The Eye

The eye has a diameter of two and a halve centimetre. It exists of three big parts:
•  the eye-wall: the outside of the eye, which works as a camera;
•  the vitreous humour: a clear, gelatinous fluid which gives the eye its form
•  the optic nerve: the cable that connects the eye to the brains.

The eye-wall too exists of three basic parts:
•  a tough outer layer: the sclera
•  a middle layer with many blood vessels and pigment: the choroid
•  an inner layer with sensitive cells: the retina

The outer layer of the eye-wall

The tough sclera is what we call the white part of the eye. It is a white, transparant membrane which protects and strengthens the eye. The tough sclera changes into the transparant cornea, which is covered with a protective conjunctiva. If compared to a camera the cornea is the window through which the rays of light enter.

The middle layer

The middle layer of the eye-wall is the choroid. It consists of many blood vessels. They tranfer the nutrients to the right place and regulates the temperature. The choroid first enters the corpus ciliare and then the iris.

The iris determines the colour of the eyes. It can be compared to the diaphragm of a camera. In the centre of the iris is an opening: the pupil. Orbicular muscles make the opening wider or smaller. When there is much light, the pupil becomes more narrow. At twilight or darkness the pupil widens. So the pupil is an opening, although it seems to be black. That colour exists because the eye doesn't reflect the light that enters.

Behind the iris is the lens. It is kept at its place by very small fibres. The lens surely is transparant. It has the shape of a globoid. The inner eye muscles can change the globoid of the lens and that way sharpen the incoming image.

Iris and lens divide the eye into the foremost and the rear chamber of the eye.

The inner layer

The inner layer of the eye-wall is the retina. If you compare it again with a camera, the retina is the shield on which the images are projected. Millions of cells are on it, which are sensible to the light. Everything they sense, they transmit to the optic nerve.

The retina has got two types of cells. The rods are especially on the side of the retina. They are only sensitive to light and darkness. The cones are more in the centre. They are more sensitive to colours. People have about one million rods and seven million cones.

Exactly behind the pupil is the most sensitive spot of the retina: the yellow spot. There are only cones. At that spot in the eye we can see the colours very well and the sight is quite clearly.

From the rods and cones the nerve-fibres leave. They join at the end of the eye, in the big optic nerve. At that spot there aren't any rods or cones. There the retina is insensitive to light and darkness. That's why that spot is called the blind spot.

Protection

The eyes are protected in a naturel way. They are safely in sockets to stand rough handling. Besides they are extra protected by the lids which can close in a reflex when danger threatens. One winks the lids about ten to fifthteen times a minute. In a smokey area, in stress or in concentration it happens even more. The importance of winking is that every time a little tear-water is spread over the eye. Tear-water protects the eyeball from drying and kills damaging bacterium. The whiskers protect the eye against dirt, small insects and sunlight, while the brows protect the eyes from rain or sweat.

Aging

The eye converts rays of light into electrical signals. The brains convert these signals and therefore we are able to see.To get a clear image we have to bend tha rays of light so that they join together exactly on the retina in the rear of the eye.

First the rays of light are bent by the surface of the cornea and eventually by the natural lens. The cornea is the outer layer of the eye. It is transparant and has a spherical shape. The rays of light which enters the eye are bent by the cornea.

The natural lens, which is flexible and can accommodate of shape, is behind the pupil and it converts the fine-tune of the rays. A strong bending of the lens is for instance needed for reading. Between the 40 th and 50 th year of life the lens will become less flexible and old person's far-sightness starts. Later on, the lens can become turbid and tough. This is a matter of cataract.