HUMAN EYE –
STRUCTURE, FUNCTIONS AND PARTS
- The human eye is spherical in shape and situated within a socket or orbit in the skull.
- It is attached to the skull by three pairs of muscle, which also control its movement.
- It is made up of three main layers; sclerotic layer, choroid and the light sensitive retina.
- Outermost white part situated at the sides and back of the eye.
- Made up of collagen fibres.
- It protects the eye and gives its shape.
- This is the transparent front part of the sclera that allows light to pass through.
- It is curved, bulging at the front. It thus reflects light rays hence helps to focus light rays onto the retina.
- The second or middle layer.
- It has many blood vessels that supply nutrients to the eye and remove metabolic wastes from the eye.
- It has dark pigments to absorb stray light and prevent its reflection inside the eye.
- Is glandular and secretes aqueous humour.
- It has blood vessels for supplying of nutrients excretion and gaseous exchange.
- It has ciliary muscles – which contract and relax to change the shape of lens during accommodation.
- Are inelastic and attach the lens onto the cilliary body holding it in position.
- Biconvex in shape, to refract light.
- Crystalline and transparent to allow light to pass through and focus it on to the retina.
- Found between lens and the cornea.
- Transparent to allow light to pass through it.
- It is watery thus helping in focusing.
- Helps maintain shape of eye ball.
- To convey nutrients and oxygen to cornea, and remove waste products.
- The coloured part of the eye has an opening – the pupil at the centre.
- Iris has circular and radial muscles which controls size of the pupil, hence the amount of light entering the eye through the pupil.
- It is a fluid.
- Found between lens and retina.
- Is viscous and gives eye the shape.
- It is transparent and refracts light.
- Retina contains light sensitive cells and is situated at the back of the eye.
- There are two types of light sensitive cells in the retina:
- Rods – are sensitive to low-intensity light and detect black and white. Nocturnal mammals have more rods.
- Cones – are sensitive to high intensity of light;
- They detect bright colour.
- Diurnal mammals have more cones.
- Fovea centralis (yellow spot) is the most sensitive part of the retina.
- Consists mainly of cones for accurate vision (visual acuity).
- Optic nerve has neurons for transmission of impulse to the brain for interpretation.
- Blind spot is located at the point where the optic nerve leaves the eye on its way to the brain.
- It is not sensitive to light it has no rods or cones.
- Eye lid is a loose skin that covers the eye. It closes by reflex action.
- Protects it from mechanical damage and from too much light.
- Prevent dust and other particles from entering eye.
- It is transparent and thin and allows light to pass through.
- It is a tough layer that is continuous with the epithelium of the eye lids.
- It protects the cornea.
- Accommodation refers to the change in the shape of the lens in order to focus images.
- Rays from a distant object would be focused at a point behind the retina if the lens were not adjusted appropriately.
- When the eye is focusing at a distant object, the cilliary muscles are relaxed and the suspensory ligaments are stretched tight.
- The lens is pulled thin, thus allowing light rays from a distant object to be properly focused on to the retina.
- When the eye is looking at near object, the ciliary muscles contract and the suspensory ligament become slack.
- The lens becomes more convex.
- This allows light rays from near object to be focused onto the retina.
Control of light intensity entering the eye
- In bright light (high intensity) the circular muscles of the iris contract.
- The diameter of the pupil decreases and less light enters.
- This protects retina from damage by too much light.
- In dim light circular muscles of iris relax (radial ones contract).
- Pupil’s size (diameter) increases, more light enters the eye.
Image formation and Interpretation
- Light rays from an object enter the cornea and are directed onto the lens through the pupil.
- They are refracted by the cornea and the lens.
- The latter brings the rays into fine focus.
- It makes the light rays converge so that an image is focused at a point on the retina.
- The image on the retina is inverted.
- This stimulate, the rods and cones on the retina and impulses generated are transmitted through the optic nerve to the brain.
- The brain interprets the image as upright.
Common Eye Defects and their Correction
- A shortsighted person cannot focus distant objects properly.
- Light rays from a distant object fall at a point in front of the retina.
- This may be due to the eyeball being too long.
- This defect can be corrected using spectacles with concave lenses.
- The lenses make the light rays diverge before they reach the eye.
- A long-sighted person cannot focus near objects properly.
- Light rays from the object are not focused on the retina.
- This may be due to the eyeball being too short.
- This defect may be corrected by using spectacles with convex lenses which make light rays converge before they reach the eye.
- Astigmatism refers to a condition in which the cornea or the lens is uneven, so that images are not focused properly on the retina.
- This defect can be corrected by wearing spectacles with special cylindrical lenses.
- Presbyopia is a condition in which light rays from a near object are not focused on the retina.
- This is caused by hardening or loss of elasticity of lenses due to old age.
- This defect is corrected by wearing convex (converging) lenses.