chapter 14: the human eye Flashcards
where is the eye ball?
- each eyeball lies in a hollow in the skull called the
> ORBIT - each eyeball is attached to the skull by
>ERECTUS MUSCLES which control eye movement
what is the iris?
(the front part of the human eye)
- a circular sheet of muscles
> contains a pigment which gives the eye its colour - the amount of light entering the eye is controlled by the two sets of involuntary muscles in the iris
> circular muscles & radial muscles
what is the conjunctiva?
(the front part of the human eye)
- a thin transparent membrane covering the sclera in front
- it is a mucuous membrane
> it secretes mucous
> helping to keep the front of the eyeball moist - it is continuous with the skin of the eyelids
what is the sclera/ sclerotic coat?
(the front part of the human eye)
- a tough, white outer covering of the eyeball
- it is continuous with the cornea
> protects the eyeball from mechanical damage
what is the pupil?
(the front part of the human eye)
- a hole in the centre of the iris
- the pupil allows light to enter into the eye
what do the eyelashes do?
(the front part of the human eye)
- they help to shield the eye from dust particles
what is the tear gland?
(the front part of the human eye)
- a gland lying at the corner of the upper eyelid
secretes tears which:
- wash away dust particles
- keep the cornea moist for atmospheric oxygen to dissolve
> dissolved oxygen diffuse into the cornea
- lubricate the conjunctiva, helping to reduce friction when the eyelids move
what are the functions of eyelids?
(the front part of the human eye)
- they protect the cornea from mechanical damage
- the eyelids can be partly closed
> squinting prevents excessive light from entering the eye and damaging the light-sensitive tissues (retina) inside - blinking spreads tears over the cornea and conjunctiva and wipes dust particles off the cornea
what is the ciliary body?
(the internal structure of the eye)
- a thickened region at the front end of the choroid
> contains ciliary muscles
> which control the curvature or thickness of the lens
what is the choroid?
(the internal structure of the eye)
- the middle layer of the eyeball between SCLERA and the RETINA
two functions:
- it is pigmented black to prevent internal reflection of the light
- contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products
what is the vitreous chamber?
(the internal structure of the eye)
- the space behind the lens
> filled with VITREOUS HUMOUR
> a transparent, jelly-like substance
> it keeps the eyeball firm and helps refract light onto the retina
what is the fovea or yellow spot?
(the internal structure of the eye)
- a small yellow depression in the retina
- it is situated directly behind the lens
>where images are normally focused - the fovea contains the greatest amount of cones > no rods
- it enables a person to have a detailed colour vision in bright light
what is the blind spot?
(the internal structure of the eye)
- the region where the optic nerve leaves the eye
> does not contain any rods or cones
> not sensitive to light
what is the optic nerve?
(the internal structure of the eye)
- a nerve that transmit nerve impulses to the brain when the photoreceptors in the retina are stimulated
what is the retina?
(the internal structure of the eye)
- the innermost layer of the eyeball
- light sensitive layer which images are formed
- it contains light-sensitive receptors or photoreceptors
> photoreceptors consists of rods and cones - cones enable us to see colours in bright light
- rods enable us to see in black and white in dim light
- photoreceptors are connected to the nerve endings of the optic nerve
what is the lens?
(the internal structure of the eye)
- it is transparent, circular, biconvex structure
> elastic and changes its shape or thickness in order to focus light onto the retina
what is the aqueous chamber?
(the internal structure of the eye)
- the space between the lens and cornea
> filled with aqueous humour, a transparent watery fluid - aqueous humour keeps front of the eyeball firm and helps to refract light into the pupil
what is the cornea?
(the internal structure of the eye)
- a dome-shaped transparent layer continuous with the sclera (white of the eye)
> it refracts/ bends light ray into the eye
> cornea causes most of the refraction of light in the eye
what is the suspensory ligament?
(the internal structure of the eye)
- a connective tissue that attaches the edge of the lens to the ciliary body
what are the functions of cones?
- enable us to see colour in bright light
- three types of cones: red, blue and green
- each cone contains a different pigment which absorbs light of different wavelengths
> all the cones together enable us to see a wide variety of colours
> cones do not work well in dim light
what are rods?
- more sensitive to light than cones
> enable us to see in dim light > only in black and white - rods sensitive to light of low intensity (dim) because they contain a pigment called visual purple
- when a person is exposed to bright light,
> visual purple is bleached
>visual purple must be reformed for a person to see in the dark - when you move from a brightly lit room to a dimly lit room
> may not be able to see objects arnd you for sometime
> takes time for visual purple to be formed again in rods - formation of visual purple requires vitamin A
> person deficient in vitamin A > may not be able to see in the dark
> night blindness
how does the iris control the amount of light entering the eye?
- to see clearly, only the right amount of light should enter the eye
- eg. more light must enter the eye in dim light
- the size of the pupil determines how much light enters the eye
-the size of the pupil controlled by two sets of involuntary muscles
> CIRCULAR MUSCLES and RADIAL MUSCLES
- circular muscles are arranged in a circle round the pupil
> radial muscles arranged radially
>circular and radial muscles are antagonistic muscles
how does the iris control the amount of light entering the eye?
in a brightly lit room and dimly lit room.
bright light:
- the circular muscles of the iris contract
- the radial muscles of the iris relax
- the pupil becomes smaller / constricts
> reduces the amount of light entering the eye
dim light:
- radial muscles of iris contract
- circular muscles of iris relax
- the pupil enlarges/dilates
> increases the amount of light entering the eye
what is the pupil reflex?
- a reflex action
> the pupil changes size in a result of changes in light intensity
> pupil becomes larger when light intensity is low
>smaller when light intensity is high - sometimes light may be so bright that decreasing the size of the pupil is not enough
> eyelids have to come closer together to screen off part of the light
>prevents excessive light from entering the eye and damaging the retina
-receptor: the retina
- effector: iris
how do we see?
- the light rays are refracted through the cornea and the aqueous humour onto the lens
- the lens causes further refraction and the rays are brought to a focus of the retina
- image on the retina stimulates either the rods or cones
> depending on light intensity
the image formed on the retina is :
- upside down (inverted)
- laterally inverted
- diminished (smaller than the actual object)
- image on the retina stimulates either the rods or cones
- nerve impulses are produced when light falls on rods and cones
> impulses are transmitted via OPTIC NERVE to the brain - brain interprets the impulses so that we see the object the right way up, front to back and the right size
what is focusing?
- the adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina
> during focusing, the thickness or curvature of the lens is adjusted
allows light to be focused on the retina
what are the changes in the eye when focusing on a distant object?
( 7 meters or more)
- ciliary muscles relax, pulling on suspensory ligaments
- suspensory ligaments become taut, pulling on the edge of
the lens - lens become thinner and less convex, increasing its focal length
- light rays from the distant object are sharply focused on the retina
- photoreceptors are stimulated
- nerve impulses produced are transmitted by the optic nerve to the brain
> the brain inteprets the impulses and the person sees the distant object
what are the changes in the eye when focusing on a near object?
- ciliary muscles contract, relaxing their pull on the suspensory ligaments
- suspensory ligaments slacken, relaxing their pull on the lens
- the lens, being elastic, becomes thicker and more convex
> decreasing its focal length - light rays from the near object are sharply focused on the retina
- photoreceptors are stimulated
- nerve impulses produced are transmitted by the optic nerve to the brain
> the brain interprets the impulses and the person sees the near object
