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Visual Impairment :
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THE EYES

Mammals have two eyes. In grazing animals, such as rabbits, these are situated on the side of the head, whereas in hunting animals, including humans, the eyes are at the front of the head.

We have a visual field of about 200o so can not see behind us, whereas animals such as rabbits have a much larger field, almost 360o.

As our eyes are in different positions in the head we get two different views of an object simultaneously. This is called stereopsis, and the brain uses the parallax due to the different views to give us very good binocular vision with depth perception which allows us to judge distance very well and gives us 3D images.

In addition our central region of about 2o is focused onto the fovea and gives us very high resolution, detailed vision, which animals like the rabbit lack.

Diagram showing field of vision of a rabbit

Diagram showing the field of vision of a human

THE MOVEMENT OF THE EYES

The eye has two types of movement; first the movement of the whole eye by the extra-ocular muscles and movement of the eye lids, and secondly, movement which controls the iris (and so the size of the pupil) and the shape of the lens. The diagram shows the extra-ocular muscles which surround the eye and enable it to move.

Diagram showing the muscles attached to the eye which allow it to be moved

Movement of the eyeballs is controlled by the 4 rectus muscles (superior, inferior, lateral and medial) and 2 oblique muscles (superior and inferior). When the muscles are all relaxed the eye is in its stabilised position and looks directly ahead. When the muscles contract they will move the eye balls. The muscles are controlled by different cranial nerves and the movement of the eyes is coordinated so that they move together, known as conjugation.

If the direction that the eyes point is not coordinated then they will look in different directions causing strabismus. This is known by a variety of terms depending on whether they eyes converge (point in) or diverge (point away from each other)

THE STRUCTURE OF THE EYE

This is a diagram showing a vertical cross section of the eye. The eye has a tough outer layer called the sclera. This has an inner layer called the choroid layer, inside of which is the light sensitive retina. The eye should be round and its shape is maintained by the fluid inside it. The region in front of the lens is filled with clear fluid called aqueous humor (because it is runny) while the area behind the lens, comprising most of the eye, is filled with jelly-like vitreous humor. The fluid in the eye is under slight pressure and helps maintain the shape of the eye. Increased pressure can result in damage to the retina causing glaucoma.

Diagram of a section through the eye showing its structure