Chapter 11: Sense Organs
For a very long time, the inside of a living eye was a closed room. A doctor could see the white of the eye, the coloured ring around its centre, and the dark circle in the middle of that ring — and nothing beyond. Look into another person’s pupil today and you will find the same thing: blackness.
In 1851, a young German physician and physicist, Hermann von Helmholtz, worked out why. Light does enter the eye. Almost all of it, however, comes back out along the very path it went in. If your own head is between the lamp and the eye you are studying, the returning light cannot reach you.
His way around the problem was simple. He set a plate of glass in front of his eye at an angle, so that it reflected lamplight sideways into the patient’s pupil while still letting him look straight through it. The blackness opened. Lit up in front of him lay the living inner lining of the eye, with blood vessels branching across it, a pale disc where the nerve left the eyeball, and, a little to one side, the small area on which we depend for our sharpest sight (Fig. 11.0-1).
He named the instrument the ophthalmoscope, and doctors have used one ever since. Twelve years later, Helmholtz turned to the ear, arguing that its coiled inner tube sorts a jumble of vibrations into separate pitches before the brain receives them at all.
Diagram
Fig. 11.0-1 — The Living Eye Seen From Within Circular field of view of the inner lining of the eyeball as seen through an ophthalmoscope; show blood vessels branching outwards from a pale rounded disc, and a small distinct spot to one side of it; label retina, optic disc (blind spot), yellow spot, retinal blood vessels.
11.1 Sense Organs And Receptors
Sit still for a moment and notice how much you already know without moving. You know how bright the room is, whether a fan is running, whether your shirt is damp, and whether you are hungry. None of that reached you by itself. Each piece of it was picked up by an organ built to detect one particular kind of change.
Organs of this kind are called sense organs — organs that detect changes around us and inside us, and pass news of them to the brain. They keep you informed of two worlds at once. The external environment is everything outside your skin; the internal environment is the condition of your own body, such as an empty stomach or an aching joint.
Awareness is useful only because it leads to action. A sense organ tells the brain what has changed; the brain then decides what the body should do about it.
11.1.1 The Major Sense Organs
Five organs do most of this work, and each is tuned to a different kind of change. The eyes are sensitive to light, the ears to sound, and the skin to touch. The tongue and the nose are both sensitive to chemical substances, and are responsible for taste and smell.
Notice that no organ in this list can do another’s job. A sound cannot be seen and a colour cannot be tasted, because the receiving cells of each organ respond to one kind of change only. This is why the loss of one sense cannot be made up by straining another.
Senses With No Organ Of Their Own
The five organs above are not the whole of your sensing. You also have a sense of balance, a sense of body movement, and the senses of hunger, thirst and pain. These have no obvious organ on the outside of the body, yet they are as real as sight, and they report almost entirely on the internal environment.
11.1.2 Receptors And Their Kinds
An eye does not, as a whole, feel light. Most of it is protective, focusing or nourishing tissue. The detecting is done by a small population of specialised cells buried inside the organ, and it is these cells that are called receptors.
What a receptor responds to is called a stimulus — any change in the surroundings or in the body that a receptor can detect, such as light falling on it, a vibration reaching it, or a rise in temperature. Each type of receptor answers to its own stimulus and ignores the rest.
The receptor by itself produces no sensation. It converts the stimulus into a nerve impulse, the impulse travels along a sensory nerve to the brain, and the sensation is felt only when the brain interprets that impulse. Strictly speaking, then, you see and hear with your brain, using the eyes and ears as instruments.
Five Kinds Of Receptors
Receptors are grouped by the stimulus they answer to, and FC 11.1 sets out the five kinds together with the organ that houses each.
Flowchart
FC 11.1 — The Five Kinds Of Receptors Receptors → photoreceptors (rods and cones of the retina) → vision Receptors → phonoreceptors (inner ear) → hearing and balance Receptors → chemoreceptors (tongue and nose) → taste and smell Receptors → thermoreceptors (skin) → sensation of hot and cold Receptors → mechanoreceptors (skin) → touch, pressure and vibration
Photoreceptors are the rods and cones of the retina of the eye. They are sensitive to light and are responsible for vision.
