My Science School

Each tooth has two main parts. The root anchors it firmly in the gum, to withstand the tremendous pressures that are exerted when you bite and chew hard foods like nuts. The crown is the visible part above the gum. It is covered with whitish enamel, which is the hardest substance in the entire body. Under the enamel is a layer of dentine, which is not quite so hard, and absorbs shocks and knocks. In the middle of the tooth are blood vessels, providing nourishment to the tooth’s parts and layers, and nerves, to detect pressure and pain.

Regular visits to the dentist are important for healthy teeth. If you don’t look after your teeth, they may go bad and decay. That means that they may have to be filled or even taken out by the dentist.

Fact File: Germs live in the holes of bad teeth. They eat the good part, which makes the holes deeper. Dentists have to drill out this germy part. The hard outside of teeth cannot grow back. Dentists have to fill the holes with metal to keep the germs out.

 

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The average person has two sets of teeth, one after the other. The first is the baby, milk, or deciduous set. Even before birth, teeth appear as tiny buds below the gums. They begin to erupt, or show above the gum from the age of a few months. By the age of about three all 20 first teeth have usually appeared. In each half (left and right) of each jaw (upper and lower), there are two incisors, one canine, and two molars.

From about the age of six years, the first teeth start to fall out. These are replaced by the adult, second, or permanent set. First are usually the front incisors and the first molars, at around seven to eight years. Last are the rear-most molars, or wisdom teeth. They appear at 18-20 years of age in some people, while in others they erupt at 40 or 50 years of age – and sometimes they never appear. In each half of each jaw, there are typically two incisors, one canine, two premolars, and three molars, making a full set of 32 teeth.

Fact File: Your back teeth are bumpy on top. You can feel it. They work together, grinding food between the bumps. These grinders need regular and careful cleaning when they finish work. Food often sticks between the bumps.

 

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Coughing is the way in which the lungs dislodge anything that blocks the air passages. Usually these are only minor blockages caused by a build-up of mucus when you have a cold or chest infection. When you cough, your vocal cords press together to seal off the air passages. At the same time your chest muscles become tense, raising the pressure in your lungs. When you release the air it rushes out, carrying the obstruction with it.

The delicate alveoli inside the lungs can be damaged by many different things, thus causing us to cough. One is tobacco smoke, which clogs the alveoli and airways with thick tar. Others are the polluting gases that hover in the air of many big cities, coming from vehicle exhausts as well as factory and power-plant chimneys. Some types of industrial dust and particles floating in the air, such as asbestos or coal-mine dust can cause considerable damage to the lungs.

Fact File: It can be quite hard to breathe when you are at the top of a mountain. At high altitudes the air is thinner so there is not so much oxygen in it. This means that you will breathe heavily if you exert yourself by climbing.

 

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Respiration, or breathing, is when you draw air in through the nose and mouth and into the lungs. Like all movements in the body, those of respiration rely on muscle power. There are two main sets of breathing muscles: the intercostal muscle and the diaphragm. Breathe in deeply and watch your ribs rise and your chest expand. Together these muscles make the chest bigger and stretch the spongy lungs inside. As the lungs enlarge, they suck in air down the windpipe. This is how we breathe in. Then the muscles relax. The ribs fall back down and the diaphragm resumes its domed shape as the spongy, elastic lungs spring back to their smaller size. The lungs blow some of their air up the windpipe. This is how we breathe out.

The movements of breathing are controlled by the brain. It sends out signals to make the muscles contract. The signals pas along nerves to the intercostal and diaphragm muscles, making them contract. This happens every few seconds throughout our life, even when you are asleep.

File Fact: The average person at rest breathes in and out about 10-14 times per minute. If you sing or play instruments like trumpets, you need lots of puff. Learn to use the muscle under your lungs to get more lung power.

 

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A baby begins as a fertilized egg – a pinhead-sized egg cell from the mother, which has joined an even smaller tadpole-shaped sperm cell from the father. Although thousands of these sperm cells may cluster around the egg cell, only one of these will actually fertilize the egg.

Non-identical twins are produced when two eggs are released at the same time, and both are fertilized. They can be the same sex, or brother and sister.

Identical twins are produced when the embryo splits into two in the early stages of its development. This produces two identical children of the same sex. Some identical twins look so alike that they can only be told apart by their fingerprints.

