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11.7.1 Vision, Disorientation, Illusion


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Eye Physiology and Anatomy

Vision is the most dominant sense, the eyes are approximately 25 times more sensitive than any other organ in the body. Although good vision is essential for pilots and is tested during the medical assessment of a pilot; perfect eyesight is not required.

Anatomy and Physiology of the Eye The eyeball lies in a bony socket within the skull (the orbit) with two eyelids which protect and clean the surface of the eye. The eyeball is connected to the skull by 6 muscles, which move the eye up and down and from side to side.

Light is refracted by the transparent cornea at the front of the eye onto the lens. The lens then focuses the remaining light onto the retina. The lens can vary its focal length by the movement of the ciliary muscle which surrounds the lens. By using a process of contraction and relaxation the lens' focal length is varied. This is the process which allows the eye to focus on both near and far objects.

  • 70% of the focussing process is refraction as light passes through the Cornea.
  • 30% of the focussing process is carried out by the variable focus lens.

Between the cornea and the lens is the iris, a circular sheet of muscle fibre; this muscle sheet gives the eye its colour. The pupil is the opening in this muscle sheet. Contraction and dilation of the iris:

  • Increases and decreases the depth of focus.
  • Controls the amount of light falling on the retina.

The retina, the light sensitive covering on the inside of the eye, contains two types of photo-receptor cells:

  • Rods: Sensitive to low light illumination and relative movement at the extremes of vision.
  • Cones: Colour sensitive the cones are associated with both vision in good light and fine detail.

The focal point of the retina is called the Fovea, this area contains cones and no rods. This is the point of highest visual acuity. Decreasing numbers of cones and increasing numbers of rods occur as the distance from the Fovea increases. Colour discrimination is limited to small areas around the central Fovea.

Both rods and cones are connected to the brain by nerve fibres (neurones) which then combine to form the optic nerve. Each cone has a single neurone; clusters of rods share the same neurone. The nerve fibres combine as the optic nerve, the blind spot.

After detection of light on the cones or rods nerve impulses travel along the optic nerve to the optic Chiasma. This is where the optic nerves from both eyes meet. From the Chiasma the impulses travel to an area of the brain known as the visual cortex, where the information from the eyes is interpreted into a usable message.