BRAIN AND NERVOUS SYSTEM

Every second, whether you are awake or fast asleep, your is busy processing information. It receives millions of signals from all parts of the body, and its sends out signals that control the way your body works. These signals travel along nerves, bundles of cells that conduct tiny bursts of electricity. The brain itself contain more than 1000 billion nerve cells, and when they are busy these cells generate enough electrical energy to power a light bulb.

Unlike other cells in your body, the cells in your brain cannot be replaced when they get worn out. As a result, the number of cells in your brain steadily decreases as you get older.

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Sweat glands
Sweat is a watery fluid that helps to keep your cool. It is made by coiled glands that open onto the surface of your skin. The palms of your hands have more sweat glands than anywhere else, up to 500 per cm2 of skin (3250 per sq in).

Fingerprints

Whenever you touch anything smooth, your fingers leave marks. These marks are called fingerprints. Fingerprints are made of sweat and body oils, and they are exact copies of the tiny ridges that cover your fingertips. Identical twins have matching fingerprints, but everybody else has their own unique pattern, which will resemble one of the basic patterns. Fingerprints are often used to identify people present at the scene of a crime. To find fingerprints, police scientists dusts the objects at a crime scene with a fine powder. This sticks to the sweat and oil that the fingers have left behind, making the marks show up. The police then see if the marks match up with the fingerprints of any of their suspects. There are four kinds of fingerprints :

• Arch
• Loop
• Whorl
• Composite

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Hair
Hair protects your head and eyes from the sun. On other parts of your body, it helps you feel things brushing against your skin. And it helps to keep you warm. Most of a hair is made of dead cells.

Can hair really stand on end?
If you are cold or frightened, your hair may feel as if it is standing on end. A tiny muscle pulls on the root of each hair, levering it int a more upright position. As each hair moves, it pushed against the skin, making a 'goosebump'. Long ago humans had lots of body hair, and warm air would have been trapped in the little pits formed by goosebumps.

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Skin ridges
These ridges are found only on fingers and toes, and on the palms of your hands and soles of your feet. They help to give you a good grip. The skin underneath them is extra thick for added protection.

Fingernails
Nails protect your fingertips (and toenails your toes), and also help you to pick things up. They are made from keratin, the same substance that makes hair. On average, a fingernail grows at a rate of about 1 mm (1/32 in) per week, four times faster than a toenail.

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Why are people different colours?
Skin gets its colour from chemicals called pigments present in skin cells. One pigment, called melanin, makes skin brown or black. Another, called carotene, gives skin a yellow colour. Pigments help to protect skin against damage by strong sunshine. People with pale skin have very little pigment and may easily get sunburned. If your skin is like this, it is important to stay covered up in strong sunshine, or to use sunblock.

Some people have more of a particular skin pigment than others. Apart from this, everyone's skin is the same.

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What is skin made of?
Skin is made of cells arranged in three main layer. The deepest layer, called the dermis, contains living cells, blood vessels and nerve endings that sense pressure, heat, cold and pain. Above this is a thin layer, called the epidermis, that produces new cells all the time. The new cells die as they are slowly pushed towards the surface, where they from a protective outer layer. Then they fall away in small flakes.

Flaking away
The surface of skin, magnified about 200 times, with flakes of dead cells forming.

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SKIN, HAIR AND NAILS

Skin grows in step with the rest of your body, and although it constantly wears away, it is also replacing itself all the time. Like hair and nails, skin helps to protect your body from the out side world. It can do this because its surface is made of dead cells. These are very tough, and they from a barrier that shields the living cells underneath. Like the surface of skin, most of each hair is also made of dead cells.

Under the surface
Each part of your body has a different type of skin. For example, the skin on the back of your arm is covered with lots of hairs, although they may be too small for you to see. The skin on the palms of your hands and the inside of your fingers has no hair at all, but it does have lots of tiny ridges and invisible openings called sweat glands.

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Sneezes and hiccups
Breathing normally happens so smoothly that we hardly notice it. But sometimes it is interrupted by a cough, a sneeze, or by hiccups. Hiccups happen when your diaphragm contracts suddenly and air rushes into your lungs. Your vocal cords, membranes lying across your trachea that you use to talk, close, making a loud squeak. Coughs and sneezes clear out dust and other particles that get trapped in your airways and the lining of your nose. Colds cause coughs and sneezes for a few days, smoking can give people cough for years because some of the smoke gets stuck in the lungs.

Drops of moisture, dust and germs shoot into the air when you sneezes.