Structure
- 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.
Sclerotic layer
- 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.
Cornea
- 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.
Choroid
- 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.
Ciliary body
- 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.
Suspensory ligaments
Are inelastic and attach the lens onto the cilliary body holding it in position.
Lens
- Biconvex in shape, to refract light.
- Crystalline and transparent to allow light to pass through and focus it on to the retina.
Aqueous humour
- 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.
Iris
- 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.
Vitreous humour
- It is a fluid.
- Found between lens and retina.
- Is viscous and gives eye the shape.
- It is transparent and refracts light.
Retina
- 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
- Fovea centralis (yellow spot) is the most sensitive part of the retina.
- Consists mainly of cones for accurate vision (visual acuity).
Optic nerve
Optic nerve has neurons for transmission of impulse to the brain for interpretation.
Blind spot
- 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
- 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.
Eyelashes
Prevent dust and other particles from entering eye.
Conjunctiva
- 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
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
Short-sightedness (Myopia)
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.
Long-sightedness (Hypermetropia)
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
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.