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Title: Human Eye
Description: Notes on the human eye, including external and internal parts of the eye, functions and uses, pupil reflex and viewing in the presence and absence of light, focusing on objects at varying distances .
Description: Notes on the human eye, including external and internal parts of the eye, functions and uses, pupil reflex and viewing in the presence and absence of light, focusing on objects at varying distances .
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Chapter 14 Notes
The eye is the sense organ responsible for sight
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External Parts of the Eye
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Iris A circular sheet of muscles that contains a pigment which gives the eye its colour
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Conjunctiva A thin white membrane covering the sclera in front
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It is continuous with the
skin of the eyelids
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It is continuous with the cornea and protects
the eyeball from damage
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The eyelids also
partially close to prevent excessive light from entering the eye and damaging the retina
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Tear Gland Lies at the corner of the upper eyelid and secretes tears which wash away dust
particles and keeps the cornea moist for atmospheric oxygen to dissolve and diffuse into the
cornea
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Eyelashes Help to shield the eye from dust particles
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It contains ciliary muscles which
control the curvature or thickness of the lens
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Cornea A domeshaped transparent layer continuous with the sclera or white of the eye
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The cornea causes most of the refraction of light that
occurs in the eye
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It is filled with aqueous
humour, a transparent, watery fluid
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Lens A transparent, circular and biconvex structure
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Retina The innermost layer of the eyeball
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It contains lightsensitive cells or photoreceptors
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Cones enable us to see colours in bright light while rods enable us to see in black and
white in dim light
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Blind spot The region where the optic nerve leaves the eye
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Optic Nerve A nerve that transmits nerve impulses to the brain when the photoreceptors in the
retina are stimulated
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It is situated directly behind the lens
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The yellow spot contains the greatest concentration of
cones but no rods and it enables a person to have detailed colour vision in bright light
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It is filled with vitreous humour, a transparent,
jellylike substance
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Choroid The middle layer of the eyeball (between the sclera and the retina)
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It contains blood vessels that bring oxygen and
nutrients to the eyeball and remove metabolic waste products
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There are three types of cones: red, blue and green
cones
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All the
cones together enable us to see a wide variety of colours
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Rods are
more sensitive to light than cones
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Rods
are sensitive to light of low intensity (dim light) because they contain a pigment called visual purple
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Visual purple must be
reformed for a person to see in the dark
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Therefore, a
person deficient in vitamin A may be unable to see in dim light
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In bright light, less light must enter the eye
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The size of the pupil is controlled by the circular
and the radial muscles
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The circular and radial muscles are called antagonistic muscles as while
one muscle contracts, the other relaxes
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The circular muscles of the iris contract
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The radial muscles of the iris relax
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The pupil becomes smaller or constricts
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In dim light
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This increases the amount of light entering the eye
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The pupil usually becomes larger when
the surrounding light intensity is low and smaller when the light intensity is high
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Thus, the eyelids have to
come closer together to screen off part of the light
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Stimulus (change in light intensity) > Receptor (retina) > Sensory neurone in
optic nerve > Brain > Motor neurone > Effector (Iris)
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Some of these reflected rays fall on the
eyes
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The lens
causes further refraction and the rays are brought to a focus on the retina
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The image formed on the
retina is upside down, laterally inverted and diminished (smaller sized)
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These impulses are transmitted via the optic nerve to the brain
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Focusing is the adjustment of the lens of the eyes so that clear images of objects at different distances
are formed on the retina
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In
focusing, the thickness of the lens is adjusted to allow light rays to be focused on the retina
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Focusing on a distant object
The lens need to be thin as only a little refraction is needed due to parallel light rays
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Ciliary muscles relax, pulling on the suspensory ligaments
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Suspensory ligaments become taut, pulling on the edge of the lens
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The lens becomes thinner and less convex, increasing the focal length
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Light rays from the distant object are sharply focused on the retina and photoreceptors are
stimulated, sending nerve impulses to the optic nerve to the brain
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Title: Human Eye
Description: Notes on the human eye, including external and internal parts of the eye, functions and uses, pupil reflex and viewing in the presence and absence of light, focusing on objects at varying distances .
Description: Notes on the human eye, including external and internal parts of the eye, functions and uses, pupil reflex and viewing in the presence and absence of light, focusing on objects at varying distances .