Chapter 6 — Sense Organs

A living organism must know what is happening around it in order to find food, avoid danger, find a mate and adjust to changes. Any change in the surroundings or inside the body that can be detected is a stimulus (plural stimuli): light, sound, heat, cold, pressure, chemicals, gravity. The organs that detect stimuli are the sense organs, and the specialised cells or nerve endings inside them that actually respond to the stimulus are the receptors. Each receptor is built to respond to one particular kind of stimulus: the receptors of the eye respond to light, those of the ear to sound vibrations, those of the nose and tongue to chemicals, and those of the skin to touch, pressure, temperature and pain.

Whatever the stimulus, the receptor's job is the same: to convert the energy of the stimulus into a nerve impulse, an electrical signal that travels along a sensory nerve to the brain or spinal cord. This conversion is called transduction. The impulses from the eye and from the ear are physically identical; what makes one a sight and the other a sound is only the part of the brain to which the nerve delivers it. The brain interprets the impulses and produces the sensation, and it also compares the information with memory to give perception: not just light, but a red mango; not just sound, but a friend's voice. A sense organ is therefore only the front end of a system that includes the nerve and the brain, and damage anywhere along the path, to the receptor, the nerve or the brain area, causes loss of that sense.

Human beings have five main sense organs, the classical five senses described by Aristotle: eyes for sight (photoreceptors), ears for hearing and also for balance (mechanoreceptors), nose for smell (chemoreceptors), tongue for taste (chemoreceptors) and skin for touch, pressure, pain, heat and cold (mechanoreceptors and thermoreceptors). Receptors in muscles and joints also tell the brain the position of the limbs, which is how you can touch your nose with your eyes closed.

Receptors show adaptation: when a stimulus continues unchanged, the receptors gradually stop responding to it. This is why you soon stop noticing the pressure of your shirt, the smell of a room or the hum of a fan, and why a strong scent seems to fade after a few minutes. Adaptation lets the nervous system ignore the unchanging and attend to what is new. Different senses adapt at different rates; smell and touch adapt quickly, pain hardly at all, which protects us.

The senses are not equally developed in all animals. Dogs smell far better than we do, eagles see farther, bats hear sounds we cannot, snakes detect the warmth of prey, and fish sense water movement along their sides. In each animal the senses are those that fit its way of life; in human beings sight is the dominant sense, occupying a larger part of the brain than any other.

The human eye is a nearly spherical ball about 2.5 cm in diameter, lodged in a deep bony socket of the skull called the orbit, which protects it from blows on all sides except the front. Only about one sixth of the eyeball is visible; the rest lies within the orbit, cushioned by fat and moved by six small muscles that turn it up, down and sideways so that both eyes point at the same object. The optic nerve leaves the back of the eyeball and carries impulses to the brain.

Several structures protect the front of the eye. The eyebrows keep sweat from running into the eyes. The eyelids, upper and lower, close over the eye to protect it from dust, bright light and injury; they close reflexly when anything approaches the eye, and they blink about 15 to 20 times a minute, spreading tears over the surface and keeping it moist and clean. The eyelashes along the edges of the lids trap dust and trigger blinking when touched. The conjunctiva is a thin transparent membrane that lines the inner surface of the lids and covers the front of the eyeball up to the cornea; it is what becomes red and inflamed in conjunctivitis. The lacrimal gland (tear gland) lies at the upper outer corner of each orbit and continuously secretes tears, a salty fluid containing the enzyme lysozyme that kills bacteria. Tears wash over the eye and drain through two tiny openings at the inner corner of the lids into the tear duct, which leads into the nose; this is why the nose runs when we cry. Tears keep the cornea moist, wash away dust, lubricate the lids and defend against infection.

Looking at a friend's eye you can see the white sclera, the transparent cornea bulging slightly at the front (visible only as a shine), the coloured ring of the iris, brown or black in most Indians and blue or green in some other peoples, and the black circle of the pupil in its centre. The pupil looks black because the inside of the eyeball is dark and light entering it does not come back out. Watch the pupil in a mirror as you switch a bright light on and off and you will see it shrink and widen; this is described in the section on adjustment to light.

We have two eyes set a few centimetres apart, facing forward. Each eye sees the world from a slightly different angle, and the brain combines the two pictures into one image with depth. This binocular vision lets us judge distance accurately, which is why catching a ball or threading a needle is hard with one eye closed. It also gives a wider field of view, and if one eye is lost the other still serves. Animals that are hunted, such as rabbits and deer, have eyes on the sides of the head to watch all round; hunters, such as cats and owls, have forward-facing eyes like ours to judge the distance to prey.

