Chapter 14 The Human Eye
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
Sense organs Sense organs are receptors that receive stimuli from the environment and inform your body of any changes in the environment
14.2 The Human Eye Upper eyelid eyelashes Tear gland pupil Conjunctiva (covers sclera) cornea Lower eyelid iris
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
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
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
Chapter 14 The Human Eye Structure and Function of the Eye (Concept Map) 19 May 2019 Concept Map page 281
Vertical Section of The Human Eye Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 9
Vertical Section of The Human Eye suspensory ligament Attaches lens to ciliary body Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 10
Vertical Section of The Human Eye suspensory ligament iris Controls amount of light entering the eye Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 11
Vertical Section of The Human Eye suspensory ligament iris aqueous humour Refracts light rays; keeps eyeball firm Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 12
Vertical Section of The Human Eye suspensory ligament iris aqueous humour pupil Allows light to enter the eye Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 13
Vertical Section of The Human Eye suspensory ligament iris aqueous humour pupil cornea Refracts light rays into pupil Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 14
Vertical Section of The Human Eye suspensory ligament iris aqueous humour pupil cornea lens Concept Map page 281 Focuses light rays onto retina Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 15
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 16
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 17
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 18
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 20
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 21
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 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 ciliary body blind spot Region over optic nerve; no photoreceptors, so insensitive to light viterous humour Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 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 viterous humour retina Light-sensitive layer containing cones responsible for colour vision in bright light and rods responsible for vision in dim light Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 24
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
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
Action of Iris in Bright Light Circular muscles of the iris contract 1 Figure 14.5 (top) page 276 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 27
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 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 Pupil constricts and this reduces the amount of light entering the eye 3 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 29
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
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 31
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 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 Pupil dilates and this increases the amount of light entering the eye 3 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 33
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
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
Stimulus (change in light intensity) Receptor (retina) Sensory neurone in optic nerve Brain Motor neurone Effector (iris)
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
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)
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
Chapter 14 The Human Eye How Do We See? 19 May 2019 Figure 14.7 page 278 Figure 14.7 page 278
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19 May 2019 41 41
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19 May 2019 42 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 Rays converge to focus on retina cornea object 2 retina 3 aqueous humour Figure 14.7 page 278 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19 May 2019 43 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 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19 May 2019 44 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 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 19 May 2019 45 45
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
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
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 48
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 49
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 50
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 51
How Eyes Focus on Distant Objects 2 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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 52
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 53
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 54
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 55
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 56
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 57
How Eyes Focus on Distant Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 58
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
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
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 61
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 62
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 63
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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 64
How Eyes Focus on Near Objects 2 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) Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 65
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 66
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 67
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 68
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 69
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 70
How Eyes Focus on Near Objects 2 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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 19 May 2019 71
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
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 73
Vertical Section of The Human Eye Pupil Reflex In bright light Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 74
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 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 In dim light Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 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 Radial muscles in iris contract . Circular muscles in iris relax. Pupil enlarges, hence more light enters the eye Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 77
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 78
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 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 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) Ciliary muscles relax Suspensory ligament become taut Lens becomes thinner and less convex Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 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 Near object Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 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 Ciliary muscles contract Suspensory ligaments slacken Lens becomes thicker and more convex Concept Map page 281 Copyright © 2006-2011 Marshall Cavendish International (Singapore) Pte. Ltd. 82