KCSE 2017 ESSAY Q8 DESCRIBE HOW THE MAMMALIAN EYE IS STRUCTURALLY ADAPTED TO ITS FUNCTION. 20 MARKS

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  • Has the eyelid; which protects the cornea from mechanical/ physical/chemical damage;Eye lid; protects the eye from bright light by reflex action;
  • Sclera/Sclerotic layer; - which contains (inelastic) collagen fibres which protects/maintains shape of the eyeball;
  • Cornea; - transparent to allow light pass through/has convex shape to refract light towards the retina;
  • Conjunctiva - (thin) epithelium for protection of cornea/has goblet cells for secretion of mucus for lubrication/ transparent to allow light pass through;
  • Choroid/choroid layer; - rich in blood vessels/highly vascularised, supplying the retina with nutrients/oxygen/ remove metabolic wastes/covered with (black) pigment cells to prevent reflection of light within the eye;
  • Ciliary muscles; have (contractile) muscles that contract and relax to alter the shape of the lens during accommodation·
  • Lens;- transparent to allow light pass through/elastic to allow adjustment of the shape of lens/ biconvex to refract light/focus light onto retina;
  • Iris; - has radial and circular muscles to alter diameter/size of the pupil, hence controlling the amount of light entering the eye; contain pigments that absorb light and stop it getting through to the retina;
  • Vitreous homour; - clear/transparent to allow light pass through/is a fluid that refracts·light rays onto the retina; maintain shape of the eye ball; support the eye;
  • Retina; contains cones, rods/ photoreceptors to perceive light/ where image is focused/ formed; has cones and rods, cones for colour vision/ bright light vision and rods for vision in dim light;
  • Optic nerve;- has sensory; neurons/nerve cells that transmit impulses to the brain;
  • Fovea (centralis); - (most sensitive part of retina) contains numerous/high concentration of cones for visual acuity/ accurate vision;has cones only.
  • Pupil;- a hole/ an aperture/opening in the iris, lets in light;
  • Suspensory ligaments;-are fibrous/inelastic fibres that hold lens in position;
  • Aqueous humour - is clear/transparent to allow light to pass through/is a fluid/liquid (exerting hydrostatic pressure) to maintain the.shape of the eyeball/refract light rays onto the lens; contain glucose for nourishment;
  • Blind spot - a point where the optic nerve leaves the eye to the brain; passage of blood vessels; since has no photoreceptors;
20 MARKS

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