What do we mean by Visual Impairment?[For non-specialist teachers and support workers to raise awareness - can be offered on outreach basis]
Presented on Thursday 22 January 2009
adapted from Power Point presentation devised by E Kennedy QTVI Falkirk.
Formation of the Eye
a) The outer layer
- Tough and provides protection.
- It is made up of back 5/6 Sclera and front 1/6 Cornea.
- Whilst remaining tough the Cornea must also stay transparent to allow light to enter. It also starts to bend the light rays.
- The Sclera provides the anchor for the muscles which move the eye.
b) The middle layer
- The Iris which controls the size of the pupil to control the amount of light reaching the retina. The colour of the iris depends on the amount of pigment it contains.
- The Ciliary body controls the suspensory ligaments and so the shape of the lens. It also secretes fluid called aqueous humour into the eye to nourish the lens and the cornea.
- The Choroid is richly supplied with blood vessels and supplies nutrition to the innermost layer of the eye.
c) The inner layer or Retina
- Cone cells are responsible for detail and colour vision and though they are found throughout the retina are in the highest concentration in the Macula - the area of best vision most detailed, that is, central vision
- Rod cells react best in dim light and give peripheral vision - less detailed.
- Chemical changes in these cells give off electrical impulses so information starts its journey to the brain for interpretation.
Pressure is needed within the eye to keep the retina in contact with the choroid (nutrition).
This is maintained by:
- Vitreous Humour - a jelly-like substance which must remain transparent to allow light to reach the retina.
- Production and drainage of the aqueous humour which brings nutrition to the lens and cornea as neither have their own blood supply as they need to remain clear.
The lens is controlled by the suspensory ligaments and is responsible for the fine focusing of the eye, bringing the object of regard into focus on the retina (upside down and back to front).
Normally, people have two eyes situated apart in the head. Each eye has a slightly different view of the same object. When the brain combines these pictures it gives depth perception.
People who have only one eye may have difficulty judging distances.
Factors necessary for sight
- Eyes working correctly
- Intact nerve pathways
- Brain working correctly