WHY YOU ARE YOU

Inside both the sperm and ovum, there are thread-like structures called chromosomes. These are made up of units of material called genes, which carry the information that makes you different from anyone else. Genes help determine all your physical and mental characteristics – the shape of your nose, how tall you are and so on.

Genes also carry the information that decides which sex you are. Men have a pair of chromosomes in most cells, called X and Y. women have a pair called X and X. In the gametes ( the sex cells, sperm and ovum) there are only single chromosomes, and  the sex chromosomes will be either X or Y in the male cell, and just X in the female cell. When a sperm and ovum join, the chromosomes from each cell pair up. If a sperm cell carries a Y, it joins with the X in the ovum and a boy will be born. If the sperm has an X and joins with a X from the ovum, a girl will be born.

You don’t simply inherit your parents’ physical features; you also inherit features from previous generations which which may not have shown up in your parents( recessive genes). The gene for blue eyes is a recessive one. It does not take effect in combination with the brown-eyes gene, but only with another blue-eyes gene.

                                               

GROWING OLDER

You can usually tell by looking at people roughly how old they are. As we get older, parts of the body change. An adult’s body is a different shape from a child’s body, as well as being larger. A young adult will usually have( proportionately) less fat and more muscle than a child. As a person gets older, the replacement of dying cells in the body becomes less efficient. In young people the skin is smooth, but as they age it becomes creased and wrinkled, and often becomes duller in appearance. Elderly people tend to be come long-sighted, so if you are short-sighted now, your vision may improve with age. A girl’s breasts grow larger as she matures. In boys the penis and scrotum increase in size. Both sexes grow pubic hair.

·        Hair colour is produced by pigments. When the body stops making these the hair turns white or grey, usually in older people. Men lose their hair more often than women, although no one is quite certain why it happens.

·        Bones grow for your first twenty years of your life. After reaching their maximum length, they then begin to shorten again. That is why old people get smaller.

·        Cancers are caused by abnormal and uncontrolled cell growth: the younger you are, the more vigorous the cell growth will be if you get it. Elderly people whose cell growth has slowed down have a better chance of surviving cancer.

A BABY IS BORN

The zygote moves from the fallopian tube to the womb or uterus, where it becomes attached to the wall. Over the next nine months the zygote will develop into a baby. Many of the organs form quite early in this development. Twenty-five days after conception, the heart is already beating inside embryo. After eight weeks the embryo looks like a  miniature person, though only 4 cm. long.

Inside the womb the embryo floats in a sac of liquid which protects it. It is joined to the mother by the umbilical cord, via the placenta. Food and oxygen needed by the developing embryo come through this from the mother, and waste products are returned through it.

The baby is ready to be born after nine months. When the woman has spasms, called contractions, labour has begun. The labour pains gradually become more frequent, until they are only two or three minutes apart.

This second stage marks the beginning of birth. The contracting muscles push the baby ( normally head first) down the uretus to the vagina. After the baby is born, the third stage follows, in which  the umbilical cord is cut and the afterbirth – placenta and membrane – is discharged.

·        Sometimes the baby cannot be born naturally, and a caesarean operation has to be performed. An opening is cut through the mother’s abdomen and uterus, through which the baby can be removed.

·        Your ‘belly button’ is where the umbilical cord used to be attached.

·        In a ‘breech birth’, the baby comes out feet or buttocks first.

THE START OF A NEW BABY

It takes just two cells coming together to start the development of a baby. One, the sperm, comes from the man, the other, the egg cell or ovum, is produced by the woman. A man starts to make sperm in his testes when he reaches puberty; girls are born with egg cells already in the ovaries. The first egg cell will be released when a girl’s monthly periods begin, probably about the age of 12 or 13. Another will be released every month thereafter, for 30 40 years.

For the sperm to get into a woman’s body, the penis has to be inserted into vagina. The penis is normally limp, but it becomes erect when a man is aroused, and can then be inserted into vagina. Millions of sperm leave the penis in a liquid called semen when the man ejaculates; then, using their tells, the sperm swim on up the fallopian tubes. If there is an egg cell there, one sperm may penetrate it; the rest will die. Conception occurs when sperm and ovum join to form a zygote.