Phonoreceptors are the receptors present in the inner ear. They are sensitive to sound and are responsible for hearing and balance.
Chemoreceptors are the receptors of the tongue and the nose. They are sensitive to chemical substances and are responsible for taste and smell.
Thermoreceptors are the temperature-sensing receptors present in the skin. They are sensitive to relative changes in temperature and are responsible for the body feeling hot or cold.
Mechanoreceptors are the receptors present in the skin. They are sensitive to different mechanical stimuli such as touch, pressure and vibration.
Because thermoreceptors report relative change rather than an absolute temperature, the same water can feel warm to a cold hand and cool to a warm one. Your skin is telling you which way the temperature has shifted, not how many degrees it stands at.
11.2 The Eye And Its Protective Structures
The eye is one of the few delicate organs that the body leaves exposed on the outside. It has to stay uncovered, because light must reach it. Everything else about its position and its surroundings is an arrangement for keeping it safe while it stays open to the world.
Before looking inside the eyeball, it is worth studying the structures that guard it. Fig. 11.2-1 shows them as they appear on the face.
Diagram
Fig. 11.2-1 — External View Of The Human Eye Front view of one human eye and the region around it, drawn as a line diagram; label eyebrow, upper eyelid, lower eyelid, eyelashes, sclera (white of the eye), iris, pupil, cornea, inner angle of the eye, conjunctiva.
11.2.1 The Orbits
The two eyes are not simply attached to the front of the face. Each one sits sunk in a deep bony socket of the skull called an orbit, so that the hard rim of the skull, and not the eyeball, meets any blow that arrives from the front or the side.
Each eye is roughly the shape of a ball, and it is not fixed in its socket. Six muscles are attached to the outside of each eyeball, and by pulling in different directions they roll it up, down and sideways. This is why you can shift your gaze without turning your head.
Both orbits face forward, side by side. That arrangement matters more than it appears to, and its consequences are taken up later in the chapter.
11.2.2 Eyelids, Eyelashes And Eyebrows
The front surface of the eye is covered by two folds of skin, the upper and the lower eyelids. The upper eyelid is the movable one. Together they protect the exposed front of the eye and can shut out light completely, which is why sleep is easier in a closed room than in a bright one.
Each eyelid carries a fringe of eyelashes along its edge, curved outwards and away from the eyeball. Because they stand out from the surface, larger particles carried on the wind strike the lashes and are prevented from falling into the eye.
Above the eye sits the eyebrow. It is not really a part of the eye at all, yet it is protective in its own way: it diverts rain water, and the perspiration that trickles down from the forehead, away from the eye.
11.2.3 The Tear Glands And The Functions Of Tears
The exposed surface of the eye would dry out within minutes if it were left alone. It is kept wet by a pair of tear glands, or lacrimal glands, one above each eye, lying at the upper sideward part of the orbit. Six to twelve fine ducts lead from each gland and pour its secretion over the front surface of the eye. The arrangement is shown in Fig. 11.2-2.
Diagram
Fig. 11.2-2 — The Lacrimal (Tear) Apparatus Front view of the right eye with the tear-producing and tear-draining structures shown in position; label lacrimal gland, ducts of the lacrimal gland, front surface of the eye, lacrimal duct at the inner angle, lacrimal sac, nasolacrimal duct, nasal cavity.
The fluid is spread across the eye by blinking. Each time the eyelids sweep down, they draw a thin film of the liquid over the whole front surface, and this film works as a lubricant — a fluid that lets two surfaces slide over each other without rubbing. Without it, every blink would scrape the eye.
The same film keeps the surface clean, because the flow of liquid washes away dust particles that have settled on it. Tears also have an antiseptic property. They contain an enzyme called lysozyme, which attacks and kills germs before they can establish an infection on the moist surface.
Tears have a fourth role that has nothing to do with protection. Strong feeling — grief, and sometimes extreme joy — makes the glands pour out far more liquid than the ducts can drain, and the eyes overflow. In this sense tears also communicate emotion, which is why a face in tears is understood without a word being spoken.
