The eyeball's tough, white outer layer is the sclera. Inside this is a soft, blood-rich, nourishing layer, the choroid. Within this, around the sides and back of the eye, is the retina. This layer detects patterns of light rays and turns them into nerve signals, which travel along the optic nerve to the brain. The bulk of the eyeball is filled with a clear jelly called vitreous humour. At the front of the eye is the dome-shaped cornea, through which light rays enter. They pass through a hole, the pupil, in a ring of muscle, the iris. Then the rays shine through the bulging lens, which bends or focuses them to form a clear picture on the retina.

Inside each side of the nose is an air chamber, the nasal cavity. Air comes in through the nostril and flows down, around the rear of the roof of the mouth, into the throat. But when you sniff, air swirls up into the top of the cavity. Here is a small patch of about 10 million specialised olfactory (smelling) cells. They have long micro-hairs, or cilia, sticking out from them. Odour particles in the air stick on to the cilia and make the olfactory cells produce nerve signals, which travel to the olfactory bulb. This is a pre-processing centre that partly sorts the signals before they go along the olfactory tract to the brain where they are recognised as smells.

The tongue is covered with dozens of pimple-like projections called papillae. These grip and move food when you chew. Around the sides of the papillae are about 10,000 microscopic taste buds. Different parts of the tongue are sensitive to different flavours: sweet, salt, sour and bitter.

Sound waves funnel into the outer ear - the flap of skin and cartilage on the side of the head. They pass along a narrow tube, the ear canal, to a small patch of rubbery skin at its end, the eardrum. The sound waves bounce off the eardrum and make it shake to and fro, or vibrate. The eardrum is connected to a row of three tiny bones linked together, the hammer, anvil and stirrup. The vibrations pass along these bones. The stirrup presses against a small, fluid-filled, snail-shaped part, the cochlea, deep inside the ear. The vibrations pass as ripples into the fluid inside the cochlea. Here, they shake thousands of tiny hairs that stick into the fluid from hair cells. As the hairs shake, the hair cells make nerve signals, which go along the auditory nerve to the hearing centre of the brain.

The sense of touch is the name given to a network of nerve endings that reach just about every part of our body. These sensory nerve endings are located just below the skin and register light and heavy pressure on the skin and also differences in temperature. These nerve endings gather information and send it to the brain