vertical Section of The Human Eye
Pupil Reflex
Focusing
The adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina.
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Concept Map page 281
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79. Vertical Section of The Human Eye
Pupil Reflex
Focusing
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Distant object
(7 metres or more)
Concept Map page 281
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80. Vertical Section of The Human Eye
Pupil Reflex
Focusing
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Distant object
(7 metres or more)
Ciliary muscles relax
Suspensory ligament become taut
Lens becomes thinner and less convex
Concept Map page 281
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81. Vertical Section of The Human Eye
Pupil Reflex
Focusing
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Distant object
(7 metres or more)
Ciliary muscles relax
Suspensory ligament become taut
Lens becomes thinner and less convex
Near object
Concept Map page 281
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82. Vertical Section of The Human Eye
Pupil Reflex
Focusing
In bright light
Circular muscles in iris contract.
Radial muscles relax.
Pupil becomes smaller, hence less light enters the eye
In dim light
Radial muscles in iris contract .
Circular muscles in iris relax.
Pupil enlarges, hence more light enters the eye
Distant object
(7 metres or more)
Ciliary muscles relax
Suspensory ligament become taut
Lens becomes thinner and less convex
Near object
Ciliary muscles contract
Suspensory ligaments slacken
Lens becomes thicker and more convex
Concept Map page 281
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