11.2.4 The Tear Ducts
Tears are made continuously, so they must also be removed continuously. At the inner angle of each eye — the corner nearest the nose — the fluid drains into a small sac. From this sac a nasolacrimal duct carries it downwards into the nasal cavity. The whole route is traced in FC 11.2.
Flowchart
FC 11.2 — The Path Of Tear Fluid Tear gland in the upper outer orbit → six to twelve ducts → front surface of the eye → spread across the eye by blinking → drained at the inner angle of the eye → lacrimal sac → nasolacrimal duct → nasal cavity
This hidden passage explains something most people have noticed. Medicine dropped into the eye is soon tasted at the back of the throat, because it does not stay on the eye — it drains down the nasolacrimal duct into the nose, and from the nose into the throat.
11.2.5 The Conjunctiva
Covering the entire front part of the eye is a thin, transparent membrane called the conjunctiva. It does not stop at the eyeball. It is continuous with the lining on the inner surface of the eyelids, so that the eye and the lids are wrapped in one unbroken sheet — which is also why a contact lens or a grain of sand cannot slip behind the eyeball.
Over the cornea, the transparent window at the front of the eye, the conjunctiva is reduced to a single layer of transparent epithelium. It has to be, because a thicker covering here would blur the light on its way in.
Taken together, the bony orbit, the eyebrows, the eyelids with their lashes, the tear glands and the conjunctiva form a protective set, each guarding the eye in its own way — against blows, water, dust, drying and germs.
11.3 Structure Of The Eyeball
The eyeball is a hollow sphere, and its wall is not a single sheet. It is built of three layers laid one inside the other — the tough outer sclerotic layer, the blood-rich middle choroid layer, and the light-sensitive inner retina. Their arrangement is set out in FC 11.3, and their positions in the whole eye can be followed in Fig. 11.3-1.
Each layer does a different kind of work. The outer one protects and holds the shape, the middle one nourishes and darkens, and the inner one detects. Read in that order, the eye stops being a list of parts and becomes a machine for catching light.
Flowchart
FC 11.3 — The Three Coats Of The Eyeball Wall of the eyeball → outer sclerotic layer → transparent in front as the cornea Wall of the eyeball → middle choroid layer → ciliary body (in front) → iris → pupil Wall of the eyeball → inner retina → rods and cones → yellow spot and blind spot
Diagram
Fig. 11.3-1 — Vertical Section Of The Human Eyeball Vertical section (L.S.) of the eyeball seen from the side, with the front of the eye on the left; label conjunctiva, cornea, aqueous chamber with aqueous humour, iris, pupil, lens, suspensory ligament, ciliary body with ciliary muscles, sclerotic layer, choroid layer, retina, yellow spot, blind spot, optic nerve, vitreous chamber with vitreous humour. All labels on straight, non-crossing lead lines.
11.3.1 The Sclerotic Layer And The Cornea
The sclerotic layer, or sclera, is the outermost coat. It is made of tough fibrous tissue and is white in colour, and it is what you are looking at when you look at the white of someone’s eye. Being tough, it holds the eyeball in shape and shields the delicate layers within.
At the front, this layer changes character completely. It bulges outward and becomes transparent over the coloured part of the eye, and this clear bulging front portion is the cornea. Light must enter somewhere, and the cornea is the window through which it does.
The change is one of appearance, not of position — the cornea is simply the front continuation of the sclerotic layer. Being transparent, it carries no blood vessels of its own, since a mesh of vessels would scatter light and cloud the view.
11.3.2 The Choroid Layer
The middle coat, the choroid, is richly supplied with blood vessels. The retina is one of the most active tissues in the body, yet it cannot carry a dense mesh of vessels across its own light-sensitive face, so the choroid lies immediately behind it and nourishes it from there.
The choroid also contains a great deal of a dark black pigment called melanin. This pigment absorbs the light that has already passed through the retina, and so prevents light rays from reflecting and scattering inside the eyeball. Without it, every bright object would be ringed by a haze of stray light bouncing off the inner walls, and no image would be sharp.
The same pigment is the reason the interior of the eye is dark. The choroid does exactly what the black lining inside a camera does, and for exactly the same reason.