The wall of the eyeball is made of three coats, one inside the other, and each has a distinct job.

The outer coat: sclera and cornea. The sclera is the tough, white, opaque, fibrous outer layer that covers about five sixths of the eyeball. It protects the delicate inner parts, keeps the eyeball's shape, and gives attachment to the six eye muscles. At the front the sclera becomes the cornea, a transparent, dome-shaped window through which light enters. The cornea has no blood vessels (it gets oxygen directly from the air and nourishment from the fluid behind it), which is why it stays transparent and why it can be transplanted from a donor without rejection. The cornea does most of the bending of light that enters the eye; the lens does the fine focusing. Because the cornea is richly supplied with nerve endings, even a speck of dust on it is painful.

The middle coat: choroid, ciliary body and iris. The choroid lies inside the sclera. It is dark brown or black because of its pigment cells, and it is rich in blood vessels. The pigment absorbs stray light and prevents reflection inside the eye, just as the inside of a camera is painted black, so that the image is sharp; the blood vessels nourish the retina. Towards the front the choroid thickens into the ciliary body, a ring of muscle (the ciliary muscle) that holds the lens by fine fibres called the suspensory ligaments and changes its shape for focusing; the ciliary body also secretes the fluid that fills the front of the eye. In front of the lens the middle coat continues as the iris, a coloured, muscular, circular curtain with a central hole, the pupil. The iris contains two sets of involuntary muscles, circular and radial, that constrict or dilate the pupil and so control the amount of light entering the eye. The colour of the iris depends on the amount of pigment in it and is inherited.

The inner coat: retina. The retina is the innermost, light-sensitive layer, lining the back two thirds of the eyeball. It is a delicate sheet of nervous tissue containing millions of receptor cells, the rods and cones, together with the nerve cells that connect them. It is the screen on which the image is formed and the place where light is converted into nerve impulses. Two special spots on the retina are the yellow spot (fovea or macula), directly behind the lens, where cones are densest and vision is sharpest, and the blind spot, where the optic nerve leaves the retina and where there are no receptors.

Inside these coats the eyeball is divided by the lens into two chambers. The small front chamber between the cornea and the lens contains a clear watery fluid, the aqueous humour, which nourishes the cornea and lens and keeps the front of the eye in shape; it is continuously produced and drained, and if its drainage is blocked the pressure rises, a disease called glaucoma. The large chamber behind the lens contains a transparent jelly, the vitreous humour, which keeps the eyeball firm and round and holds the retina in place against the choroid.

Behind the iris hangs the lens, a transparent, elastic, biconvex structure (thicker in the middle than at the edges) made of layers of transparent protein fibres, rather like the layers of an onion. It has no blood vessels and is held in position by the suspensory ligaments attached to the ciliary body. The lens divides the eye into the aqueous chamber in front and the vitreous chamber behind.

The eye works on the same principle as a camera. Light from an object enters through the cornea, passes through the aqueous humour, the pupil and the lens, and then through the vitreous humour to fall on the retina. The cornea and lens together form a converging lens system that bends (refracts) the rays so that they meet on the retina and form an image. The image formed on the retina is real, inverted (upside down and reversed left to right) and smaller than the object, exactly as in a camera. We do not see the world upside down because the brain, from infancy, learns to interpret the inverted image the right way up. Experiments in which people wore goggles that turned the image upright on the retina showed that after a few days the brain adjusted and the world looked normal again.

For a clear image the rays from an object must be brought to a focus exactly on the retina, neither in front of it nor behind. Since the distance from the lens to the retina is fixed, the eye can only manage this by changing the focal length of the lens, which it does by changing the lens's shape. A fatter, more curved lens bends light more and has a shorter focal length; a thinner, flatter lens bends light less. This ability of the eye to change the curvature of its lens so as to focus objects at different distances is called accommodation, and it is the subject of the next section.

The light falling on the retina is absorbed by the rods and cones, which contain light-sensitive pigments. The pigment molecules change shape when they absorb light, and this change triggers a nerve impulse. Impulses from about 13 crore receptors are collected by about 10 lakh nerve fibres that gather at the back of the eye to form the optic nerve. The two optic nerves cross partly at the base of the brain so that the left half of each retina reports to the left side of the brain and the right half to the right. The impulses reach the visual area at the back of the brain (the occipital lobe), where they are interpreted as the sight of the object, upright, in colour and in three dimensions.

The sharpness of the image depends on the part of the retina it falls on. When you look directly at something its image falls on the yellow spot, where cones are packed most densely, and you see it in fine detail and full colour; objects at the edge of your field of view fall on the peripheral retina and are seen only vaguely. That is why you turn your eyes to look at what interests you.