Only one in 83 pregnancies results in twins.

Fact File: The bones of a baby’s head are not fully fused at birth, allowing the skull to pass through the mother’s birth canal. The bones gradually become joined, but a gap at the top of the skull, called the fontanelle, may not close up for several months.

 

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At about 280 days after the baby’s inception, the mother starts to feel strong tightening pains, called contractions, in her womb when the birth is near. These contractions become stronger and the neck of the cervix starts to open. As the contractions continue, the baby’s head moves down and eventually emerges through the cervix and vagina.

A breech birth is different to a regular birth because the buttocks engage in the pelvis instead of the head. This makes delivery more complicated as the largest part, namely the head, is delivered last. A breech birth is encountered about once in every thirty deliveries. Because the head in such cases is the last part of the child to be delivered and because this part of the delivery is the most difficult, the umbilical cord may be compressed while the after-coming head is being born, with the result that the child may be deprived of oxygen.

Fact File: Sometimes a baby cannot be born normally through the vagina and so it has to be surgically removed from the mother’s womb. This operation is called a caesarean section.

 

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The part of the brain that analyzes messages coming from the receiver cells in the nose is closely connected with the limbic system, that part of the brain that deals with emotions, moods and memory. It is called the primitive brain, sometimes even the ‘smelling brain’. The connection explains why smells are richly supplied with emotional significance. The smell of fresh-baked bread may bring on instant pangs of hunger, while the scent of perfume may remind you of a loved one. On the other hand, unpleasant smells such as rotten eggs, produce revulsion and sometimes even nausea.

Certain smells will bring memories of long forgotten special occasions flooding back. This is because the areas of the brain which process memories are also closely linked to the limbic system, which in turn is linked to the areas in the brain that control the sense of smell.

Fact File: Aromatherapy is the art of using the perfumed essential oils of plants to treat the body and mind. The perfume passes over the nerve cells in the nasal passage and a message is sent to the brain.

 

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Close your eyes and touch something, such as your clothes, a table, a car or even your own skin. Stroke it gently. What does it feel like? Is it hard or soft, hot or cold? The surface may be smooth, bumpy, gritty, furry or hairy. It could be dry, moist, or slimy. Your skin continuously passes huge amounts of information to the brain. It monitors touch, pain, temperature and other factors that tell the brain exactly how the body is being affected by the environment. Without this constant flow of information, you would keep injuring yourself accidentally, which is what happens in some rare diseases where the skin senses are lost. Senses in the skin are measured by tiny receptors at the ends of nerves. There are several different types of receptor. Each type can detect only one kind of sensation, such as pain, temperature, pressure, touch and so on.

Fact File: Sometimes we need drugs, or analgesics, to control a pain. Some drugs, such as aspirin, work by preventing the sensation of pain from reaching the brain.

 

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Compared to other sensations (in particular smell) our taste sense is not very sensitive. It has been estimated that a person needs 25,000 times as much of a substance in the mouth to taste it as is needed by the smell receptors to smell it. However, despite this, the combination of the four types of taste buds responding to the basic tastes of salt, sour, bitter or sweet, enable a wide range of sensations to be determined as the brain analyzes the relative strength of the tastes. If we were to lose our sense of smell, almost all taste sensation would be lost as well. Eating oysters, for instance, where the smell is so important, would become a dull and totally ‘tasteless’ experience. That is why we sometimes lose our sense of taste when we have a cold because our nasal passages become blocked. You will gradually lose your sense of taste as you grow older, which is one reason why elderly people may no longer enjoy their food so much.

Fact File: The human sense of smell is very poor compared to that of animals, such as dogs. Some dogs are able to identify and follow the smell of a person’s perspiration.

 

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Tastes are detected by thousands of taste buds scattered along the tip, sides, and back of the tongue. There are also some near the lips, on the roof and sides of the mouth, and in the upper throat. Each taste bud is tiny – a microscopic bunch of about 50 cells which have furry, frilly tips. There are four main types of taste: sweet, sour, salty and bitter and these can be detected by different areas of the tongue. When molecules land on the frilly tip, the tastebud cells make nerve signals. These signals pass long small nerves which gather into two main nerves – the seventh and ninth cranial nerves. These signals then travel along them to the gustatory, or taste, area in the brain. You have around 10,000 taste buds on your tongue.

