HUMAN EYE –

STRUCTURE, FUNCTIONS AND PARTS

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.

 

See also:

EFFECTS OF DRUGS ABUSE ON THE HUMAN HEALTH

THE ROLE OF ENDOCRINE SYSTEM IN HUMAN BEINGS

SPINAL CORD

FUNCTIONS OF MAJOR PARTS OF THE HUMAN BRAINNERVOUS SYSTEM

NERVOUS SYSTEM