A camera focuses by moving its lens backward or forward. The eye cannot move its lens, so it changes the lens's shape. This is accommodation, the adjustment of the curvature of the lens to focus objects at different distances on the retina, and it is done by the ciliary muscle acting through the suspensory ligaments.

When you look at a distant object (more than about 6 metres away), the rays reaching the eye are nearly parallel and need little bending. The ciliary muscle relaxes, the ring it forms widens, the suspensory ligaments are pulled taut, and they stretch the elastic lens so that it becomes thinner and flatter, with a longer focal length. The eye is at rest; this is why looking at distant scenery is restful. When you look at a near object, such as a book, the rays diverge strongly and must be bent more. The ciliary muscle contracts, the ring narrows, the suspensory ligaments go slack, and the lens, freed from their pull, bulges by its own elasticity into a thicker, more convex shape with a shorter focal length. Close work therefore keeps the ciliary muscle contracted, which is why long hours of reading tire the eyes.

Two limits matter. The far point is the farthest distance at which an object can be seen clearly with the ciliary muscle relaxed; for a normal eye it is infinity. The near point or least distance of distinct vision is the closest distance at which an object can be focused with the ciliary muscle fully contracted; for a normal young adult it is about 25 cm. Bring a finger slowly towards your eye and note where it blurs: that is your near point. It is nearer in children and recedes with age, as the lens hardens and loses its elasticity, until by the age of 45 or so most people cannot focus close work without spectacles; this is presbyopia. The range between the near and far points is the range of vision.

The eye also adjusts to the amount of light, through the pupil reflex. The pupil is the hole in the iris, and two sets of involuntary muscles in the iris change its size. In bright light the circular muscles of the iris contract and the pupil constricts to as little as 2 mm, cutting down the light that enters so that the retina is not dazzled or damaged. In dim light the radial muscles contract and the pupil dilates to as much as 8 mm, admitting more light. The reflex is automatic and takes about a second. When you step from bright sunlight into a dark cinema you can see nothing for a while; the pupil widens and the rods slowly become more sensitive, a process called dark adaptation that takes several minutes. Coming out again into sunlight, the pupil shrinks quickly and the glare fades.

Doctors test the pupil reflex by shining a torch into a patient's eye; a pupil that does not constrict indicates damage to the nerve pathway or the brain. In cats the pupil closes to a vertical slit, in goats to a horizontal one; the human pupil remains round.

The retina contains two kinds of light-sensitive receptor cells, named after their shapes.

Rods are long, thin cells, about 12 crore of them, spread over the whole retina except the yellow spot and most numerous towards the edges. They contain a purple pigment called rhodopsin (visual purple), which is very sensitive and responds even to very dim light, but they cannot distinguish colours; they report only shades of grey. Rods are responsible for vision in dim light (night vision or scotopic vision) and for detecting movement at the edges of the visual field. Rhodopsin is bleached by bright light and slowly regenerated in the dark, which is why it takes several minutes in a dark room before you can see well. Rhodopsin is made from vitamin A, and a shortage of vitamin A in the diet causes night blindness, the inability to see in dim light; carrots, papaya, green leafy vegetables, milk, eggs and fish liver oil prevent it. Nocturnal animals such as owls and cats have retinas rich in rods.

Cones are shorter, thicker cells, about 60 to 70 lakh of them, concentrated at the yellow spot (fovea), where there are no rods at all. They need bright light to work and they give colour vision and sharp, detailed vision. There are three kinds of cone, containing pigments most sensitive to red, green and blue light respectively; every colour we see is produced by the brain from the relative stimulation of the three kinds. A person who inherits a defective or missing cone pigment is colour blind, most commonly unable to tell red from green; the condition affects about 8 percent of men and less than 1 percent of women because the gene lies on the X chromosome. Colour-blind people can lead normal lives but are not allowed to drive trains or fly aircraft, where coloured signals matter. Cones give a sharp image because each cone at the fovea has its own nerve fibre to the brain, while many rods share one fibre.

The blind spot is the small area where the optic nerve leaves the retina. It has no rods or cones, so light falling on it is not seen. You can find it: draw a cross and a dot about 8 cm apart on a card, close your left eye, look at the cross with your right eye and move the card slowly towards you; at a certain distance the dot vanishes, because its image has fallen on the blind spot. We do not normally notice the blind spot because the other eye covers that part of the field and because the brain fills in the gap.

