The Human Eye

The human eye Sense organs are receptors that receive stimuli and inform the body of changes in the environment.

The structure of the human eye Each eyeball lies in a hollow in the skull called the orbit and is attached to the skull by rectus muscles which controls eye movement.

Cornea Dome-shaped transparent layer continuous with the sclera or the white part of the eye. Specialised form of the conjunctiva. Refracts or bends light rays into the eye.

Conjunctiva A thin transparent membrane covering the sclera sc lera in front. Secretes mucus in order to keep the front of the eyeball moist.

Iris The amount of light entering the eye is controlled c ontrolled by the two sets of involuntary muscles in the iris, the circular muscles and the radial muscles.

Pupil Allows light to enter the eye.

Eyelids Protects the cornea from mechanical damage. Can be close partially, preventing excessive light from entering the eye and damaging the light-sensitive tissues inside. This is known as squinting. Blinking spreads tears over the cornea and conjunctiva and wipes dust particles off  the cornea.

Eyelashes Shields the eye from dust particles.

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Tear glands Secretes tears which wash away dust particles, keep the cornea moist for atmospheric oxygen to dissolve which diffuses into the cornea, and lubricates the conjunctiva reducing friction when the eyelids move.

Internal structure of the eye

Sclera The ‘white of the eye’ which protects the eye from mechanical damage.

Choroid The middle layer of the eyeball. Pigmented black to prevent internal reflection of light. Contains blood vessels that bring oxygen and nutrients to the eyeball and remove metabolic waste products.

Ciliary body Contains ciliary muscles which control c ontrol the curvature or thickness of the lens.

Lens Transparent, circular, and biconcave structure. Elastic and changes its shape s hape or thickness in order to refract light onto the retina.

Suspensory ligament Attaches the edge of the lens to the ciliary body. body. Chapter 14 - The human eye

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Aqueous chamber Space between the lens and the cornea. Filled with a transparent, watery fluid known as aqueous humour which keeps the front of the eyeball firm and helps to refract light into the retina.

Vitreous chamber Space behind the lens. Filled with vitreous humour which is transparent and jelly-like. Vitreous humour keeps the eyeball firm and helps to refract light into the retina.

Retina The light-sensitive layer on which images are formed. Contains photoreceptors which are light-sensitive. They consists of rods and cones. Cones enable us to see colours in bright light. Each cones contains a different pigment which absorbs light of different wavelengths, working together to allow us to see a variety of colours. They do not work well in dim light. Rods enable us to see in black and white in dim light. They contain a pigment called visual purple. When the eye is exposed ex posed to bright light, all the visual purple is bleached and must be re-formed for a person to see in the dark. Photoreceptors are connected to the nerve-endings from the optic nerve.

Fovea (yellow (yellow spot) Small yellow depression in the retina which is situated directly behind the lens. Images are normally focused at the yellow spot. Contains the greatest concentration of cones, but no rods. Hence, the yellow spot enables a person to have detailed colour vision in bright light.

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

Blind spot The region where the optic nerve leaves the eye. Does not contain rods nor cones, therefore it is not sensitive to light. The size of the pupil is controlled by two sets of involuntary muscles (circular and radial muscles) in the iris.

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Controlling the amount of light entering the eye The size of the pupil determines how much light enters the eye. In bright light, the circular muscles of the iris contract and the radial muscles relax. The pupil becomes smaller or constricts, reducing the amount of light entering the eye. In dim light, the circular muscles of the iris relax and the radial muscles contract. The pupil enlarges or dilates, increasing the amount of light entering the eye. The ciliary and radial muscles are antagonistic a ntagonistic muscles because when one set contracts, the other set relaxes.

Sight Process • • • •

Light rays are refracted through the cornea and the aqueous humour onto the lens Lens causes further refraction and the rays converge to a focus on the retina Image on retina stimulates either the rods or cones, depending on light intensity Image formed on retina is - Inverted Inverted - Laterally inverted - Smaller in size than the actual object

Focus Distant object 1) Ciliary Ciliary muscles relax, pulling pulling on the suspensory suspensory ligaments 2) Suspensory ligaments become taut, pulling on the edge of of the lens 3) Lens becomes thinner thinner and less convex, increasing its focal length 4) Light rays from the distant object object are sharply focused on the retina retina 5) Photorecep Photoreceptors tors are stimula stimulated ted 6) Nerve impulses produced are transmitted transmitted by the optic nerve to the brain brain which interprets the impulses and the person sees the distant object Near object

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