11.3.2.1 The Ciliary Body
Towards the front of the eye the choroid does not simply end. It thickens and expands into a ring of tissue called the ciliary body, which lies at the junction of the choroid and the iris and is itself a part of the choroid.
The ciliary body contains smooth muscles, the ciliary muscles, arranged in a circle. When these muscles contract and relax they alter the shape of the lens — the transparent focusing body lying just behind the pupil. This adjustment is what lets the eye view objects at different distances, and it is taken up in detail in 11.4.2.
11.3.2.2 The Iris And The Pupil
In front of the ciliary body, the choroid continues forward as a thin coloured curtain called the iris, named from the Latin word for rainbow. The iris partly covers the lens and leaves a circular opening at its centre. That opening is the pupil.
The pupil looks like a structure but is really a hole. When you say a person has brown, black or blue eyes, you are describing the colour of the iris, since the pupil has no colour of its own.
The iris is not a fixed curtain. It carries two sets of muscles: radial muscles, which run outwards like spokes and widen the pupil when they contract, and circular muscles, which run round the opening and narrow it. Between them they adjust the size of the pupil, and in this way regulate the amount of light entering the eye.
The adjustment needs no decision on your part. In dim light the pupil is dilated so that as much light as possible gets in; in bright light it is constricted, so that the retina is not flooded. Fig. 11.3-2 compares the two.
Diagram
Fig. 11.3-2 — The Pupil In Dim Light And In Bright Light Two front views of the same eye side by side; in A the pupil is wide, in B the pupil is small. Label iris, pupil, radial muscles and circular muscles in each; mark A — in dim light, radial muscles contract and circular muscles relax; B — in bright light, circular muscles contract and radial muscles relax.
The pattern in which these muscle fibres are arranged differs in every individual and does not change through life. This is why the iris can be used to tell one person from another.
11.3.3 The Retina
The innermost of the three layers is the retina, and it is the only one sensitive to light. Everything else in the eye exists to deliver light to this layer cleanly and in focus.
The retina contains two types of sense cells, called rods and cones after the shape of their inner ends. Both convert light into nerve impulses, but they are not two copies of one instrument — they work under different conditions and report different things.
11.3.3.1 Rods And Cones
The rod cells have rod-like inner ends. They are sensitive to dim light but do not respond to colour, so what they report is shape and movement in shades of grey. They contain a pigment called rhodopsin, also known as visual purple, and they are distributed almost throughout the retina.
The cone cells have conical inner ends. They work only in bright light and are responsible for colour vision. They contain the pigment iodopsin, also known as visual violet, and they are mostly confined to one small region of the retina, the yellow spot.
Diagram
Fig. 11.3-3 — A Rod Cell And A Cone Cell Two sensory cells of the retina drawn side by side and greatly enlarged; label rod cell, cone cell, rod-shaped inner end, conical inner end, nucleus, and the nerve fibre leaving the base of each cell.
The two kinds differ in several ways at once, and Table 11.1 sets them out feature by feature.
11.3.3.2 The Yellow Spot — The Area Of Best Vision
Rods and cones are not spread evenly. A particular spot on the retina, called the macula lutea — from macula, a spot, and luteum, yellow — or simply the yellow spot, lies at the back of the eye almost at the centre, on the horizontal axis of the eyeball.
This spot contains the maximum number of sensory cells, and cones in particular. Packing the detectors closely together is what makes fine detail possible, in the way that a picture built from many small dots shows more than one built from few. The yellow spot is therefore the region of brightest vision and of colour vision, while the rest of the retina, with fewer cones and more rods, gives a less detailed picture.
This has a consequence you can test on this page. Only the small part of a line that falls on the yellow spot is sharp at any instant, so the eyes must keep moving to bring each new group of words onto that spot.
11.3.3.3 The Blind Spot — The Area Of No Vision
Every sensory cell of the retina must send its impulses out of the eyeball. Their nerve fibres run across the inner face of the retina, converge at one point, and bundle together to leave the eye as the optic nerve.
At that point of exit there is room for nothing else. The blind spot lies to one side of the yellow spot, on the nasal side of the retina, and it contains no sensory cells at all. Light falling here produces no impulses and therefore no image, which is why it is called the point of no vision.
