STRUCTURE AND FUNCTION OF PARTS OF THE HUMAN EYE

Biology

STRUCTURE AND FUNCTION OF PARTS OF THE HUMAN EYE

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.
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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.
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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.
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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

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