Category Human Body

We take a grip on things by using our muscles to exploit the development of the human thumb. The importance of this thumb is that it can be moved across the palm of the hand to touch the index finger. In contrast the digits of a monkey’s hand are more rigid. This difference explains why a human being can handle an instrument with such precision.

     The mechanism of the hand is operated by way of the wrist, a hinge joint composed of eight small bones (carpals) packed firmly together to give elastic stability. Below the wrist project five small, long bones (metacarpals) which give the palm firmness. Four of the bones have hinge joints and are connected to the thumb allows it to move round and meet the fingers and palm, thus providing a firm grip.

    Possession of such a hand has enabled man to form a society and culture based on the use of tools. It is thought the hand developed from the five-rayed forepaw of an early vertebrate. However, occasionally a child is born with an extra tiny thumb or little finger and some people believe that the forepaw was originally seven-rayed.

    The human thumb has length longer in proportion than that of an ape. About 92% of human being is right handed, but apes tend to use both hands with equal ease.

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Inside the womb of a pregnant woman or animal a placenta is formed, through which the unborn baby is nourished. The baby is connected to the placenta by the umbilical cord, which is joined to the unborn child at the navel.

     The umbilical cord is the baby’s lifeline. Everything the child needs to survive will pass through it- air, blood and nourishment. At most it is no more than an inch wide and, perhaps, only a foot long.

    Once the baby is born the placenta, which serves a purpose only during the pregnancy, will be discarded.

      The umbilical cord will be cut with scissors a few inches away from the newborn baby’s stomach. This is quite painless because the umbilical cord has no nerves. The baby will now breathe on its own.

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The body temperature of a human being is an indication of his physical condition, so that an abnormally high or low temperature is generally a sign that something is wrong.

     The normal temperature is usually given as 36.90 C. (98.40 F.), but as the body temperature varies throughout the day, anything between approximately 36.70 C. (OR 98.10 f.) and 37.20 C. (or 99.00 F.) may be taken as normal.

    For instance, the temperature rises after a large meal, during hot weather and after violent exercise. Your temperature is at its lowest at night when you are asleep.

    Control of body temperature is exercised by a centre in the brain which ensures that a balance exists between heat production and heat loss. A raised temperature is often the sign of bacterial or virus infection. It may be due to heatstroke, to certain types of brain injury or disease or to shock.

     A very high temperature, or fever, may begin with a “rigor” (an attack of shivering and cold), in which the whole body may tremble uncontrollably and the teeth chatter. Although at this stage the skin feels cold and clammy, the temperature within the body is raised. Soon the skin becomes hot and dry, pulse and breathing rate are speeded up and there is a feeling of exhaustion, aching muscles. Headache, thirst and perhaps delirium and loss of the sense of time.

    Finally this stage is succeeded by profuse sweating and a gradual relief of the symptoms.

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The brain directs and coordinates movements and reflexes, registers sensations and is the supreme nervous organ by which man acquire knowledge and the power to use and adapt it. It shapes our personalities, and without it we would be more helpless than the tiniest human baby.

       There are three main parts of the brain: the forebrain (or cerebrum), the midbrain and the hindbrain. They have the consistency of soft jelly and are protected by three membranes (meanings), a tough outer envelope called the dura and a water fluid (cerebrospinal fluid) which acts as a support and a cushion. The brain is connected to the spinal cord, and its surface is highly convoluted.

      The cerebrum which forms nearly nine-tenths of the brain is divided into two halves (hemispheres). Generally the left half of the cerebrum controls the right half of the body, and the right half of the cerebrum controls the left half of the body. Some areas are connected with the special senses of man, but there are so-called “silent areas” which scientists believe are connected with memory and the association of ideas. The thalamus, a mass of grey matter which is buried in the cerebrum, is the source of instinctive feelings and emotion.

    The midbrain is concerned with eye-movements, while the hind-brain contains the nerve cells responsible for breathing, heart action, and digestive juices and so on. The cerebellum, a part of the hindbrain, plays an important role in the execution of the more highly skilled movements.

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Genes and chromosomes

Inside the nucleus of a human cell are 23 pairs of chromosomes. Along these chromosomes are “genes”. Genes control everything about you, including the way you look. Each of your cells has the same genes. You inherit two copies of every gene – one from your mother and one from your father. That is why you look like your parents.

Inheriting eye colour

The gene for brown eyes is known as a “dominant” gene – you only need to inherit one brown eye colour gene for your eyes to be brown. The gene for blue eyes is weaker. In general, a child will only have blue eyes if both inherited eye colour genes are blue. A man carries a gene for brown eyes and a gene for blue eyes, while the woman carries only a gene for blue eyes. One of their daughters has blue eyes because she has inherited two blue eye colour genes. The other daughter has brown eyes because she has inherited at least one brown eye colour gene.