The eye

• Our survival, indeed our very existence, depends on our reactions to stimuli coming from the environment. We feel, hear, see, taste and smell our surroundings every living moment. The main sense organs in humans are the skin, ears, eyes, tongue and nose.
• Eye


  A light-sensitive organ that enables us to 'see' small variations of colour, shape, size, brightness and distance
• How we see


  1. Light rays from the object travel in a straight line to the eyeballs

  2. They pass through the structures at the front of the eyeball, through the pupil

  3. Light stimulates light-sensitive cells of the retina

  4. Impulses sent along the optic nerve to the brain

  5. Brain forms an image of size, shape, colour and distance away of the object
• Cornea


  • Bends (or refracts) the light towards the retina
• Lens


  • Can vary the amount of bending or refraction and thus ensure the accurate focusing of the image on the retina
• Iris


  • Composed of circular and radial muscles and controls the size of the pupil which then varies the amount of light that enters the eye
• Accommodation


  The adjustment of the lens for focusing on near and distant objects
• Accommodation


  1. Ciliary muscle contraction and relaxation affects the tension in the ligaments which changes the shape of the lens

  2. When focusing on a distant object, the ciliary muscles relax, lens flattens, less refraction

  3. When focusing on a near object, the ciliary muscles contract, lens becomes more rounded, more refraction
• Depth of focus


  When the pupil is small (in bright light) the eye has a greater depth of focus
• Retina


  • Photosensitive layer at the back of the eye, made up of rods and cones
  • Rods are sensitive to light and dark only, function best in low light
  • Cones are sensitive to colour, function best in high light, located around the fovea
• Blind spot


  Point where optic nerve leaves eye, lacks photoreceptors and is insensitive to light
• Sight defects


  • Long-sightedness
  • Near-sightedness
  • Cataract
  • Glaucoma
• Long-sightedness (hypermetropia)


  Eyeball too short or lens too flat, light from distant objects can focus on retina but light from near objects focused behind retina
• Near-sightedness (myopia)


  Eyeball too long or lens too curved, light rays from distant object bent more than necessary and focus in front of retina
• Astigmatism


  Surface of lens or cornea curved irregularly
• Cataract


  Lens becomes opaque, light cannot pass through
• Glaucoma


  Too much fluid gathers in front of lens, causing increased pressure in aqueous humour which can damage optic nerve
• Cataract


  Occurs when the lens becomes opaque and light cannot pass through
• Glaucoma


  Occurs when too much fluid gathers in front of the lens, causing a build-up of pressure in the aqueous humour, which can damage the optic nerve
• How we hear


  1. Sound waves reach the ear and the pinna directs them into the auditory canal

  2. The sound waves travel down the ear canal to the ear drum

  3. The ear drum vibrates when hit by the sound waves

  4. This causes the ear ossicles to vibrate

  5. The vibrations pass to the membrane covering the oval window which leads to the inner ear

  6. The vibrations start up pressure waves in the fluid of the cochlea

  7. Hair cells in the cochlea vibrate in response to the pressure waves

  8. Nerve impulses are generated which pass along the auditory nerve to the brain
• Tympanic membrane (ear drum)


  A thin membrane which is pulled taut and separates the outer and middle parts of the ear
• Vestibular apparatus


  • Responsible for our sense of balance and information about the position and movement of our body
  • Made up of the semicircular canals which detect movement of the head, and the utricle and saccule which detect the position of the head
• How the vestibular apparatus detects movement and position


  1. Receptors inside the semicircular canals, utricle and saccule are hair cells that deflect on movement

  2. This causes a message to be sent to the brain
• The semicircular canals are at right-angles to each other, in the three planes, so that any movement of the head, and therefore the body, is detected
• The utricle responds to vertical movements of the head and the saccule responds to lateral or sideways movement of the head