The Eye

• The eye


  A sense organ containing many receptor cells. The receptor cells are contained in the retina, and the rest of the eye is essentially adapted to ensure that appropriate quantities of light are focused on these receptor cells.
• Conjunctiva


  • Thin transparent protective outer covering (prevents microorganisms entering the eye)
• Retina


  • Inner coat of eye where light-sensitive cells are found
• Iris


  • Controls how much light enters eye
• Pupil


  • Space in iris through which light passes
• Cornea


  • Transparent front part of eye that allows light to enter eye; causes some refraction (bending)
• Lens


  • Refracts (bends) light
• Optic nerve


  • Contains neurones carrying nerve impulses from the light sensitive cells in the retina to the brain
• Aqueous humour


  • Clear fluid found in the front of the eye that provides support and allows light to pass through
• Vitreous humour


  • Clear fluid found in the back of the eye between the lens and the retina that provides support and allows light to pass through
• Focusing the light rays


  Light is focused on the retina by the lens. This allows both near and far objects to be seen clearly.
• Accommodation


  The term used to describe the way in which the lens changes shape to focus light rays on the retina, irrespective of how far away an object is.
• How the lens changes shape

  The ciliary muscle (a ring of muscle running round the inside of the eye) is attached to the lens by suspensory ligaments. The shape of the lens is determined by whether the ciliary muscle is contracted or relaxed.
• Most of the refraction (bending) of light rays takes place in the cornea. The lens then 'fine-tunes' the light rays so that they focus on the retina.
• Eye


  A sense organ which contains the receptors for sight
• Retina


  • The part of the eye which contains the receptors
• Other parts of the eye

  • Protect it against damage
  • Focus light rays on the receptors in the retina
  • Control the intensity of light that enters the eye
• Conjunctiva


  Front part of the tough outer coat, the sclera. It is convex and transparent.
• Cornea


  Transparent front part of eye that allows light to enter
• Pupil


  Space in which light passes through
• Iris


  Pigmented - decides the colour of your eyes - so light cannot pass through. Its muscles contract and relax to alter the size of its central hole or pupil.
• Lens


  Transparent, bi-convex, flexible disc behind the iris attached by suspensory ligaments to the ciliary
• Aqueous and Vitreous humour

  Help maintain the eye shape pushing the retina to the eye wall
• Retina


  • The lining of the back of eye containing two types of photoreceptor cells-rods - sensitive to dim light and black and white - and cones-sensitive to colour. A small area called the fovea in the middle of the retina has many more cones than rods.
• Optic nerve


  Bundle of sensory neurones at back of eye. Carries impulses from the eye to the brain
• Passage of light through the eye

  1. Conjunctiva

  2. Cornea

  3. Pupil

  4. Lens

  5. Retina

  6. Optic nerve
• Accommodation


  1. Ciliary muscles

  2. Suspensory ligaments
• Ciliary muscle relaxes


  Lens is pulled thin, suspensory ligaments are pulled tight
• Ciliary muscle contracts


  Lens thickens, suspensory ligaments relax
• Pupil enlarges (dilated)


  In dim light, sufficient light falls on the retina
• Pupil gets smaller (constricts)


  In bright light, protects the sensitive cells in the retina from damage
• Neurones


  Nerve cells
• Controlling light levels entering the eye

  1. Iris contracts

  2. Iris relaxes

  3. Pupil gets smaller

  4. Pupil gets larger
• Iris


  Coloured part of the eye surrounding the pupil
• Pupil


  Opening in the centre of the iris that allows light to enter the eye
• Bright light


  Pupil gets smaller, letting less light enter the eye
• Low light levels


  Pupil gets larger, letting more light enter the eye
• It is important that the amount of light entering the eye is carefully controlled - too little light and an image will not form on the retina, while too much light will damage the retina's light-sensitive cells
• Insulin is a hormone
• Blood glucose levels are kept at the right level by insulin