You can find your own blind spot using the chart in Fig. 11.3-4. Close your right eye, hold the page at arm’s length and look steadily at the circle with your left eye. From that distance you can see both the circle and the square. Now bring the page slowly towards you, keeping the left eye fixed on the circle. At about 15 cm (centimetres) from the eye the square disappears, because its image has fallen on the blind spot. Repeat it with the left eye closed and the right eye fixed on the square, and this time the circle will disappear.
Diagram
Fig. 11.3-4 — Chart For Experiencing The Blind Spot Two solid black figures printed on one horizontal line about 8 cm apart, a small square on the left and a small circle on the right, each with a short horizontal rule extending outwards on either side of it.
11.3.4 The Lens And Its Suspension
The lens lies just behind the pupil. It is a transparent, flexible, biconvex crystalline body — biconvex meaning that it bulges outwards on both faces, the shape that brings light rays together.
Its transparency comes from the way it is built. The lens is made of lens fibres, long thin cells that have lost their nuclei, so there is almost nothing inside them to obstruct or scatter light.
The lens is not fixed to the eyeball directly. It is held all round by fine fibres called the suspensory ligament, which fasten it to the ciliary body as spokes hold a hub. Because the ciliary muscles can tighten or slacken this ligament, and because the lens is itself somewhat elastic, its shape can be changed for viewing objects at different distances.
11.3.5 The Two Chambers Of The Eye
The lens, together with the structures that suspend it, divides the inner cavity of the eyeball into two chambers — the aqueous chamber in front of the lens and the vitreous chamber behind it. Neither is empty; both are filled with fluid, and it is fluid pressure that keeps the eyeball firm.
The aqueous chamber is the front chamber, lying between the cornea and the lens. It is filled with a clear watery liquid called aqueous humour, from aqueous, watery, and humour, a fluid. The aqueous humour serves in two ways: it keeps the lens moist and protects it from physical shock, and it refracts light — that is, it bends the light rays passing through it, helping to bring them to a focus.
The vitreous chamber is the larger cavity behind the lens. It is filled with a transparent, jelly-like, thicker fluid called vitreous humour, from vitreous, glassy. The vitreous humour also serves two purposes: it helps to keep the shape of the eyeball, and it protects the retina and its nerve endings.
The two fluids are easily confused, and Table 11.2 separates them.
Between them, the two fluids explain why a healthy eyeball is firm to the touch and springs back into shape after a knock.
11.4 How Do We See?
All the parts described so far do nothing on their own. Seeing happens only when they act in a fixed order, and the order matters — light must be let in before it can be focused, and it must be focused before there is anything worth sending to the brain.
11.4.1 The Four Steps In Seeing An Object
Seeing an object may be divided into four major steps.
Entry of light rays. Light rays from the object enter the eye through its transparent structures — the conjunctiva, the cornea, the aqueous humour, the lens and the vitreous humour. Every one of these must stay clear, because the light has to pass through all of them in turn.
Focusing of the image. First the curvature of the cornea converges the light rays to some extent. The lens then converges them further, so that they meet on the retina and form an image there. The image on the retina is inverted and real.
Transmission of nerve impulses from the retina to the brain. The light energy of the image produces chemical changes in the sensitive cells, the rods and cones. These changes generate nerve impulses, which travel along the optic nerve and reach the visual area of the cerebrum, where they give the sensation of sight.
Interpretation by the brain. The brain interprets the image in many ways. It sees objects upright, for instance, even though the image formed in the eye is inverted.
An image is called real when the light rays actually meet to form it, so that it could be caught on a screen. In the eye, the retina is that screen. The whole route, from the object to the sensation, is set out in FC 11.4.
Flowchart
FC 11.4 — The Path Of Light And Of The Impulse Object → conjunctiva → cornea → aqueous humour → pupil → lens → vitreous humour → retina (rods and cones) → chemical change → nerve impulse → optic nerve → visual area of the cerebrum → sensation of sight
The image itself is small, upside down and reversed left to right, as shown in Fig. 11.4-1. Nothing in the eye ever turns it the right way up.
