EYE

Eyelids Protect cornea from mechanical damage Close eye partially (squinting) prevents excessive light Blinking spreads tears over eyes so dust is wiped off Eyelashes Shield eyes from dust particles Tear gland Secret tears – washes dust particles, keep cornea moist for atmospheric O2 to dissolve Lubricates conjunctiva to reduce friction when eyelids move Cornea Dome-shaped transparent layer able to refract/bend light rays into eye Iris Circular sheet of muscles, contain pigment to make eye’s color Controlled by 2 sets of involuntary muscles: circular and radial muscles Pupil Hole in centre of iris that allows light to enter eye Conjunctiva Mucus membrane covers sclera, secret mucus to keep front of eyeball moist Sclera Tough, white outer covering of eyeball (continuous with the cornea) Choroid Black pigmented middle layer of eyeball – prevent internal reflection of light Contain blood vessels that carry O2 and nutrients to eyeball + remove metabolic waste Retina Contain light-sensitive cells photoreceptors – consist of rods and cones Ciliary body Contain ciliary muscles – control curvature and thickness of lens Suspensory ligaments Attaches the edge of lens to ciliary body Lens Transparent, circular, biconcave structure Its shape/thickness can be changed to refract light onto retina Aqueous chamber Space between lens and cornea filled with aqueous humour (transparent watery fluid) – keeps front of eyeball firm, refracts light into pupil Vitreous chamber Space behind lens filled with vitreous humour (transparent jelly-like substance) – keeps eyeball firm, refracts light onto retina Blind spot Region where optic nerve leaves the eye Doesn’t contain rods/cones, therefore not light-sensitive

Fovea/yellow spot Small yellow depression where images are focused Contain cones not rods, enable detailed colour vision in bright light Optic nerves Transmit impulses to brain upon stimulation of photoreceptors Under bright light Under dim light Circular muscles (of iris) Contract Relax Radial muscles Relax Contract Pupil Constricts, become smaller Dilates, enlarges Effect Less light enters eye More light enters eye Close object Distant object Ciliary muscles Contract, relaxing pull on suspensory ligaments Relax, pulling on suspensory ligaments Suspensory ligaments Slacken, relaxing pull on lens Become taut, pulling on edge of lens Lens Thicker, more convex Decrease focal length Thinner, less convex Increase focal length Light rays focused on retina Photoreceptors are stimulated Nerve impulses are transmitted to the brain, which is interpreted by the brain to allow the person to see the object