Persistence of vision is the fact that an image formed on the retina persists for about a sixteenth of a second after the object is removed. If a series of slightly different pictures is shown faster than this, the eye cannot separate them and sees continuous motion. This is the basis of cinema, where 24 pictures are projected every second, and of television and animation. A glowing incense stick whirled in a circle in the dark appears as a complete ring of fire for the same reason.

In a normal eye the cornea and lens focus rays from a distant object exactly on the retina when the ciliary muscle is relaxed, and accommodation brings near objects into focus down to 25 cm. If the eyeball is too long or too short, or the lens too strong or too weak, the image falls in front of or behind the retina and vision is blurred. Such refractive errors are the most common defects of vision and are corrected by spectacle lenses.

Myopia (short sight, near sightedness) is the inability to see distant objects clearly, while near objects are seen well. A myopic child can read a book but cannot read the blackboard. It is caused by the eyeball being too long from front to back, or by the lens or cornea being too curved (too strong). Rays from a distant object are brought to a focus in front of the retina and have spread out again by the time they reach it, so the image is blurred; the far point is not at infinity but at some finite distance. Myopia is corrected by spectacles with a concave lens (diverging lens, minus power), which spreads the incoming rays a little before they enter the eye so that the eye's own lens brings them to a focus farther back, exactly on the retina. Myopia often begins in school years and increases until the eye stops growing; long hours of close work and too little time outdoors are thought to make it worse.

Hypermetropia (long sight, far sightedness) is the inability to see near objects clearly, while distant objects are seen well. A hypermetropic person holds a book at arm's length to read it. It is caused by the eyeball being too short, or by the lens or cornea being too flat (too weak). Rays from a near object would come to a focus behind the retina, so the image on the retina is blurred; the near point is farther than 25 cm. The eye can partly compensate by accommodating even for distant objects, which strains the ciliary muscle and causes headaches. Hypermetropia is corrected by a convex lens (converging lens, plus power), which bends the rays inward before they enter the eye so that the eye focuses them on the retina.

A third refractive error, astigmatism, arises when the cornea is not evenly curved, like the back of a spoon rather than a ball, so that lines in one direction are focused and lines in another are blurred; it is corrected by a cylindrical lens. Refractive errors are measured by an optometrist with a chart of letters of decreasing size and are stated in dioptres, the reciprocal of the focal length of the correcting lens in metres. Besides spectacles, contact lenses and laser surgery on the cornea can correct these defects.

Presbyopia (old sight) is the loss of the power of accommodation with age. From about the age of 40 the lens gradually loses its elasticity and hardens, and the ciliary muscle weakens, so the lens can no longer bulge enough to focus near objects. The near point recedes from 25 cm to 50 cm, 100 cm or more, and reading, sewing and other close work become difficult, although distance vision may remain good. It happens to nearly everyone and is not a disease. It is corrected by convex lenses for reading. A person who is also myopic needs different corrections for far and near, and is given bifocal spectacles, in which the upper part of each lens is concave for distance and the lower part convex for reading.

Cataract is the clouding of the lens, which becomes opaque, milky or yellowish so that light cannot pass through clearly. Vision becomes hazy, as if through frosted glass, colours fade, bright lights cause glare, and finally sight is lost. Cataract is the leading cause of blindness in India. It is usually a result of ageing, the proteins of the lens gradually clumping together, but it can also follow injury, diabetes, long exposure to strong sunlight, smoking, or certain medicines, and a few babies are born with it. Cataract cannot be cured by medicine or spectacles. It is treated by a short surgical operation in which the cloudy lens is removed and replaced by an artificial plastic intraocular lens; sight returns almost immediately and the patient usually needs only reading glasses afterwards. Free cataract camps run by the government and charities restore the sight of lakhs of people every year.

Glaucoma is a rise in the pressure of the fluid inside the eye because the aqueous humour is not draining properly. The pressure damages the optic nerve and destroys vision gradually from the edges inward, often without pain, so it is called the silent thief of sight; it is detected by measuring eye pressure and is controlled by eye drops or surgery. Damage from glaucoma is permanent, so regular eye checks after 40 are important.

Conjunctivitis is an infection or allergy of the conjunctiva: the eye becomes red, itchy, watery and sticky. The infectious kind spreads by touch and towels and needs hygiene and drops. Trachoma, a chronic bacterial infection of the conjunctiva and cornea, was once a major cause of blindness in India and is now nearly eliminated. Xerophthalmia is drying and damage of the conjunctiva and cornea due to vitamin A deficiency; it begins with night blindness and can end in blindness, and is prevented by vitamin A rich food and the vitamin A drops given to children in health programmes. Squint is a misalignment of the two eyes, correctable in childhood. Retinal detachment, the peeling of the retina from the choroid after injury, and diabetic damage to the retina are serious conditions needing urgent treatment.

