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Biology 14 Eyes Cheat Sheet by

Growth and development

Human eye

Eyebrows
prevent sweat from running into the eyes
Eyelash
traps dust and prevent it from running into the eyes
Eyelid
protect eyes from dirt and strong light
 
spread tears over eye surface when blinking
Tear gland
produces tears (sodium, chlori­de..) kills bacteria
Tear duct
drains tear into nasal cavity

Process of seeing

1
light rays from object enters the eye
 
refracted and focused onto retina
by
cornea, aqueous humour, lens , vitreous humour
cornea
most of the refraction
lens
fine focusing
2
real and inverted image formed on retina
3
photor­ece­ptors stimulated by light generate nerve impulse
 
nerve impulse travel along optic nerve to visual centre
4
visual centre in cerebrum interpret nerve impulse as an upright image of object
 

structure

Orbit
socket in the skull
 
attached by 3 pairs of eye muscles
3 eye muscles
allow eyeball to rotate in different direction
Conjun­ctiva
transp­arent membrane
 
keeps front of eyes lubricated and moist

Types of photor­ece­ptors

Rod cells
Cone cells
more numerous
less numerous
sensitive to low light intensity
sensitive to high light intensity
important for dim light vision
important for bright light vision
black and white vision
color vision (red, green, blue)

DIstri­bution of photor­ece­ptors on retina

Rod cells
periphery of the retina
 
none at yellow spot and blind spot
Cone cells
concen­trated at yellow spot
 
a few present on periphery of retina
 

Functions

Sclera
tough white coat
 
protects inner structure
 
maintain shape of eyeball
 
surface for the attachment of eye muscle
Cornea
transp­arent layer of tissue
 
allows light to enter the eye
 
a curved surface to help refract and focus light on retina
 
no capill­aries since it obtains nutrients from aqueous humour
Choroid
middle layer of eyeball
 
contains black pigment that absorbs light
 
pigment reduces reflection of light within the eye and helps form sharp images
 
rich in capill­aries as it supplies nutrients to sclera and retina
Iris
continuous with the choroid
 
made up of muscles which controls the size of pupil to regulate the amount of light entering the eyes
 
contains pigment that determine the colour of iris
Pupil
an opening at centre of the iris
 
allows light to enter the eyes
 
size is controlled by iris
Retina
innermost layer of eyeball
 
contains many photor­ece­ptors and nerve fibres
 
photor­ece­ptors : rod or cone cells
Optic nerves
nerve fibres in retina grouped
 
transmits nerve impulses from photor­ece­ptors to cerebrum of brain
Yellow spot
high density of cone cells
 
no rod cells
Blind spot
no photor­ece­ptors
Lens
transp­arent , elastic , biconvex
 
refracts and focuses light on retina
 
thickness adjusted by ciliary body
 
living cells with no nuclei
 
no capill­aries as it obtains nutrients from aqueous humour
Suspensory ligament
connected to ciliary body
Ciliary body
contains ciliary muscles
 
controls tension of suspeo­nsory ligaments
 
controls thickness of lens
Aqueous humour
watery fluid produced by ciliary body between cornea and lens
 
supplies nutrients and o2 to cornea and lens by diffusion
Vitreous humour
jelly like fluid between lens and retina
Both
refracts lights on retina
 
maintain shape of eyeball
 

Drawing ray diagrams

Light rays (distant)
parallel
Light rays (near)
come from the same point of object
Reminders
add arrows
 
dotted lines behind retina

Seeing in dim light

Circular muscles of iris
relaxes
Radial muscles
contracts
Pupil
dilates
Result
more light enters eyes
Importance
allow photor­ece­ptors to be stimulated so a clear image forms

Seeing in bright light

Circular muscles of iris
contracts
Radial muscles
relaxes
Pupil
constricts
Result
less light to enter eye
Importance
prevent photor­ece­ptors from being damaged by bright light

Near objects

Ciliary muscles
contract
Tension
reduced
Suspensory ligaments
slackened
Lens
thicker (more convex)
 
refracts more light
Eye strains
ciliary muscles contra­cting for a long time

Distant objects

Ciliary muscles
relaxes
Tension
increases
Suspensory ligaments
tightened
Lens
thinner (less convex)
 
refracts light less

Short sighted

Vision problem
cannot see distant object clearly
Cause
lens too thick
 
eyeball too long
Image
formed in front of retina
Correction
concave lenses (diverges light)

Long sighted

Vision problem
cannot see near objects clearly
Cause
lens too thin
 
eyeball too short
Image
formed behind the retina
Correction
convex lens (converge light)

Color blindness

Problem
cannot distin­guish colors
Cause
deficiency of one or more cone cells
Correction
no cure
 
wear lenses
 

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  • Published: 18th October, 2022

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