Sense of Vision
The eyes are light sensitive organs
placed inside bony sockets called ‘orbits’ .
How does the eye turn?
There are six sets of muscles which are capable of turning the eye
Fig – 1 .View of the human eye.
1. Eye lids
Eyelids protect the eye from, mechanical, chemical damage.
They cover the eye by reflex action in the presence of a brilliant light protecting the inner tissues.
2 .Sclera
This is the the tough outer covering of the eye. It consists of collagen fibres.
This maintains the shape of the eye and offers protection.
3. Cornea
This is the front part of the sclera.
It is transparent and overlaps the iris and the pupil.
This layer refracts the light prior to entering the lens.
4. Iris.
This consists of muscular tissue with coloured pigments.
Two sets of muscles, circular and radial control the size of the pupil according to the light available.
Iris controls the amount of light entering the eye
Iris colours are due to these pigments.
Eumelanin - brown/ black
Pheomelanin – red / yellow.
Fig.– 2 Muscle movements of the eye.
5. Lens and focusing.
The crystalline biconvex structure separates the aqueous humour and vitreous humour.
Ciliary muscles and suspensory ligaments can control the shape of this.
It is by this process that an image is focused on to the retina.
Under Bright light. | Under Low light. |
Circular muscles contract. Pupil small.
Radial muscles relax.
| Circular muscles relax. Pupil large.
Radial muscles contract(Shorten) |
Fig. – 3 Vertical section of the eye.
- Do you agree, if somebody says
- "almost the same amount of light enters the eye under bright and dim lighting conditions".?
6. Pupil.
This is the window of the eye, through which the light enters.
It appears black as light goes right in without being reflected.
Light is absorbed by the cells in the retina, at the back of the eye.
7. Retina.
When an ophthalmologist uses an ophthalmoscope to look into your eye he sees the retina as in this figure.
Fig– 4. Ophthalmoscope view of the eye.
In the center of the retina is the ‘optic nerve’, a circular white area measuring about 2 x 1.5 mm. across.
From the centre of the optic nerve, radiate the major blood vessels of the retina.
A section of the retina shows that the ganglion cells lie closest to the lens and the ‘rods and cones’, which sense the light (photo sensors) lie right at the back.
They convert the image that falls on the retina to electrical signals and send to the brain with the help of ganglion cells.
‘Rods’ are supposed to give black and white vision even in very poor light.
Three types of ‘cones’ recognize the colours.
Fig. 5 A section of retina showing light sensitive cells.
Fig. 6 The structure of the eye.
Fovea
This is an oval shaped region , only 0.5mm in length.
It has a slight reddish colour.
In spite of the colour it has no blood vessels but completely filled with cones; No rods.
Fine details of the image are recognized here.
Choroid
This is the layer that supplies blood to the retina by a network of fine capillaries tinted black to reduce the scattering of light inside the eye.
Aqueous humour
The clear liquid that fills the region between the cornea and the lens. This is secreted by the ciliary body and is rich in salts.
Vitreous humour
A jelly like substance supporting the eye ball.
Ciliary muscles
Smooth muscle fibres consisting of circular and radial muscles which are capable of changing the shape of the lens.
Suspensory ligaments
Attaches the muscles to the lens.
Optic nerve
The sheath of nerve fibres connecting the retina to the brain.
Blind spot
The opening in the retina for the optic nerve. This region has no light sensitive cells and so named as the blind spot.
FUNCTIONING OF THE EYE
Fig.- 7 How a camera works.
The eye works in a way quite similar to the camera.
When the light comes from an object, the rays are refracted in a manner to produce an image as shown in Fig. – 7 .
In a camera, the image is clearly brought to the film by shifting the lens toward the object or the image. This effort is called focusing.
Method of Focusing the Eye.
In the eye focusing the image on the retina is achieved by a method called the‘accommodation’.
In a glass lens the focal length does not change as it depends on the curvature.
In the eye-lens the curvature can be changed.
When you wish to look at a closer object, what you should do is to increase the curvature of the lens.
This is performed by the contraction of the ciliary muscles.
As the lens is soft and flexible it becomes thicker, increasing the curvature to shorten the focal length.
This will make the image fall on the retina.
Lens is not the only part that refracts the light.
The cornea also refracts to some extent, but the lens does the finer adjustments.
In old age, ciliary muscles become week and the lens would not compress to become thick.
Then only people have to wear glasses.
- Fig. 8. Correcting the short sightedness and far sightedness.
1.0 Name the numbered cages.
Q. 2.0 Find suitable words or phrases for the numbered spaces.
|
Name
|
Structure
|
Function
|
|
1………
|
A biconvex
structure inside the eye.
|
Focuses the image on the
retina.
|
|
Choroid
|
Network
of capillaries tinted black..
|
2
|
|
3………..
|
Smooth
muscle fibres consisting of circular and radial muscles.
|
Change
the shape of the lens.
|
|
4………..
|
The
sheath of nerve fibres at the base of the eye.
|
Passes
nerve impulses to the brain.
|
|
5………..
|
An
oval shaped region , only 0.5mm.A slight reddish colour. It has no blood
vessels but completely filled with cones; No rods.
|
A
highly sensitive region of the retina.
|
|
Sclera
|
A tough cover of the eye consisting of
collagen fibres.
|
6.……………..
………………
|
|
7……….
|
A layer of cells at the back of the eyeball.
|
Converts light to nerve impulses.
|
|
8……….
|
A small area at the beginning of the
optic nerve.
|
No special function.
|
|
Vitreous humour
|
A jelly like substance supporting the eye ball.
|
.9……………….
|
|
10. …….
|
Crowded in the central region of retina.
|
Specializes
in colour perception.
|
3.0 Multiple choice questions.
1. What happens to a ray of light passing through cornea and lens.
A- Accomodation, B - Focusing. C- Reflection. D- refraction
2. What function is performed when the muscles change the shape of the lens?
A= Accommodation. C- Protecting D - Increasing the pupil size, E - Decreasing the pupil size.
3. Mother said , " She got blue eyes". What part of the eye gives colour?
A - Cornea. B. -Irs. C. - Pupil. D.- Retina.
4. Which is the most light sensitive region of the eye.
A- Sclera. B.- Fovea C.- Choroid D - Lens.
5.What gives vision even in very poor light?
A - Rods. B - Cones. C- Blind spot. D- Conjunctiva.
6.What causes the blind spot?
A - Fovea B - Aqeous humour C Vitreous humour D - Optic nerve.
7. Area filled with blood capillaries.
A Sclera B - Pupil C- Choroid D.- Retina.
8. Why in all eyes the pupil is black?
A - It reflects light. B- It absorbs light C.- It refracts light D - It absorbs light.
9. In a healthy normal eye the image is described as follows.
A - A real, upright image formed on the retina.
B- A virtual image upright image formed on the retina
C- A diminished real inverted image on the retina.
D - A diminished real inverted image before the retina.
10 .What is the advantage of having two eyes.
A - Easy to judge the distance. B .- Clear vision. C- Good vision in poor light
D- If one fails the other can see.
For answers visit Answer page- Biology.
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