Fact File: Smell enters the cavity behind the nose when you inhale. They also rise up from the back of the mouth as you eat, which is why smell is such an important part of enjoying food.

 

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If the eye is not exactly the right shape, or the lens cannot focus properly, you cannot form a clear image on the retina. In this case you may need to wear glasses to correct your vision. A short-sighted person can see nearby objects very clearly. A long-sighted person can see things in the distance while nearby objects are blurred. Contact lenses are an alternative to wearing glasses and many people prefer these. They are thin plastic discs that rest on the surface of the cornea. They act like the lenses of ordinary glasses. Most modern contact lenses are made from very soft material that does not cause any discomfort to the eye. Some lenses are only worn for just one day and then they are thrown away. It can be difficult to get used to wearing contact lenses and to putting them in the eye without scratching the delicate cornea. However, many people now prefer them to wearing glasses.

Fact File: Film and television images consist of a series of rapidly changing still images, yet we see them as continuous motion. There is a slight delay between each of the images that appear on the screen. However, because this delay is so short, our brain is able to fill in the gaps and provide a complete picture of what is happening.

 

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The eyes are the body’s windows on the world. Like a video camera they detect a moving picture of the world and turn it into tiny electrical signals. These signals are nerve impulses which go to the brain to be sorted. In ancient times people thought that light shone out of their eyes onto what they looked at. Now we know that light rays pass from an object into the eye. Every second or two the eyelids blink and sweep tear fluid across the conjunctiva, washing away and dust and germs.

The image that forms on the retina when light passes through the lens is actually upside down. This is because of the way in which light rays are bent by the eye’s lens. The brain automatically turns the image the right way up, but you are never aware that this is happening.

Fact File: There are six muscles to control the movements of each eye. Muscle (a) swivels it away from the nose; (b) towards the nose; (c) rotates it upward; (d) downwards; (e) moves it down and outwards and (f) moves it upwards and outwards.

 

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Referred pain is a pain that’s source is in one place of the body but we feel it on another part of the body. Internal organs and structure are well supplied with nerves, but pain is widely spread and poorly located compared with skin sensations. Most of the pain is caused by stretching and contracting, as in the pain of colic. Internal pain will cause stimulation of local nerves in a portion of the spinal cord, and this makes it appear that the pain is coming from the skin which is supplied by the sensory nerves. The heart (1) and the oesophagus (2) refer pain to the neck, shoulders and arms. The uterus (3) and pancreas (4) refer pain to the lumber region. The kidneys (5) refer pain into the groin. Pain from the diaphragm may be referred to the shoulders as the phrenic nerve is formed from the spinal nerves in the neck, which also supply the shoulders.

Fact File: Some areas of the skin are densely packed with nerve endings, as in the finger-tips, while others, as on the back, have comparatively few.

 

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The liver has two vital roles to play: making (or processing) new chemicals, and neutralizing poisons and waste products. The liver is the largest organ in the body weighing between 1.36 and 1.81 kg (3 and 4 lb). It is only possible for the blood to get back to the heart and lungs from the stomach by first passing through a system of veins in the liver, known as the portal system.

A variety of things including viruses, drugs, environmental pollutants, genetic disorders and systemic diseases can affect the liver and stop it functioning properly. However, the liver has a marvelous capacity to renew itself and will usually return to normal once the causes are removed or eliminated.

Fact File: If the kidneys do not work properly, they can become ‘furred’ up with hard crusts and crystals of chemicals from the urine. These deposits are called kidney stones. They can be removed by an operation, dissolved by drugs, or shattered into tiny fragments by high-energy ultrasonic sound waves.

 

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The appendix is a narrow tube-like piece of gut resembling a tail, which is located at the end of the large intestine. The tip of the tube is closed; the other end joins on to the large intestine. It is only found in humans, certain species of apes and in the wombat. Other animals have an organ in the same position as the appendix that acts as an additional stomach, where the fibrous part of plants and cellulose is digested by bacteria. It seems that a we evolved through the ages and began to eat less cellulose in favour of meat, a special organ was no longer needed for its digestion.

Fact File: The only time we are aware of the appendix is when it becomes infected and this is known as appendicitis. For the most part it is a useless part of the large intestine with no known function.

 

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