India has a large number of blind people, and many cases could have been prevented by early treatment or by a good diet. Eye donation after death allows the corneas of one person to restore sight to two people with corneal blindness; eyes must be removed within six hours of death, and pledging one's eyes is a simple act that gives sight to others.

The ear is the organ of hearing and also of balance. Most of it is hidden inside the temporal bone of the skull; the part we see is only the collecting funnel. Each ear has three regions, the outer, middle and inner ear.

The outer (external) ear consists of the pinna (auricle), the flap of skin-covered elastic cartilage on the side of the head, and the auditory canal (external auditory meatus), a tube about 2.5 cm long leading inward from it. The pinna collects sound waves and funnels them into the canal; animals such as dogs and rabbits can turn their pinnae towards a sound, but ours are nearly fixed. The skin of the canal has hairs and glands that secrete wax (cerumen), which traps dust and insects and keeps the canal supple. The canal ends at the ear drum (tympanic membrane), a thin, taut, oval membrane that separates the outer ear from the middle ear and vibrates when sound waves strike it.

The middle ear is a small air-filled cavity in the bone beyond the ear drum. Across it stretches a chain of three tiny bones, the smallest in the body, called the ear ossicles: the malleus (hammer), attached to the ear drum, the incus (anvil), and the stapes (stirrup), whose footplate fits into a small membrane-covered opening in the inner ear called the oval window. The ossicles act as levers that pass the vibrations of the ear drum to the oval window and, because the oval window is much smaller than the ear drum, they concentrate the force and increase the pressure about twenty times, so that the vibrations can move the fluid of the inner ear. The middle ear is connected to the back of the throat (the pharynx) by the Eustachian tube, which is normally closed but opens when we swallow or yawn to equalise the air pressure on the two sides of the ear drum. This is why the ears pop when a bus climbs a ghat road or an aeroplane descends, and why a cold can spread from the throat to the middle ear and cause earache.

The inner (internal) ear lies deep in the temporal bone and is a system of fluid-filled tubes and chambers, the membranous labyrinth, enclosed in a bony labyrinth of the same shape. It has two parts with different jobs. The cochlea is a coiled tube, like a snail shell with two and a half turns, filled with fluid and containing the actual organ of hearing, a strip of sensory hair cells resting on a membrane along its length (the organ of Corti). The vestibular apparatus, concerned with balance, consists of three semicircular canals set at right angles to one another in three planes, and two small sacs, the utricle and saccule, in the vestibule between the canals and the cochlea. Nerve fibres from the cochlea and the vestibular apparatus join to form the auditory nerve (eighth cranial nerve), which carries impulses to the brain.

Sound is a vibration of the air (or water) that spreads out as waves of alternating pressure. Hearing is the process by which these pressure waves are turned into nerve impulses. It happens in a chain of steps, each part of the ear passing the vibration to the next.

• The pinna collects the sound waves and directs them into the auditory canal.
• The waves travel down the canal and strike the ear drum, which vibrates at the same frequency as the sound. A loud sound makes it vibrate more strongly, a high-pitched sound more rapidly.
• The vibrations of the ear drum are passed to the malleus, which is attached to it, and from there through the incus to the stapes. The lever action of the three ossicles and the difference in area between the ear drum and the oval window together amplify the vibrations about twenty times.
• The footplate of the stapes pushes in and out on the oval window, setting up pressure waves in the fluid of the inner ear. A second membrane, the round window, bulges outward to allow the fluid to move.
• The waves travel through the fluid of the cochlea and make the membrane carrying the sensory hair cells vibrate. The hairs bend against an overlying membrane, and this bending generates nerve impulses. Different parts of the cochlear membrane respond to different frequencies: the part near the oval window to high notes, the part near the tip of the coil to low notes, so the pitch of a sound is recognised by which hair cells fire, and its loudness by how strongly they fire.
• The impulses travel along the auditory nerve to the hearing area of the brain in the temporal lobe, where they are interpreted as sound: its pitch, loudness, quality and, by comparing the two ears, its direction.

The human ear can hear sounds between about 20 and 20,000 vibrations per second (hertz). Sounds below 20 Hz are infrasound and above 20,000 Hz ultrasound; dogs hear up to about 45,000 Hz, which is why they respond to a silent dog whistle, and bats use ultrasound up to 1,00,000 Hz to find their way in the dark. The upper limit of human hearing falls with age. Loudness is measured in decibels: a whisper is about 30 dB, normal talk 60 dB, heavy traffic 80 dB, a loudspeaker at a festival or a firecracker 110 to 140 dB. Sounds above about 85 dB heard for long, and any sound above 120 dB, damage the hair cells of the cochlea, which do not regrow, causing permanent deafness. Loud music through earphones is a growing cause of hearing loss in young people.

