The eye, the optical nerve and the brains enable a person to visually perceive the surroundings.
The visual system has an optical part, a part in which an image is converted into electrical signals, a part for transferring these signals and a part to observe the image.
The optical part
The eye contains a number of lenses. The cornea is the front-most transparent part of the eye. The refraction power of the cornea is fixed. In order to get both a clear image of a tree far away and the letters in a book, the eye has the lens, which has a variable refraction power. The lens is located behind the iris and hangs from a muscle that runs around the lens. The force applied by this muscle makes the lens flatter or rounder. A flat lens gives less refraction and so makes it possible to see far away, but a round lens gives stronger refraction and enables a person to read. Because of the great elasticity of the lens, a person can focus on all distances. The eye also contains a diaphragm, usually called the iris. The pupil, the black round in the centre of the coloured iris, is the opening of the diaphragm. The size of the pupil depends on the amount of light that is falling into the eye. When there is a lot of light, the diaphragm closes and the pupil gets smaller, whereas with little light, the pupil will be larger.
an object that we focus our eyes on is sharply reflected on the retina by the lens system. Here the light is converted into electrical signals that are transferred to the brains. This conversion takes place in specialised cells in the retina: the rods and cones.
Rods and cones are light-sensitive cells. These cells contain a substance, the photopigment. The composition of the photopigment changes when light falls on it. Thus light is converted into a chemical change in the cell. This chemical change makes the rods and cones issue an electric signal that can be transferred to the brains. All signals of the millions of light-sensitive cells in our retina together enable our brains to observe the original image of the object. When we first look at a house and then to a tree, we will immediately see the tree. The image of the house will not linger. This means that the photopigment must be able to react to changes in light very quickly. In order to be able to do this, the photopigment must be in an optimum condition. The retina consists of a number of layers.
The outer shell is a carpet of pigmented cells with a thickness of one cell. The pigment in these cells is not photopigment. This layer is the retinal pigment epithellium. The rods and cones are lying against the retinal pigment epithellium. Close cooperation between the light-sensitive cells and the cells in the retinal pigment epithellium ensures that the photopigment is in the required optimum condition. Rods and cones continuously produce new pigment. The newly made photopigment can respond to changes in light at the required fastness. The cells of the retinal pigment epithellium nibble away old and used photopigment from the rods and cones, break this up into the various parts it consists of and give the parts back again to the rods and cones. This closes the circle, for the parts are used again to produce new photopigment.
The photopigment in the rods is not the same as the photopigment in the cones. This difference determines, among other things, the different functions of both cell types. Rods and cones are not randomly spread across the retina. The cones are mainly found in the centre of the eye, the rods are lying around them. Their number decreases the larger the distance from the centre. The colour of the centre distinguishes itself by a slightly different composition than the rest of the retina; it is more yellow and therefore it is also called the yellow spot (macula lutea). The cones make us see very small details and colours. Cones need a lot of light to be able to properly react.
When there is little light, the rods come into action. The rods help us to see in the dark. If we want to clearly see a bird in a tree, we manoeuvre our eyes such that the image of the bird falls exactly on our yellow spot. While we are fixing our eyes on the bird, we still see the tree and the surroundings of the tree as well. What we observe in the surroundings without moving our head or eyes, we call our field of vision. The image of the surroundings of the bird falls outside the yellow spot and is therefore not observed by the rods. In summary, we use our cones to be able to see clearly and to distinguish colours, provided that there is enough light (daylight, well-lit room). We use rods when there is little light and, additionally, the rods mainly determine the size of our field of vision.
The image perception
Influenced by light, the rods and cones transfer an electrical signal to the nerve cells. A nerve cell is specialised in transferring this type of signals. For this it is provided with long offshoots, nerve fibres, that guide the signal over sometimes large distances. These nerve cells and their fibres form the inner layer of the retina. The fibres come from all parts of the retina together at one central point and there leave the eye. This point does not contain light-sensitive cells and has no part in observing things. This explains its name: blind spot. Its official name is: papilla.
The rear-most part of the brains, the optical cortex, picks up the signals coming from the eyes and makes observation possible. This makes us aware of our surroundings and enables us to respond to all visual information that is so abundantly present in our society.