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.

THE EAR

The ear constantly picks up sounds, keeping us informed of what is happening around us. Sounds are caused by vibrations in the ear. The outer part of the ear is called the auricle or pinna. It traps sounds, and funnels them down the ear passage until they reach the eardrum. When the vibrations touch the drum, they make it vibrate. Three small bones behind the eardrum pick up the vibrations and pass them to another tight membrane, the oval window; thus they enter the inner ear and pass into the cochlea. Here, nerve endings pick up the vibrations and send them along the auditory nerve to the brain.

The brain changes the vibrations into sounds. In a street, you can hear a large variety of sounds; people walking and talking, traffic moving and workmen working. The brain can distinguish between different kinds of vibration, if they are loud enough. If the sounds are too loud, however, they can damage the eardrum.

·        Air pressure inside the ear is adjusted by the Eustachian tube to match the pressure on the outside. If the outside pressure changes too quickly, eg in an aeroplane, it will be painful – until your ears ‘pop’, equalizing the pressure.

·        The ears contain a kind of built-in spirit level, which helps you keep your balance, Nerve endings in the liquid in the semicircular canals of the inner ear pass information to the brain, which works out the position and movement of your head. Whizzing the body round and round upsets this, so that you feel dizzy.

THE BRAIN

Of the 30,000 million nerve cells in the body, some 14,000 million makeup your brain. The brain has three jobs to do. First, it receives information from sensory nerves about your body, eg stomach pains, or about the world outside. Second, the brain sends out instructions to all parts of the body – eg via motor nerves to the muscles in your hand. The third task of the brain  is to store and process the information it receives, so that, for example, you can understand and remember the words you see here.

Different areas of the brain are responsible for different activities. The largest part is called the cerebal cortex. This deals with receiving sensations and with conscious thought or control of the body. The cerebellum and other parts are responsible for unconscious control of the body.

·        The lobes on the sides, top and front of the brain are divided into the two cerebral hemispheres.

·        The left of the brain controls the right side of the body, and vice versa. This is because the nerves from the spinal cord cross over in the lower section of the brain.

·        At the base of the brain is a small part called the hypothalamus. This controls ‘automatic’ activities such as breathing, and maintains body temperature at about 37 degree centigrade.

·        At birth, your head was relatively large, weighing about 350g- almost one-tenth of the weight of the body. I n an adult the brain weighs on  average about 1400g – about one – fiftieth of the weight of the body.

THE NERVOUS SYSTEM

We have a very good nervous system, which tells us what goes on in every part of our body. The brain controls the system and is like a telephone exchange, with the nerves- the wires- receiving and sending messages all over the body. The two main parts of the nervous system are the central nervous system and the peripheral nervous system consists of the brain and the spinal cord. The peripheral nervous system is made up of the nerves which send messages to the brain from the senses and which take messages from the brain to the glands and muscles. Every muscle and sense organ has to be connected.

·        Nerves are made up of cells called neurons. These have thread-like arms called dendrites which pass impulses down the cell’s one long arm or axon, to other cells. Impulses consist of very weak electrical currents.

·        Because the verves endings are continually receiving information from their surroundings, they are called receptors.

·        Messages travel along  nerves as electrical impulses at speeds of up to 400km/hr.

·        In reflex actions, messages travel to and from the spinal cord without the brain being involved.

WASTE

Food which cannot be digested travels through the alimentary canal and leaves our bodies as faeces. Other wastes, for example worn-out tissues and cells, leave the body via the skin as sweat, or the lungs as carbon dioxide, or leave the bloodstream via the kidneys to the excreted as urine.

Kidneys act as filters. The renal artery, which brings blood to the kidneys, is divided into a large network of capillaries. These filter out waste matter and impurities dissolved in water. Useful materials and, after cleansing, most of the water, are returned  into the blood. Waste liquid or urine travels down the ureters to the bladder, where it is stored  before being excreted through the urethra. Urine contains various wastes dissolved in water, including urea( a product of poisonous ammonia, neutralized by the liver). The kidneys also control the balance of water and other substances in the body. So the most of the body’s waste are removed through the lungs and kidneys. When kidneys fail, kidney machines are used to filter the blood and take out poisonous waste materials.

·        The lungs get rid of waste materials, including water and carbon dioxide. If you breathe on a cold mirror, you can see the droplets of water which you breathe out.

·        Sweating through the skin is another way of excreting waste. Liquid is passed from the blood vessels into the sweat glands, and then out through the pores in the skin. We get rid of about a litre of water each day through the skin. In hot weather, we lose more water this way. Sweating is one way the body keeps cool.

·        Each kidney contains a million microscopic tubes with filters, called nephrons. Liquid waste passes through these via collecting ducts into the ureters, which carry it to the bladder.