2 INSTRUCTIONAL OBJECTIVE state the function of the different parts of the eyedescribe how a focused image of near and distant object is produced on the retinadescribe the pupil reflex in response to bright and dim light
3 LOCATION In sockets (orbits) of skull moved by six muscles optic nerve connects it to brain
6 Sclerotic layer thick wall (the 'white' of the eye) protective layer around eyeballFn: Provides protection against mechanical injuryCornea‑ transparent allows lightto enterFn: bends (refracts) lightinto pupil towards lens
7 Choroid layerFn: pigmented black, absorbs light so as to prevent reflection within the eyecontains network of blood capillaries which brings nutrients to the eye and also removes metabolic wastes.
8 Ciliary bodyFn: contains ciliary muscles which alters the shape of lens‑ muscles attached to lens by suspensory ligamentsIrisconsists of 2 sets of muscles:radial muscle and circular muscle-these two muscles work in opposite waysFn: Controls the amount of light falling on the retina.
9 How muscles of iris work? When muscles contract, they become shorter.Therefore, when radial muscles contract, they pull together and become shorter.Therefore the size of the pupil widens or dilates.When circular muscles contract, the circumference becomes smaller. This causes the size of pupil to be smaller or constrict.
10 Eye in Dim LightRadial muscles contract, become short, pupil enlarges or dilates.Circular muscles relax, becomes longer, circumference becomes bigger, pupil enlarges or dilates.Did you notice the muscles work in opposite ways, ie. radial contract, circular relax.
11 Eye in Bright LightRadial muscles relax, become longer, pupil enlarges or dilates.Circular muscles contract, becomes short, circumference becomes smaller, pupil constricts.Did you notice the muscles work in opposite ways, ie. radial relax, circular contract.
12 Animation on size of pupil Notice the size of the pupil before and after shining the pen-light.This formula will help you:IN BRIGHT LIGHT = 3CCircular muscles Contract, pupil Constricts
13 Lens ‑ transparent, elastic and biconvex Attached to suspensory ligamentThe stretching of the suspensory ligaments causes it to change shape.When looking at far objects, the lens becomes thin or less convex.When looking at near objects, the lens becomes fat, more convex.Fn: bends light rays and brings them to a focus on retina
14 Retinalight sensitive layer, contains photoreceptors (light receptors)Fn: when stimulated by light from the image, convert the light energy into electrical impulse.photoreceptor cells connected to optic nerve which carries sensory impulses to brain
15 Yellow Spot‑ central part of retina‑ Images normally focused on this spot- high concentration of photoreceptor cells ---- precise visionBlind Spot‑ part of retina where optic nerve is attached‑ no photoreceptor cells present here‑ images falling here is not registered
16 HOW DOES THE EYE WORK Light falling on an object is reflected. Some of these light rays falls on the eyeThe light rays are refracted through the cornea and aqueous humour onto the lensThe lens bends the rays and cause it to focus on the retina.Photoreceptors in retina stimulated.Impulse produced and transmitted via optic nerve to brain
17 ACCOMMODATIONadjustment of the lens so that clear images of objects at different distances are focused on the retina
18 ciliary muscles contract tension on suspensory ligament slacken Near Objectsciliary muscles contracttension on suspensory ligament slackenlens more convex ie.short and fatlight rays bent (refracted) moreImage on retinaNear object focused on retina
19 Can you write down the steps when looking at far objects?
20 PUPIL REFLEXimmediate response of the eye in controlling the size of the pupil due to changing lighting conditions.
21 HOW THE PUPIL REFLEX WORKS Light enters pupil and falls on retinaPhotoreceptors stimulatedImpulse producedImpulse carrying information about lighting condition (dim or bright)is transmitted along sensory neurone in optic nerveImpulse reaches brainTransferred to relay neurone and thenMotor neuroneMotor impulse carried from brain to muscles of irisIf dim light, circular muscles relax and radial muscles contract --pupil dilatesIf bright light, circular muscles contract and radial muscles relax pupil constrict
22 The value of having TWO eyes Two eyes view the same object from two different positions.This provides vision in 3-Dimensions, the ability to judge distance.Survival value