Deafness has several causes. Conductive deafness arises when sound cannot reach the inner ear: wax blocking the canal, a perforated ear drum, or ossicles stuck by infection; it is often curable. Nerve deafness results from damage to the hair cells or the auditory nerve by noise, infection, some drugs or age, and is usually permanent, though hearing aids amplify sound and cochlear implants can bypass the damaged cells. Middle ear infection (otitis media), common in children after colds, causes earache and discharge and must be treated to prevent damage to the ear drum.

Besides hearing, the inner ear tells the brain the position and movement of the head, and this information, combined with what the eyes see and what the muscles and joints report, keeps us balanced. The organs of balance are the vestibular apparatus: the three semicircular canals and the two sacs, utricle and saccule.

The semicircular canals are three fluid-filled loops set at right angles to each other, one horizontal and two vertical, so that among them they cover the three planes in which the head can turn: nodding, shaking and tilting. At the base of each canal is a swelling, the ampulla, containing a tuft of sensory hair cells embedded in a jelly cap. When the head turns, the canal turns with it but the fluid inside lags behind because of its inertia, so it presses on the jelly cap and bends the hairs, and the hair cells fire impulses to the brain. The canal that lies in the plane of the movement responds most, so the brain knows in which direction and how fast the head is turning. The semicircular canals thus detect rotational movement of the head. When you spin round and stop, the fluid keeps moving for a while, the canals report a turn that is not happening, and you feel giddy; dancers avoid this by fixing their eyes on one point.

The utricle and saccule detect the position of the head with respect to gravity and straight-line movement such as starting, stopping and going up in a lift. Each contains a patch of hair cells covered with a jelly in which are embedded tiny crystals of calcium carbonate called otoliths (ear stones). Gravity pulls the heavy otoliths downward, bending the hairs in a direction that depends on how the head is tilted; when the body accelerates, the otoliths lag behind and bend the hairs the other way. The brain reads the pattern of impulses and knows whether the head is upright, tilted or moving.

All this information goes along the vestibular branch of the auditory nerve to the brain, which compares it with what the eyes report and with the sense of position from the limbs, and sends instructions to the muscles to keep the body upright. The cerebellum at the back of the brain coordinates these adjustments, and most of them happen without our being aware of them. Balance is disturbed when the vestibular apparatus and the eyes disagree: in a car or a boat the eyes see a steady interior while the inner ear feels the motion, and the conflict produces the nausea of motion sickness; looking out at the horizon, so that eyes and ears agree, relieves it. Infection or disease of the inner ear causes vertigo, a spinning sensation with loss of balance, and some people suffer giddiness when the otoliths become dislodged.

The organs of balance are old in evolution: even fish have otoliths, and the semicircular canals are found in all vertebrates. In human beings they make possible walking on two legs, riding a bicycle and standing on one foot with the eyes closed, which you can test: it is much harder than with the eyes open, showing how much sight contributes to balance.

Smell (olfaction) is a chemical sense: it detects molecules of substances that have evaporated into the air and entered the nose with the breath. Everything that has a smell, a flower, a cooking curry, petrol, a rotting fish, is releasing tiny amounts of volatile chemicals into the air, and the nose has receptors that respond to them.

The nose has two nostrils that open into the two nasal cavities, separated by a partition, the septum. The cavities are lined by a moist mucous membrane with hairs near the entrance that filter dust, and the passages are warmed and moistened by blood vessels; this is the nose's job as the entrance of the respiratory tract. High up in the roof of each nasal cavity, above the main air stream, lies a small patch of yellowish tissue about the size of a postage stamp, the olfactory epithelium. It contains about 50 lakh olfactory receptor cells, which are specialised nerve cells. Each receptor cell has a tuft of fine hairs (cilia) projecting into the layer of mucus that covers the epithelium, and its other end is a nerve fibre that passes up through tiny holes in the bone above the nasal cavity to the olfactory bulb of the brain, which lies just above.

For a substance to be smelt it must be volatile, so that its molecules reach the nose in the air, and soluble in the mucus, so that they can reach the receptor hairs. When a molecule of the right shape binds to a receptor on the cilia, the cell fires an impulse to the olfactory bulb, and from there impulses pass to the smell area of the brain. Human beings have several hundred different kinds of olfactory receptor, each responding to a group of related chemicals, and by combining their signals the brain distinguishes several thousand different odours, from jasmine to burning rubber. Because the receptors lie above the main air current, we sniff to draw air up to them when we want to smell something carefully; a cold blocks the passages with mucus and swelling and dulls the sense of smell.

