1. Coordination and Response in Plants and Animals
The Structure and Function of the Eye

2. Receptors, Effectors and the Central Nervous System
Syllabus Objectives
Relate the structure of the eye in man to its function as a sense organ
Explain sight defects and their corrections

3. Introduction The human eye is a highly specialized sense organ
It detects the stimulus of light
It is protected by a bony socket (orbit) in the skull
Tear glands under the upper eyelids secrete tears
Which moistens
Lubricates
Protects the eye as blinking occurs
The eye is made up of 3 layers:
Sclera/sclerotic coat
Choroid layer
Retina

4. Sclera/sclerotic coat
Tough outer white layer
Protects the eye
Continues over the front of the eye as transparent structure
cornea (helps refract light rays which enter the eye)
The conjunctiva is a thin transparent membrane
Lines the eyelids and the exposed part of the front of the eyeball

5. Choroid layer Pigmented middle payer
Prevents reflection of light in the eye
Contains numerous blood vessels which supply nutrients to the tissues in the eye
Forms ciliary body and the iris at the front of the eye
Ciliary body is connected to the lens by suspensory ligaments

6. Choroid layer cont’d
The ligaments and the ciliary muscles work together to adjust the shape of the lens
The lens a structure that is
elastic
transparent and
Biconvex
The lens bends light rays to a point of focus on the retina

7. Choroid layer– the iris and pupil
In front of the lens is the iris
The coloured part of the eye
It is the circular disc of muscles (circular & radial muscles)
It surrounds a central hole – the pupil
The iris controls the amount of light entering the eye by adjusting the size of the pupil

8. Choroid layer cont’d
In front of the lens there is a transparent liquid – aqueous humour
Behind the lens is a transparent jelly-like material – vitreous humour
The aqueous and vitreous humours help maintain the shape of the eyeball

9. Retina The inner layer contains light sensitive cells
Rods (sensitive to dim light. Allows black and white vision)
Cones (sensitive to bright light. Gives colour vision)
The fovea or yellow spot is directly behind the pupil
Only contains cones

10. Retina – rods and cones
There are more rods towards the outer edges of the retina
Rods and cones send impulses to the brain along the optic nerve
There are no light sensitive cells at the point where the optic nerve leaves the eye – blind spot
If light rays fall on this spot no impulses will be sent to the brain

11. Functioning of the Eye - Introduction
The eye is the sense organ which receives the stimulus of light
Light rays from objects enter the eye and are refracted (bent) by the cornea
These light rays pass through the aqueous humour, the lens and the vitreous humour where further refraction occurs
The light rays then fall on the retina where an image is formed
The rods and cones then sends information to the brain along the optic nerve
The brain interprets what is seen

12. The Functioning of the Eye - Accommodation
Light rays from an object spread out in all directions
Rays from distant objects are almost parallel to each other
Rays from near objects are more spread out
For an object to be seen clearly and in detail
Rays must be bent (refracted) to be focused exactly on the retina
Light rays from near objects therefore need more refraction than rays from distant objects

13. The Functioning of the Eye - Accommodation
The eye changes the degree of refraction by changing the shape of the lens
This results in the production of clear images of both near and distant objects
The shapes of the lens is altered by the contraction and relaxation of the ciliary muscles
The amount of bending caused by the cornea, aqueous and vitreous humours is fixed
The ability of the eye to form sharp images on the retina of near and distant objects is called accommodation!

14. The Functioning of the Eye - Accommodation
When viewing distant objects the lens gets thinner (less convex)
Relaxation of the ciliary muscles causes increased tension on the suspensory ligament
The suspensory ligaments now pull on the lens making it long and thin
A thin lens is less refractive and therefore focus distant objects

15. The Functioning of the Eye - Accommodation
When viewing near objects the lens gets thicker (more convex)
Contraction of the ciliary muscles causes a release of tension on the suspensory ligaments
Because the lens is elastic, it becomes short and fat
This increases the refractive power of the lens and allows near objects to be focussed

16. The Control of Light Entering The Eye
The amount of light that enters the eye is determined by the size of the pupil
The diameter of the pupil is in turn controlled by the muscles in the iris
The iris has muscles arranged in 2 layers:
Circular and
Radial muscles
These 2 sets of muscles are antagonistic in action

17. The Control of Light Entering The Eye – In bright light
The diameter of the pupil decreases
This prevents damage to the retina
The circular muscles of the iris and the radial muscles relax
Allowing only a small amount of light to enter the eye

18. The Control of Light Entering The Eye – In dim light
The radial muscles of the iris contract and the circular muscles relax
The diameter of the pupil increases
More light enters the eye
Allowing a brighter image to be formed in dim light

19. Sight defects and their corrections – normal vision
A normal eye can accommodate to objects at different distances by changing the shape of the lens and its refractive power

20. Sight defects and their corrections – short sight
The eyeball is longer than normal (from back to front) or the cornea too curved
Light from a near object is clearly focused on the retina
Rays from a distant object are focused in front of the retina and produce a blurred image
A concave lens which converge the rays from a distant object and bring them to a point of focus on the retina

21. Short sight correction

22. Sight defects and their corrections – long sight
This occurs when the eyeball is shorter (from back to front) than normal or the lens is too fat
Light from a distant object is clearly focused on the retina
However, the image from a near object is focused behind the retina and produces a blurred image
A convex lens converges the light rays and causes the image from a near object to be focused on the retina

23. Long sight correction

24. Aging and sight defects
With age the lens become hardened and less elastic
Ability to accommodate for near vision (reading) decreases
Reading glasses with convex lens corrects this condition

25. Cataracts This is the most common cause of blindness
It results when normal transparent lens becomes opaque
This prevents light from passing through and reduces vision
It can be surgically treated by
removing the damaged lens and replacing it with artificial lens implanted into the eye or
using spectacles or contact lens

26. Glaucoma
The aqueous humour in front of the lens is usually drained away from the eye so that it does not build it
When the flow of aqueous humour becomes blocked, too much of it collects in the eye and the pressure increases within the eyeball
The high pressure causes damage to the optic nerve and normal vision is reduced, eventually leading to blindness in most cases
This condition can be controlled with the use of eye drops and or tablets which help to lower the pressure
Surgical treatment and lasers can also be used to drain the fluid from the affected eye