·        A woman has two ovaries. An egg is normally released from one ovary one month, and from the other ovary the next month. If two eggs are released at the same time, twins may develop.

·        Sometimes a fertilized egg divides into two, and when this happens identical embryos will develop. Twins will be born.

·        When a sperm penetrates an ovum, the tail falls off and the head becomes bigger. When the sperm reaches the nucleus of the ovum  a zygote is formed, which grows as the cells divide.

FEELING

Some parts of the body are more sensitive to feelings than others. The lips, tongue, hands and feet are among the most sensitive. That is because these parts contain many nerve endings. Touch something with your finger, and you can feel it almost  immediately; the nerve endings in your finger have sent an electrical signal to your brain via the spinal cord. Your brain distinguishes between nerve messages indicating, for example, tickle, Itch, cold, heat, touch, pressure or pain, and registers the intensity of the signals. A strong signal, for example if you touch a hot saucepan, you will pass straight back from the spinal cord to your arm muscle – so that you pull your hand back as a reflex action at about the same time as the message reaches your brain.

The sensory nerves have specialized nerve endings to pick up different sensations, such as heat or cold. They can also  have axons ( eg those coming from the nerve endings for pressure) than  along thin ones; some axons can even be over a more long, such as those leading from the base of the spinal cord to the big toe.

·        Each person has a different pain threshold. The bath temperature that seems fine to one person may be painfully hot to another.

·        One pain can suppress another. If you stub your toe, biting your lip or thumping your fist  on a table may help reduce the pain.

·        Pain can sometimes be felt in a different place from its actual source. For example, a pain may appear to your brain to be coming from your neck or shoulders, when in fact it comes from the diaphragm. This results from a ‘crossed line’ between neighbouring nerves.

·        People who have lost a limb sometimes think they still feel pain in it, because the brain is still getting signals from the stump.

TASTE AND SMELL

There are a large number of taste buds all over the surface of the tongue. Food is mixed with saliva in the mouth and some of the mixture goes through the taste pores, stimulating sensory cells. As dissolved material touches these, signals are sent to the brain.

Taste and smell are closely linked. Both combine to identify the flavor of food. With a cold we can still taste whether food is sweet, sour, salt or bitter, but without the smell, the flavor is not the same. The upper part of the nose contains many olfactory( smelling) cells. Chemical substances in the air are dissolved in mucus in the nose. The smell is then picked up by  the olfactory cells and the brain is informed. New smells are learned by the brain and stored for future recall.

·        A well- trained person is said to be able to detect up to 10,000 different smells. The brain remembers them so that we can recall them at a later date.

·        Of our 9,000 or so taste buds, most are on the tongue. Others are on the roof of the mouth and the back of the throat.

·        We sniff when a smell reaches our nose to get more of the scent to the olfactory cells. If the smell is unpleasant, we stop sniffing. These cells also tell us when we smell dangerous things such as gas.

·        Nerves for taste and smell send messages to an area quite low down at the front of the brain.

THE EYE

The eye is a complicated structure which works rather like a camera. We see things because light falls on them. Light bounces off objects and passes through a sort of protective windscreen called the conjunctiva, then through another slightly curved transparent membrane called the cornea.

The light rays are bent gently as they pass through the pupil, an adjustable aperture in the centre of the coloured iris, and then go through the lens. The lens shape can change to help us focus on things at different distances. The convex lens turns the light rays  upside –down. When these rays reach the retina, at the back of the eyeball, they are upside- down and back to front. The retina is made up of light-sensitive cells. These stimulate nerve endings, which join together to form the optic nerve. When messages reach the brain, the images are turned the right way up and interpreted.

·        There are two types of cells in the retina, called rods and cones after their shapes. Rods are more sensitive to small amounts of light than cones, but they can only see black and white. Cones pick up colours other than black and white.

Some cone cells see red, some green and some blue. The brain combines their messages to recognize the whole range of visible colours – just as all the colours are made from red, green and blue light on a colour TV screen.