- Sclera - Tough white outer coating (the white part of your eye is actually the sclera. The main function of the sclera is to protect the eyeball.
- Cornea - This is actually the front part of the sclera but unlike the sclera it is transparent. The function of the cornea is to refract light rays into the eye.
- Conjunctiva - This is a thin epithelium which protect the cornea.
- Vitreous humour and aqueous humour - The liquid behind the lens is jelly-like and is called vitreous humour while the aqueous humour in front of the lens is watery. The function of both the vitreous humour and the aqueous humour is to keep the spherical shape of the eyeball. In addition, the aqueous humour also functions in providing nourishment to the non-vascularised lens and cornea.
- Lens - Transparent structure (flexible and can change its shape during accomodation) which refract light rays on to the retina.
- Suspensory ligament - To hold the lens in place. Plays a very important role in accomodation.
- Iris - Iris gives colour to your eyes. It consists of the radial and the circular muscles. The iris controls the size of the pupil, thus controlling the amount of light entering the eyes. These muscles of the iris work antagonistically.
- Pupil - Pupil is a hole at the centre of the iris. The function is to allow light to enter the eye.
- Choroid - The second layer of the eye. It is highly vascularised to provide nourishment to the eye. It is also pigmented black (absorbs light) so as to prevent internal reflection in the eye.
- Ciliary body - It produces aqueous humour. It contains circular muscles which helps to alter the size of the lens during accomodation.
- Retina - The internal lining at the back of the eye is the retina. It contains light sensitive cells (the cones and the rods) which respond to light.
- Fovea (yellow spot) - The part of the retina which is very sensitive to light since rods and cones are highly concentrated here. When you focus on an object, the image of the object will fall on to this region.
- Blind spot - This region of the retina contains no light sensitive cells so object will not be seen here if light falls on to this region.
- Optic nerve - This contains nerve fibres which transmit electrical impulses to the brain.
- Tear glands - These are glands present under the top of the eyelid. The function is to produce tear fluid which helps to prevent friction when blinking. The tear fluid also wash away any dust particles or foreign bodies. It contains lysozyme which kills bacteria.
2. ACCOMODATION
- Viewing distant image - Ciliary muscles relax, the suspensory ligaments become taut pulling the lens outwards. As a result the lens becomes thinner. This will in turn, increased the focal length hence enable you to view distant object.
- Viewing near object - Ciliary muscles contract, the suspensory ligaments become slacken. Hence there is no pulling force to pull the lens outwards. As a result the lens becomes thicker and this will decrease the focal length. Hence you are able to focus at near object.
3. PUPIL REFLEX
The following figure shows the iris which control the size of the pupil thus controlling the amount of light entering the eye.
In Bright Light AND In Dim Light- The retina (due to the presence of rods and cones) is very sensitive to light. When light falls on to the retina, the light (stimulus) will stimulates the retina. An electrical impulse will then be transmitted to the brain along the sensory nerve fibres in the optic nerve which contains sensory neurones. At the synapse between the sensory neurones and the relay neurones, acetylcholine will be released which stimulates the relay neurones in the brain to produce an electrical impulse. The electrical impulse is then transmitted along the relay neurones. Again at the synapse between the relay neurones and the motor neurones, acetylcholine will be released which stimulates the motor neurones to produce an electrical impulse. The electrical impulse is then transmitted along the motor neurones to the effector (which in this case are the radial and circular muscles of the iris).
- In dim light, the radial muscles contract and the circular muscles relax. This causes the pupil to dilate and hence allows more light to enter the eye.
- In bright light, the radial muscles relax and the circular muscles contract. This causes the pupil to constrict and hence allows less light to enter the eye (thus protects the delicate light sensitive cells in the retina)
