Every person has five important senses, which are rather like five different gates to a city, through which·messengers can come bringing news of what is going on in the outside world; for that is what our senses do-we see what is going on around us through our eyes, we hear the noises that are made by means of our ears, we smell things through our noses, taste things with our tongues, and feel warmth, cold, pain and so on in our skins.
The human eyeball is round in shape, and consists of three layers-the outer layer is a tough, white coat, which is transparent at the front, so that the light can pass easily through it; the middle layer does not go all the way round, but the front part of it is called the iris, and it has a small opening in the middle called the pupil ; the innermost layer of the eye is the retina, and consists of a large number of sensitive cells.
Light, like sound, travels in waves; and when light waves enter the eye they pass through the outer layer, and through the pupil. If we are looking at a bright light the pupil closes up to the size of a pin-head, so as not to let too much of the light in; if we are looking at something in a dim room, then the pupil opens wide to let in more light.
Behind the pupil is the lens, which is like a magnifying glass which can change its shape- it does this by means of a series of tiny muscles which are loose and relaxed if we are looking at faraway objects, but are tight if we are looking at things close to us, thus giving the lens a new shape and making it easy for us to see things in focus, that is, sharply and distinctly. (It is, of course, a strain on the eye muscles to keep them tight, and that is why it is always a good plan, when we are reading for any length of time, to relax the eye muscles now and again by looking at a distant object.) The light rays pass on through the lens and strike the retina, and from there a message goes to the brain telling us what the eye sees.
The eye is very much like the camera-that, too, has a lens to let in the light, with an opening that can be adjusted according to the brightness of the day, so as to let through less or more light; the lens of the camera can be made to bring near or distant objects into focus, and at the back of the camera is a sensitive film which records what is in front of the camera when light is let in.
Sound travels in waves and it is by means of the ear that we receive these waves and transform them into messages to be sent on to the brain. That part of the ear that we can see on people's faces is called the external ear. Sound waves reach a piece of stretched skin called the ear-drum, and make it vibrate- it quivers very quickly, and these vibrations start a chain of tiny bones inside the ear moving in such a way that they pass on the vibrations through some fluid to the inmost part of the ear, a sort of spiral tube, called the cochlea.
The cochlea consists of a series of nerve -cells arranged in such a way that sounds of a certain kind reach special places in the cochlea- high sounds, like the top notes of a violin, in one place, and low ones, like the deepest notes of an organ, in another. Some animals, such as the dog, can hear sounds that are too high for us to hear at all.
From the cochlea messages travel along special nerves to the brain, and so we know what the sounds are.
Attached to the cochlea are three other important nerve-centers called the semicircular canals, which help us to keep our balance. These canals are filled with fluid and so arranged that the fluid starts to move whenever we move our heads; the fluid touches various nerves and messages are sent to the brain to tell us in what position our head is and thus make it easy for us to keep our balance.
If you turn round and round very quickly the fluid in the canals is very much disturbed and you will feel dizzy.
The ear works very similarly to the microphone in the mouthpiece of a telephone. That, too, has a drum, in this case a flat disk of metal called a diaphragm, which vibrates when sound waves reach it. These vibrations cause changes in an electric current flowing through carbon granules at the back and make the current stronger or weaker. This current is sent over wires to a distant receiver, which turns it back into sound.
Just as our eyes and our ears have special nerve-cells in them which send messages to special parts of the brain, so our noses and our tongues have special nerves joined to the brain to tell us what we smell or taste.
Things like fruit give off tiny particles which enter the nose and reach the special nerves at the top, and these send on messages to the part of the brain concerned with smell. The tongue has some very special nerve-cells connected with the taste buds, which serve to tell us if something in our mouth is sweet or sour, bitter or salt.
The skin also has various nerve-cells, which tell us various things-there are special nerves attached to the ends or roots of the hair, others which send messages about the temperature of the skin, whether it is hot or cold, and many more which send messages of pain when we get hurt.
Every muscle in the body is also provided with nerves, which send messages to the brain whenever the muscles are moved-in this way we know whereabouts our legs and arms and so on are. If you shut your eyes you can still touch the point of one forefinger with the point of the other, because the nerves from the muscles in the arm help to tell us where the arms are and how much we should move them. This also helps us to keep our balance.
The various nerve-cells in the eyes, ears, nose, tongue and skin send messages to the brain which tell of sensations of light, sound, smell, taste, heat, cold or pain. But probably we have seen or heard or smelt or felt some object before us many times before, so we know something about it. In this way a sensation becomes what is called a perception.
For example, if you hold an orange before a very young baby, all that his eyes tell him is that there is a colored blob in front of them-it is only later on when he knows what an orange is that he perceives this colored blob as an orange. For all that our senses tell us is that something is of a certain color, smell and feel, but because we have come across these things before we perceive them to be various objects.
Sometimes, if we are expecting something to happen, we may make a mistake in our perceptions- if you are waiting for a friend in the street, and you see someone who looks rather like him, you may perceive him as your friend-and when he turns out not to be, you will feel very surprised.
We seem to prefer shapes which are simple and complete, also ones which are regular, or symmetrical, and which "look good" or are well-proportioned. But very often this is not possible and something we perceive may be ambiguous, that is, it may be seen in one of two ways; or something we perceive may be so influenced by the things surrounding it that it looks different from what it really is-this is called an illusion. On the opposite page are some simple illusions.