The eye

• Conjunctiva


  Mucus membrane covering sclera, secretes mucus to keep front of eyeball moist
• Cornea


  Dome-shaped transparent layer that refracts light into pupil
• Pupil


  Hole in centre of iris allowing light to enter eye
• Iris


  Circular sheet of muscles that controls pupil size
• Eyelash


  Shields eye from dust particles
• Eyelid


  • Protects cornea from mechanical damage
  • Squinting prevents excessive light entry
  • blinking spreads tears over the eyes so dust can be wiped off
• Tear gland


  Secretes tears to wash away dust, keep cornea moist, lubricate conjunctiva
• Sclera


  • Tough, white outer covering that protects inner structures
  • maintains eyeball shape
• Choroid


  • Black pigmented middle layer that prevents internal light reflection
  • Contains blood vessels that transports oxygen and nutrients to eyeball and remove metabolic waste products from the eyeball
• Retina


  • Innermost layer
  • contains photoreceptors (rods and cones)
  • connected to nerve fibres from the optic nerve
  • images are formed and detected here
• Sclerotic coat (sclera)


  • Protects the delicate inner structures
  • Eye muscles (rectus muscles) attached to this layer facilitates the movement of the eyeball
  • Maintains the shape of the eyeball
• Choroid


  Black pigmented middle layer which prevents the internal reflection of light
• Retina


  • Innermost layer of the eye wall
  • Contains light-sensitive cells known as photoreceptors, which consist 'rods' and 'cones'
  • Connected to nerve fibres from the optic nerve
  • Images are formed and detected here
• Cones


  • Responsible for colour vision and formation of sharp images
  • Three types - red, blue and green
  • Each type contains different pigment, which absorbs light of different wavelengths
  • They work together to enable us to see a variety of colours in bright light
• Rods


  • Stimulated even by very dim light
  • Enable us to see in dim light, but only in black or white
  • Contain a pigment called visual purple (requires vit. A)
  • Visual purple is bleached when exposed to bright light and nerve impulses cannot be transmitted to the brain
• Fovea (yellow spot)


  • A small yellow depression where images are focused
  • Contains highest concentration of cones but no rods
  • Enables a person to have detailed colour vision in bright light
• Blind spot


  • Point where the optic nerve leaves the eye
  • Does not contain photoreceptors, therefore not sensitive to light
• Optic nerve


  Transmits nerve impulses to the brain when photoreceptors are stimulated
• Lens


  • Transparent, circular and biconvex structure
  • Shape or thickness can be changed in order to refract light onto the retina
  • Focuses light onto the retina
• Suspensory ligament


  Attaches the edge of the lens to the ciliary body
• Ciliary body


  Contains ciliary muscles which control the curvature and thickness of lens
• Aqueous chamber


  • Space between the lens and the cornea
  • Filled with aqueous humour, a transparent, watery fluid
  • Aqueous humour keeps the front of the eyeball firm and refracts light into the pupil
• Vitreous chamber


  • Space behind the lens
  • Filled with vitreous humour, a transparent jelly-like substance
  • Vitreous humour keeps eyeball firm and refracts light onto the retina
• The circular muscles and radial muscles are antagonistic.
• Focusing


  Adjustment of the lens of the eye so that clear images of objects at different distances are formed on the retina
• 1. Choroid