Chapter

5

The Eye and Vision

Function and Structure		75
The Cornea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		75
The Iris and Pupil . . . . . . . . . . . . . . . . . . . . . . . . . . . .		75
The Lens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 76
The Retina		76
The Fovea and Visual Acuity . . . . . . . . . . . . . . . . . . . . . . . .		77
Light and Dark Adaptation		78
Night Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		78
The Blind Spot		79
Stereopsis (Stereoscopic Vision) . . . . . . . . . . . . . . . . . . . . . .	.	80
Empty Visual Field Myopia . . . . . . . . . . . . . . . . . . . . . . . .	.	80
High Light Levels . . . . . . . . . . . . . . . . . . . . . . . . . . . .		80
Sunglasses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	.	80
Eye Movement . . . . . . . . . . . . . . . . . . . . . . . . . . . .	.	81
Visual Defects . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 81
Use of Contact Lenses . . . . . . . . . . . . . . . . . . . . . . . . . .		83
Colour Vision . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 84
Colour Blindness . . . . . . . . . . . . . . . . . . . . . . . . . . . .		84
Vision and Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . .		84
Monocular and Binocular Vision		84
Questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	.	85
Answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		. 88

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Vision and Eye The 5

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The Eye and Vision		5

Function and Structure

The eye is the organ which receives electromagnetic waves within the visual spectrum from the external world and passes them to the brain for interpretation into an image. The basic structure is similar to a simple camera with an aperture, a lens, and a light sensitive screen called the retina.

To be able to keep tracking a moving object, the eyes need to act in harmony with one another which means coordinated control of the muscles of the two eyes by the brain. In a fatigued person, this coordination sometimes fails and the result is that quite differing images are transmitted from each eye. Subsequently double vision occurs.

The Eye and Vision 5

Figure 5.1 The structure of the human eye

The Cornea

Light enters the eye through the Cornea, a clear window at the front of the eyeball. The cornea acts as a fixed focusing device and is responsible for between 70% and 80% of the total focusing ability of the eye. The focusing is achieved by the shape of the cornea bending the incoming light rays.

The Iris and Pupil

The amount of light allowed to enter the eye is controlled by the iris, the coloured part of the eye, which acts as a diaphragm. It does this by controlling the size of the pupil, the clear centre of the iris. The size of the pupil can change rapidly to cater for changing light levels.

If the eye observes a close object the pupil becomes smaller and, if the object is at a distance, the pupil becomes larger.

The amount of light can be adjusted by a factor of 5:1. But this factor is not sufficient to cope with the different light levels experienced between full daylight and a dark night and a further mechanism is required. In reduced light levels a chemical change takes place in the light sensitive cells on the retina which enable them to react to much lower light levels.

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The Lens

After passing through the pupil the light passes through a clear lens. Its shape is changed by the muscles (ciliary muscles) surrounding it which allow the final focusing onto the fovea. This change of shape is known as accommodation. The power of accommodation can be affected by the aging process or fatigue. When a person is tired, accommodation is diminished, resulting in blurred images.

In order to focus clearly on a near object, the lens is thickened. To focus on a distant point, the lens is flattened.

The image is inverted and reversed by the lens onto the retina. However the brain perceives the object in the upright position because it considers the inverted image as normal.

The Retina

The retina is a light sensitive screen lining the inside of the eyeball. On this screen are lightsensitive cells which, when light falls on them, generate a small electrical charge which is passed to the visual cortex of the brain by nerve fibres (neurones) which combine to form the optic nerve. The optic nerve enters the back of the eyeball along with the small blood vessels needed to bring oxygen to the cells of the eye.

The light sensitive cell receptors of the retina are of two types - rods and cones. The centre of the retina is called the fovea (see below) and the receptors in this area are all cones. Moving outwards, the cones become less dense and are gradually replaced by rods, so that in the periphery there are no cones.

Vision through the functioning of the rods is called scotopic vision whereas vision through the operation of the cones is known as photopic vision.

Mesopic vision is when both the rods and cones are in operation.

Cones

The cones are used for direct vision in good light and are colour sensitive. Each cone has its own neurone and thus can detect very fine detail. The human eye is capable of distinguishing approximately 1000 different shades of colour.

Rods

The maximum density of rods is found about 10° from the fovea. Several rods are connected to the brain by a single neurone. The rods can only detect black and white but are much more sensitive at lower light levels. As light decreases, the sensing task is passed over from the cones to the rods. This means that in poor light levels we see only in black or white or varying shades of grey.

Rods are responsible for our peripheral vision. At night time, with a dimly lit flight deck, the colour of instruments must be bright enough for cone vision to be used.

Rods are also sensitive to movement and the movement of an object to the side of us is quickly picked up.

Rods and cones are the nerve endings of the optic nerve. Thus, as an extension of the brain, they are very much affected by a shortage of oxygen, excess of alcohol, drugs or medication.

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