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The eye is enclosed in a bony cavity called the socket. It is about 2.5cm in diameter and is spherical in shape. It can be turned in different directions by six muscles. These muscles are attached at one end to the wall of the socket and at the other to the eyeball. The eyeball contains liquid and semiliquid internal sections. In the wall of eyeball there are three main layers. The outermost layer is tough sclera and it protects the soft internal parts of the eye. In front of it there is a circular, transparent area, called the cornea. In front of the lens is water solution called aqueous humour. It is surrounded by the retina.

Vitreous humour, a jelly-like substance is behind the lens. There are lachrymal glands, which secrete tear fluid and lubricate sclera. To the outer surface of the eye the eyelids provide protection. When something is about to enter the eye, the reflex action causes the eyelids to close. The middle layer is called choroid and is supplied by blood vessels. This extends throughout the eye except in front where it forms a hole. It is called the pupil of the eye. Circular band called iris surrounds the pupil. The eye colour is because of iris. The third and the innermost layer is the retina. It lines most of the internal eyeball and is a very sensitive layer. We are able to see because of this layer.

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When light bounces off objects and passes through the cornea, we see things. The cornea is a slightly curved surface and bends the light rays. The light rays then pass through the iris. Depending on how strong the light is the muscle adjusts the size of the pupil. The light rays now come to the lens, having gone through the pupil. The actual shape of the lens is very important. According to the distance of the object from the eye the muscles attached to it adjust this shape. At the back of the eye it ensures that the light rays remain focused on the retina. It is convex because of the shape of the lens and the image that reaches the retina is back to front and upside down. Rods and cones are the large numbers of two types of cells, retina is made up of. The rods are more sensitive than the cones but they register only black and white images. Other than black and white, the cones help us to see colours. The cones work properly only when there is enough light.

On the centre of the retina, most of the light entering is likely to fall. This part, called the yellow spot, has only cones. We see very well when the light is bright. To have only colour-sensitive cone cells in the central part of the retina is rather a disadvantage in the dark. From the rods and cones the nerves arise and join together to form the optic nerve. This sends impulses to the brain to be interpreted as shapes and colours. Two slightly different images are received at the same time because we have two eyes. From these two, the brain creates one composite image. This allows us to see things in three dimensional form. The brain also turns the images received on the retina so that they appear the right way up.

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The only time we are likely to see tears is when we cry. The liquid which makes tear is always secreted by the tear glands. Tears are useful because they lubricate the sclera and help it remove dust particles from the surface of the eye. The fluid passes into small tubes from the eye surface to the lachrymal ducts and from these into nose.

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Rods and Cones are the two kinds of light-sensitive cells the retina contains. In each eye there are about 60,00,000 cells and 12,00,00,000 rod cells. The cone-shaped cells respond to different light wavelengths and to bright light, so they enable you to see colours. The rod-shaped cells are sensitive to dim light, but not to colour. The yellow spot has only cone cells.

Only when we are fast asleep do we dream. We cannot see through the eyes during sleep. But our brain is fully active. Without passing through light sensitive cells, the images are entering the brain. To differentiate colours, the brain has no cells. And so, whether it is a sweet dream or a nightmarish one, we see only black and white pictures.

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In the uppermost part of the nasal passages are the organs of smell located. These are away from the direct path of the stream of air drawn in through the nostrils. The olfactory cells- the cells involved in smell are embedded in the mucous membrane. A long filament of protoplasm leads from each cell to the surface of the mucous membrane. Above this surface there are tiny hairs project, which form the filament. From the other end of the olfactory cell, the olfactory nerve connects it to the olfactory lobe of the brain.

In the olfactory area, the surface of the mucous membrane is bathed in fluid. Before the olfactory cells can be stimulated, the particles that are to be smelled must dissolve in this fluid. By eddies are the particles of the odour- releasing substances carried upward to the area. These eddies arise as the currents upward to the area. These eddies arise as the currents of cold air from the outside meet the warm air already in the nasal passages.

As we become aware of a faint odour, we sniff vigorously to sharpen the sense of smell. To penetrate the nasal passages, this causes a reinforced stream of cold air. The upward currents become stronger and more molecules of the odorous substance come in contact with the olfactory area. The sense of smell in man is generally quite rudimentary.

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The structure of the ear includes a large flap or the outer ear, called auricle or pinna. It is followed by ear passage to the trympanum or the eardrum. It is a thin membrane stretched tightly across the eardrum called the hammer, anvil and stirrup because of their shapes. The hammer touches the eardrum, so that when this vibrates it makes the hammer move. The hammer in turn is connected to the anvil, which also receives the vibrations. Behind the three bones there is a second drum, called the oval window or fenestra ovalis. The stirrup fits into this and so, the oval window picks up vibrations. The inner ear contains cochlea, three semicircular canals and two small sacs.

Cochlea, a long coiled or spiral chamber looks rather like a snail's shell. It is a very complicated organ in the inner ear. The cochlea has fluid in it, and there are also hair cells with nerves. They make it move, as the vibrations go through the liquid. The hairs pick up these movements and signals are sent along the nerve fibres. To form the auditory nerve, the nerve fibres join together, which takes the signals to the brain.

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The process of hearing starts with pinna. As the sound vibrations falls on the pinna, it traps them and directs them down the ear passage to the eardrum. The vibrations travel along the ear channel and reach the eardrum that vibrates. The hammer Picks up the vibrations and passes them to the anvil.

The anvil is connected to the stirrup, which picks up the sound waves and these reach the oval window. This is at the end of the liquid-filled cochlea. They make it move, as the vibrations go through the liquid. The organ of Corti picks up the movements and turns them into signals so that they can be sent to the brain. As the sound we hear, it is the brain, which interprets the signals. So it the brain and not really the ear which hears.

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To deal with all the hard work they have to do, the teeth have got to be tough. They are all made up in the same way, although they are of different shapes. The tooth is alive and has a supply of blood and nerves. If anything goes wrong, there is some warning through these nerves.

The enamel is a hard outer covering which is very strong and can stand up to great wear and tear. Most of the tooth consists of dentine. The long roots of the tooth hold it firmly in the jawbone.