The structure of the eye
The eyeball, the main part of the eye, rests in the bony eye socket in the skull, which provides strong protection on
three sides. The wall of the hollow eyeball consists of three layers: the tough protective sclera, the choroid and the
retina.
The sclera, which is thick and white, covers the sides and the back of the eyeball. Six muscles are attached to it,
which allow the eye to be moved within its socket. The muscles are coordinated by the brain, so that both eyes
always move in the same direction. At the front of the eye the sclera is transparent (durchsichtig) and is called the
cornea. It is covered by a thin membrane, the conjunctiva, which acts as a barrier against infections, secreting oils
and mucus. The dark coloured choroid is the second layer of the eye, lies just inside the sclera and is filled with
blood vessels which provide the eye with food and oxygen. The dark pigments in the choroid prevent reflection,
which would disturb vision.
The iris, which is part of the choroid, lies behind the cornea. It is the iris which gives the eye its colour. It is shaped
like a disc, with an opening in the middle, the pupil, through which light enters the eye. The iris is composed of
muscle fibres (Muskelfasern) which are arranged both radially (strahlenförmig) and in a circular (kreisförmig) way.
When the circular muscles contract, the pupil becomes smaller, allowing less light to enter the inside of the
eyeball. When the radial muscles contract, the pupil becomes wider, letting more light in. Just behind the pupil lies
the lens, which is curved (gewölbt) and transparent. It can change its shape like a bag filled with water.
The space between the cornea
skull
and the lens is filled with a watery
fluid, the aqueous humour, which
nourishes (versorgen) the cornea
and the lens and gives the front of
the eye its form and shape.
After passing through the lens,
the light travels through the
vitreous body, a colourless
vitreous body
jellylike substance, which fills the
eyeball and helps to keep its
shape. The light rays then fall
onto the retina, which is the place
where perception takes place.
The optic nerve penetrates
(durchdringen) the sclera and
carries the visual information from
the eye to the brain in the form of
electric impulses.
Figure 1 The structure of the eye and its position in the skull
Tasks:
1) Read the text and underline all important words which will help you to label figure 1.
2) Label figure 1 on an extra sheet of paper. Check your answers with the help of p. 155.
3) Copy figure 1 (make a drawing!) using different colours.