Diagram
Fig. 11.4-1 — Formation Of The Image On The Retina Simple ray diagram of the eye in section with an upright arrow-shaped object placed to the left; show two rays from the top of the object and two from the bottom passing through the cornea and lens, crossing, and meeting on the retina to form a small inverted image. Label object, cornea, lens, retina, inverted real image.
11.4.2 Accommodation
To see an object clearly, its image must be in sharp focus on the retina of each eye. The difficulty is that objects are not all at the same distance, and rays from a near object and a far object cannot both be brought to a focus by a lens of one fixed shape.
The eye solves this by changing the lens itself. The process of focusing the eye to see objects at different distances is called accommodation, and it is brought about mainly by a change in the curvature of the elastic lens, making it thinner or fatter.
For distant vision, the lens is more flattened, that is, thinner. For near vision — nearer than 6 metres (m) — the lens becomes more convex, that is, more rounded. These changes in the shape of the lens are brought about by the ciliary muscles, and Fig. 11.4-2 shows both states.
Beyond about 6 m the rays reaching the eye are so nearly parallel that no further adjustment is needed, which is why distance vision feels effortless while close work does not.
In the normal condition, with the ciliary muscles relaxed, the lens remains stretched by the suspensory ligaments and is less convex. This flattened shape is suited to viewing distant objects.
When you look at a nearby object, the ciliary muscles, which are circular, contract and tend to pull the ciliary body slightly forward. This releases the tension on the suspensory ligament and makes it loose. The lens is elastic, so once the pull on it is released it springs into a thicker, more rounded shape.
Diagram
Fig. 11.4-2 — Accommodation Of The Eye Two ray diagrams of the eye, one above the other. A — distant object: ciliary muscles relaxed, suspensory ligament tight, lens pulled thin, rays from a distant object focused on the retina. B — near object: ciliary muscles contracted, tension in the suspensory ligament released, lens thicker and more rounded, rays from a near object focused on the retina. Label ciliary muscles, suspensory ligament, lens, retina and the point of focus in each.
Notice the sequence carefully, because it is easy to reverse: muscles contract → ligament slackens → lens thickens. A muscle contracting does not squeeze the lens; it lets the lens go.
11.4.3 Light And Dark Adaptation
When you walk from a brightly lit street into a dark hall, you can see almost nothing for a short while. Slowly your vision improves until you can make out the seats and the people around you. This improvement is called dark adaptation.
Two changes bring it about. The visual purple of the rods, which had been broken down by the bright light outside, is regenerated, so the rods become sensitive again. At the same time the pupil dilates, permitting more light to enter the eye.
The reverse happens on the way out. A person with dark-adapted eyes who steps into bright sunlight after the show experiences a dazzling effect for a short period. Within a few seconds normal viewing returns, and this recovery is called light adaptation.
Here the changes run the other way. The visual purple of the rods is bleached, which reduces their sensitivity, and the pupil constricts to reduce the amount of light entering the eye. The partial closure of the eyelids in dazzling light serves the same purpose, which is why you screw up your eyes without being told to. The two processes are compared in Table 11.3.
11.4.4 Colour Vision
Colour vision is possible only through the cones of the retina, and the cones are stimulated only in bright light. Below that level of brightness the eye still works, but it works with rods alone — and rods do not report colour.
This is why a garden looks so different by moonlight. You cannot make out the red, violet or purple flowers in it on a moonlit night, because at that light level only the rods are functioning and not the cones.
11.5 Common Defects Of The Eye
An eye works well only while four conditions hold: the image must fall exactly on the retina, the parts in front of the retina must stay transparent, the sensory cells must have their pigments, and the two eyes must point at the same object. Each of the common defects described below is a failure of one of these conditions.
11.5.1 Myopia — Short-Sightedness
Myopia, also called near-sightedness or short-sightedness, is a condition in which near objects can be seen clearly while distant objects appear blurred. In it, the image of a distant object is formed in front of the retina instead of on it, so what reaches the retina is a spreading patch of light rather than a sharp picture (Fig. 11.5-1).
