suspensory ligaments
sclera
The Eye
choroid
cornea
fovea
pupil
aqueous humour
optic nerve
lens
iris
retina
ciliary body
vitreous humour
The eyes move in unison and are positioned at the front of the head. They are housed in
bony sockets called orbits for protection. They are anchored in the orbits by muscles
attached to the outer part of the eye. The muscles are also responsible for the movement of
the eye.
The front surface of the eye is moist, smooth ad curved. Our field of vision is restricted and
our range of vision is limited. The effectiveness of the eye may vary. The pupil size alters
according to the intensity of light.
Function and structure
External Features
Eye lids
cover and protect the eye. Closing the eye lids can be both voluntary
and involuntary. Regular blinking (closing and re-opening of the eye
lids) distributes fluid throughout the eye. This stops the eye from
drying and also removes dust.
Conjunctiva
this is a thin layer (singer layer of epithelial cells), which lines the
inside of the eyelid and the front of the eyeball. It is transparent.
Tear glands
these are open under the eyelids. They secrete a mixture of sodium
bicarbonate, sodium chloride and enzymes that kill bacteria.
Function of tears
protection (i.e. enzyme)
washes away dust
keeps surface of eye moist
excess fluid drains into the nasal cavity by a duct on the inside of the
eye
Muscles of the eye attached to the outer wall of the eye
(sclerotic layer) and inner wall of the orbit
they anchor the eye and are used for
movement
Internal features
There are three layers to the outer wall of the eye:
Sclerotic coat
it is a tough, non-elastic, fibrous layer, which covers the whole of the
eye except where the optic nerve joins the back of the eye. It is
modified at the front of the eye to form the cornea.
Cornea
it is a transparent disc (so that light rays can pass
through). It has a curved surface and begins the
refraction of light.
Choroid
this is living tissue. It has blood vessels supplying glucose and oxygen
to the eye. It is deeply pigmented (in man it is black so that it can
absorb all the light. In animals, it is not so pigmented). This absorbs
and prevents the reflection of light.
Retina
this is the inner layer at the back of the eye where the image is
formed. It contains two types of light sensitive cells: rods and cones.
BUT there are no light sensitive cells where the optic nerve joins the
back of the eye. This is known as the blind spot.
Rods are sensitive to light of low intensity (black and
white image). They are scattered fairly evenly
throughout the retina except on the fovea, where
only cones are situated.
Cones are sensitive to high light intensity (colour
image). They are highly concentrated in they in
the fovea – where only cones are situated.
Other features
Blind spot
No sensitive cells (i.e. cones/rods) are at the blind spot. This is because
all of the axon fires from the sensory receptors join to form the optic
nerve, which passes the eye and a hole in the skull to the brain. If an
image falls on the blind spot, no impression is recorded because there
are no light sensitive cells here. We are not aware of this blank in our
vision because we have two eyes, which scan the same field.
Aqueous humour
a fluid-like medium at the front of the eye between the lens and cornea
Vitreous humour
a jelly-like medium that makes up the main body of the eye between
the lens and the retina.
Functions of both humours:
refract light to provide image for retina
provide food for eye
provide pressure outwards to give the eye a shape
Crystalline lens
the cornea begins the refraction of light and the lens continues this
process. The lens is held in position by suspensory ligaments, which
are attached to ciliary muscle at the front edge of the choroid. The
shape of the lens is altered by the contraction and the relaxation of the
muscles of the ciliary body.
Ciliary body
this has muscle and is positioned at the thickened front end of the
choroid.
Suspensory ligaments
these are non-elastic
Focusing on an object
“Accommodation” is the ability of the eye to receive light from far or near objects and to
focus a sharp image on the retina.
Far object (the eye is in its resting state)
-
the ciliary muscles are relaxed
the suspensory ligaments are pulled taut (tight)
lens is long and thin – i.e. less refraction
Near object
-
the ciliary muscles are contracted
the suspensory ligaments are slack (loose)
lens is fatter and shorter – i.e. greater refraction
The iris – light and dark
The iris is muscle. It is the pigmented/coloured part of the eye, which can be seen (e.g.
brown, blue, green etc.). In the centre of the iris is the pupil (the gap). The size of the gap
(pupil) is altered by the contraction and relaxation of two muscles (radial and circular) in the
iris. The size of the pupil alters in response to the amount of light. In the dark the pupil is
larger, in brightness the pupil is smaller. This is to retain a constant amount of light in the
eye (homeostasis) so that the eye is not damaged by bright light.
This is an example of
muscles working in
Dim light: radial muscles contract
antagonistic harmony.
circular muscles relax
pupil gets larger
The iris remains the
same size.
Bright light: radial muscles relax
circular muscles contract
The colour of the iris is
pupil gets smaller
determined by genes.
Fovea (centralis)
There is a small depression (dip) in the centre of the retina called the fovea. In this part of
the retina, there are no rods – only cones. The fovea has the highest concentration of light
sensitive cells in the retina and therefore, gives the best interpretation of the image.
The living parts of the eye include:
lens
cornea
They all obtain their nutrients from the humours.
choroid
conjunctiva
Image formation
The image formed on the retina is:
1.
smaller
2.
upside down
3.
a real image – it is formed by light rays
The brain receives and interprets the image (via nerve impulses from the optic nerve) and
converts it into the upright position.
Eye Experiments
1.
The blind spot experiment: take a long word on a piece of paper. Hold it out in front
of you. Close one eye. Read the letters one by one. As you read along the word,
when you reach one of the letters, the first letter will disappear but when you
continue onto the next letter, the first letter will reappear again.
2.
Shut one eye and rotate the other one. Both eyes are moving.
3.
Close one eye. Take two objects the same shape and colour, but different in size. Try
to get them level. It is difficult because both eyes are needed to judge distance.
4.
Angle of the eyes: Tell a partner to focus on a spot far, and then a near spot.
Measure the distance between the pupils in both cases. For far focussing, the distance
is greater than for near focussing.
5.
Effect of light on pupil size: Using a torch/light, see the difference in the size of your
partner’s pupil in bright light.
6.
Weak/strong eye: look to a tree in the distance. Place a pen directly on top of the
tree using one eye. Open the other eye. Do the same with the other eye. Whichever
eye moves the pen the least is the strongest.