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Parts of the eye

Controls the amount of light entering the eye
A hole in the centre of the iris where light enters
Transp­arent layer, refracts light rays into pupil
Refracts light rays (aqueo­us/­vit­reous)
Transp­arent, circular, biconvex structure, elastic, change shape to refract and focus light onto the retina
Ciliary body
Contains ciliary muscles that contra­ct/­relax to control the curvature of the lens
Pigmented black to prevent internal reflection of light, contains blood vessels to bring nutrie­nts­/oxygen and remove metabolic wastes
Cones; colours in bright light. Rods; black and white in the dark
Largest concen­tration of cones, where images are usually focused
Blind spot
Devoid of photo-­rec­eptors and is insens­itive to light
Protects eye from physical damage, prevents excessive light from damaging tissues inside the eye
Tough, white outer covering, protects the eyeball from physical damage, covered by conjuctiva
Tear glands
Secretes tears to wash away dust particles, lubricate the eye, keep conjuctiva and cornea moist


Side view

Pupil reflex

Reflex action is an involu­ntary response to a specific stimulus, without conscious control. It involves the brain as the reflex centre.

Pathway: Stimulus -> photo-­rec­eptors in retina -> sensory neurone in optic nerve -> brain -> motor neurone -> effector

Lens shape


1 Focusing on a distan­t/near object
2 Ciliary muscles relax/­con­tract
3 Suspensory ligaments become taut/s­lacken
4 Pullin­g/r­elaxing on the edge of the lens
5 Lens become less/more convex, increa­sin­g/d­ecr­easing focal length
6 Light rays from the object are sharply focused on the retina
7 Photo-­rec­eptors are stimulated
8 Nerve impulses produced are transm­itted by the optic nerve to the brain, which then interprets the impulses and the person sees the object.

Distant; ciliary relax, lens becomes thinner
Near; ciliary contract, lens becomes thicker

Iris muscles


1 Change in light intensity
2 Stimulus is detected by photo-­rec­eptors located in the retina
3 An electrical impulse is generated, transm­itted by sensory neurones in the optic nerve to the brain
4 At the brain, the impulse is transm­itted across a synapse to the relay neurone, then across another synapse to the motor neurone
5 Motor neurone transmits the impulse to the circular and radial muscles of the iris
6 Circular muscles contra­ct/­relax, while radial muscles relax/­con­tract. This causes the pupil to constr­ict­/dilate, thereby reduci­ng/­inc­reasing the amount of light entering the eye

Bright: Circular contract, radial relax, pupil constrict
Dim: Circular relax, radial contract, pupil dilate


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