The sense of smell is closely linked to memory and emotion, because the olfactory pathways connect directly to the parts of the brain concerned with them; a smell from childhood can bring back a whole scene. Smell adapts rapidly: within a minute or two of entering a smelly room the receptors stop responding to that odour, though they can still detect a new one. Smell protects us by warning of spoiled food, smoke, leaking gas (cooking gas is deliberately given a smell for this reason) and poisonous fumes, and it stimulates appetite and the flow of saliva and digestive juices. In many animals smell is the master sense: dogs track by it, moths find mates by it over kilometres, and salmon find the river of their birth by its smell.

Loss of the sense of smell is called anosmia and can follow head injury, infections such as influenza and certain viral illnesses, nasal polyps, or ageing. Because much of what we call the taste of food is actually its smell, people with anosmia find food flat and tasteless.

Taste (gustation) is the second chemical sense. It detects substances dissolved in the saliva in the mouth, and its receptors are on the tongue, with a few on the palate and the throat.

The tongue is a muscular organ covered by a moist mucous membrane. Its upper surface is rough because of thousands of tiny projections called papillae, which you can see in a mirror as small bumps; the large ones in a V at the back are easily visible. Embedded in the sides of most papillae are the taste buds, about 10,000 in an adult, fewer in old age. Each taste bud is a small flask-shaped cluster of 50 to 100 taste receptor cells with a tiny pore opening on to the surface of the tongue. The receptor cells bear fine hairs that project into the pore, and nerve fibres wrap around their bases. Taste cells are replaced every ten days or so, which is why the tongue recovers quickly after being scalded by hot tea.

For a substance to be tasted it must be dissolved in saliva; a dry tongue cannot taste, as you can check by drying your tongue with a cloth and placing a sugar crystal on it. The dissolved molecules enter the pore, bind to the receptor cells, and the cells send impulses along cranial nerves to the taste area of the brain. There are only five basic tastes: sweet (sugars), salty (sodium and other salts), sour (acids, as in lemon and tamarind), bitter (many alkaloids such as quinine, and bitter gourd), and umami, the savoury taste of glutamate in meat broths, tomatoes and cheese, recognised more recently. Bitter taste is the most sensitive, being detected at very low concentrations, which protects us because many poisons are bitter; sweet and salty signal energy and minerals. The old textbook map of the tongue, with sweet at the tip, salty and sour at the sides and bitter at the back, is only roughly true: all tastes can be detected on all parts of the tongue where there are taste buds, though the tip is somewhat more sensitive to sweet and the back to bitter.

The rich variety of flavours we enjoy, the difference between mango and pineapple or between coffee and tea, comes not from these five tastes alone but from smell. While we chew, volatile molecules from the food rise through the back of the mouth into the nasal cavity and stimulate the olfactory receptors; the brain combines taste and smell into flavour. That is why food loses its flavour during a cold and why holding the nose makes different foods taste alike. Temperature, texture and the burning sensation of chilli (which is detected by pain and heat receptors, not taste buds) also contribute to what we experience as taste.

Taste serves to choose food and reject what is harmful, and the taste of food starts the flow of saliva and gastric juice that prepares the body to digest it. Taste is dulled by smoking, by very hot or very spicy food, by some medicines and by ageing, and the number of taste buds falls in old age, which is one reason elderly people often find food bland and add more salt.

The skin is the largest organ of the body, covering about 1.5 to 2 square metres in an adult, and it is also the largest sense organ. Besides protecting the body, keeping water in, regulating temperature by sweating and making vitamin D, it contains millions of sensory receptors that tell the brain what is touching the body and how the surroundings feel. The skin has an outer epidermis of stratified epithelium and an inner dermis of connective tissue, and the receptors lie mostly in the dermis and at the base of the epidermis.

The skin has several kinds of receptor, each responding to a different stimulus, and together they make up the sense of touch, which is really a family of senses.

• Touch receptors, close to the surface, respond to light contact and let us feel the texture of a cloth, a light breeze, or an insect walking on the arm. Free nerve endings wrapped around the base of each hair respond when the hair is moved.
• Pressure receptors, lying deeper in the dermis and in the tissue beneath, respond to firm pressure and vibration.
• Heat receptors and cold receptors are separate; they respond to a rise or a fall in skin temperature rather than to a fixed temperature, which is why lukewarm water feels warm to a cold hand and cool to a hot one.
• Pain receptors are free nerve endings, found everywhere, that respond to anything that damages or threatens to damage the tissue: cuts, burns, strong pressure, chemicals. Pain is unpleasant by design; it forces us to remove the cause and protect the injured part, and people born without pain sense injure themselves constantly.

