B. Side View of the Eye

Functions of the different parts of the human eye

I. Cornea

• Transparent layer continuous with the sclera.

• Refracts light rays into pupil. Maximum refraction of light rays due to the largest difference in optical density between air and the cornea.

• In front of the cornea lies the conjunctiva membrane. At times, when bacteria infects the conjunctiva membrane, sore eyes result.

II. Pupil

• Opening through which light enters.

• Size of the pupil is regulated by the action of the circular and radial muscles of the iris.

III. Aqueous humour

• Consists of a watery substance.

• Functions: 1. Maintains the firmness of the eyeball at the front region. 2. Has a different optical density from the cornea and this helps to refract light rays.

IV. Iris

• Consists of circular and radial muscles of the iris.

• Contains coloured pigment that gives the eye its characteristic colour. Colour of this pigment is affected by the genes of the individuals.

• Function: Circular and radial muscles of the iris function antagonistically to control the size of the pupil.

V. Lens

• Elastic, transparent and biconvex structure that contains water and proteins.

• Curvature determined by the action of the ciliary muscles and pull of the suspensory ligaments.

• The curvature of the eye lens affects refraction of light rays from near and distant objects to form a focussed image on the retina.

VI. Suspensory ligaments

• Fibres that attach the edge of the lens to the ciliary body.

• The tautness of the suspensory ligaments is affected by the action of the ciliary muscles.

VII. Ciliary body

• Contain ciliary muscles that control the curvature of the lens.

• Action of ciliary body is affected by the autonomic ("automatic") nervous system.

VIII. Sclera

• Tough, white outer covering.

• Protects the eye from injury.

IX. Choroid body

• Contains a black pigment known as melanin to prevent reflection of light within the eyeball. This function is important to prevent multiple images from forming within the eye.

• Contains choroid blood vessels that supply the eye ball with nutrients and oxygen.

X. Retina

• Light-sensitive layer that

• Consists of photoreceptors - (i) rods that allow vision in dim light. Rods allow vision in black, white and grey colours. Rods contain a pigment known as phodopsin (or visual purple) that is bleached in the presence of bright light. When we walk from a bright room to a dim room, it takes some time before our eyes can adapt and view the surroundings. The time lag is due to the regeneration of the phodopsin (or visual purple) pigment and (ii) blue, green and red cones that allow vision in bright light. Perception of colour is determined by the different numbers of each cones stimulated while intensity of colour is affected by the frequency at which nerve impulses are formed from each cone.

XI. Vitreous Humour

• Consists of jelly-like substance.

• Functions: 1. Refracts light rays and 2. Keeps the eyeball firm at the back region of the eye.

XII. Yellow spot or fovea

• Point where images are usually focused.

• Contains only cones. Rods are absent.

XIII. Blind spot

• No rods or cones.

• Image is not formed if light rays are converged at the blind spot.

XIV. Optic Nerve

• Transmits nerve impulses to the brain when photoreceptors in the retina are stimulated.