The receptors are not evenly spread. The fingertips, lips and tongue are the most sensitive parts, packed with touch receptors and given a large share of the brain's sensory area; the back, upper arms and thighs are much less sensitive. You can measure this with the two-point test: touch the skin with the two points of a divider and find the smallest separation at which they are felt as two. On a fingertip two points 2 to 3 mm apart are felt separately; on the back they must be 40 to 70 mm apart before they are distinguished. Because of this sensitivity the fingertips are used for reading Braille, for examining objects in the dark and, by doctors, for feeling a pulse or a swelling.

Impulses from the skin receptors travel along sensory nerves to the spinal cord and up to the sensory area of the brain, where each part of the body has its own region, sized according to its sensitivity rather than its actual size. Some touch responses are reflexes handled by the spinal cord alone, such as jerking the hand away from a hot vessel before the brain has registered the heat. Touch receptors adapt quickly, so we stop feeling clothes and a wristwatch, but pain receptors adapt little.

Touch is the first sense to develop in the embryo and the last to fade in old age. Loss of skin sensation, as in leprosy or in the feet of some diabetics, is dangerous because injuries go unnoticed and become infected; patients must inspect their hands and feet daily. Skin sensation also carries social meaning, in a handshake, a pat on the back or a mother's touch.

The sense organs are delicate and mostly cannot be replaced, so caring for them is a lifelong duty. The habits are simple.

Eyes. Read and write in good light, with the book about 25 to 30 cm away and the light falling from behind or the side rather than into the eyes. Take breaks from close work and screens: every 20 minutes look at something 20 feet away for 20 seconds. Never rub the eyes with dirty hands, and never touch the eyes after touching an infected person's eyes or towel. Wash the eyes with clean water; do not use kajal or eye drops shared with others. Do not look at the sun, an eclipse, welding arcs or bright lasers. Wear protective glasses when working with tools, chemicals or fireworks; every Deepavali eye hospitals fill with children injured by crackers. Eat food rich in vitamin A: carrots, papaya, mango, spinach and other green leaves, milk, eggs and fish. Have the eyes tested when there is any blurring, headache after reading or difficulty seeing the blackboard, and wear the spectacles prescribed; children should have an eye test every year. After 40, have a check for glaucoma and cataract.

Ears. Never put pins, matchsticks, pencils or ear buds into the ear canal; they push wax deeper, injure the canal and can pierce the ear drum. Wax is normally cleared by the ear itself; if it blocks the canal a doctor removes it. Keep the ears dry and treat colds and throat infections promptly so that they do not spread to the middle ear through the Eustachian tube. Avoid loud noise: keep earphone volume low, stand away from loudspeakers and crackers, and use ear plugs or muffs in noisy workplaces. A slap on the ear can rupture the drum. Do not pour oil into the ear for pain; see a doctor for any discharge, earache or reduced hearing.

Nose, tongue and skin. Do not pick the nose or insert objects; treat colds and sinus infections. Keep the mouth clean, avoid very hot food that scalds the tongue, and avoid tobacco in every form, which damages taste and causes cancers of the mouth. Keep the skin clean and dry, treat cuts and wounds, protect it from strong sun, and never ignore loss of sensation in any patch of skin, which can be a sign of leprosy, a curable disease.

Living without a sense. When one sense is lost, the others become sharper through use and training, and the brain reassigns some of the area it used for the missing sense. Blind people learn to move about by sound and touch, using a white cane to feel the path, and they read by Braille, a system of raised dots invented in 1824 by the Frenchman Louis Braille, himself blinded at the age of three. Each Braille character is a cell of up to six raised dots arranged in two columns of three; the 63 possible patterns stand for letters, numbers, punctuation and common words, and are read by passing the fingertips over the embossed lines. Braille books, signs and now computer displays give the blind access to education and work; the Indian rupee notes carry raised marks so the blind can identify them. The deaf communicate by lip reading and by sign language, in which hand shapes and movements form words, and children who are deaf from birth can learn to speak with training and hearing aids or cochlear implants. Helen Keller, who was both deaf and blind from infancy, learnt language through touch alone and became a writer and public speaker. Society's part is to build ramps, tactile paving, audible signals and inclusive schools, and to treat people with sensory disabilities as equals.