There are two possible reasons for myopia. Either the eyeball is lengthened from front to back, so that the retina lies further away than it should, or the lens is too curved, so that it converges the rays too strongly. Both reasons may occur together.
Correction of myopia. The rays are being brought to a focus too soon, so they must be made to spread a little before they enter the eye. A suitable concave, or diverging, lens does this: it causes the light rays to diverge before they strike the lens of the eye, and the image then forms further back, on the retina itself. The power of such glasses is written with a minus sign.
Diagram
Fig. 11.5-1 — Myopia And Its Correction By A Concave Lens Two ray diagrams of the eye, one above the other. Upper: parallel rays from a distant object brought to a focus in front of the retina, with the eyeball drawn slightly elongated; caption position marked ‘image formed in front of the retina’. Lower: the same eye with a concave (diverging) spectacle lens placed in front of it, the rays diverging first and then focusing exactly on the retina. Label concave lens, cornea, lens, retina, point of focus.
11.5.2 Hyperopia — Long-Sightedness
Hyperopia, also called far-sightedness or long-sightedness and known by the older term hypermetropia, is the opposite condition: there is difficulty in seeing near objects. In it, the image of a near object falls behind the retina, and the picture on the retina is again blurred (Fig. 11.5-2).
The reasons are likewise the opposite of those for myopia. Either the eyeball is shortened from front to back, so that the retina lies too close, or the lens is too flat and does not converge the rays enough.
Correction of hyperopia. A convex, or converging, lens is required. It converges the rays before they enter the eye, so that they meet a little earlier — on the retina. The power of such glasses is written with a plus sign.
Diagram
Fig. 11.5-2 — Hyperopia And Its Correction By A Convex Lens Two ray diagrams of the eye, one above the other. Upper: rays from a near object brought to a focus behind the retina, with the eyeball drawn slightly shortened; caption position marked ‘image formed behind the retina’. Lower: the same eye with a convex (converging) spectacle lens placed in front of it, the rays converging first and then focusing exactly on the retina. Label convex lens, cornea, lens, retina, point of focus.
The two defects are best held apart by asking one question first — where does the image fall? Everything else follows from the answer, as Table 11.4 shows.
11.5.3 Astigmatism
Astigmatism is a defect in which some parts of an object are seen in focus while other parts appear blurred at the same time. It arises from an uneven curvature of the cornea: instead of being equally curved all round, the cornea is more sharply curved in one direction than in another, so rays lying in different planes are not focused together.
It is corrected by cylindrical lenses, which are curved in one direction only and can make up for the uneven curvature of the cornea.
Diagram
Fig. 11.5-3 — A Chart For Testing Astigmatism A radial chart on a white ground: eight equally spaced groups of parallel black lines arranged like the spokes of a wheel around a small central gap, each group at a different angle so that every direction is represented. No labels or lead lines.
11.5.4 Presbyopia
Presbyopia is a condition affecting older people, who cannot see near objects clearly. With age the lens gradually loses its flexibility. Since accommodation for near vision depends on the lens springing into a rounder shape, a stiff lens can no longer make that change, and the result is a kind of far-sightedness.
This too is corrected by a convex lens, which is why reading glasses become common in later life.
11.5.5 Cataract
In cataract, the lens itself turns opaque. Light can no longer pass through it freely, and vision is cut down — in an advanced cataract, to total blindness. Nothing is wrong with the retina; the light simply never reaches it clearly.
The defect is corrected surgically. The clouded lens is removed, and the focusing work it used to do is taken over either by spectacles with highly convex lenses, which compensate for the missing lens, or, in the newer technique, by a small plastic lens implanted behind or in front of the iris.
11.5.6 Night-Blindness
Night-blindness is a condition in which a person has difficulty in seeing in dim light, as at night. Only the rods function at low light levels, and the rods can work only if they hold their pigment. In night-blindness the visual purple of the rods is not formed, so the rods cannot respond and dim light produces no sensation.
The usual cause is a deficiency of vitamin A, which is required for the synthesis of the pigment. Bright-light vision, which depends on the cones, may remain quite normal in the same person.
11.5.7 Colour Blindness
Colour blindness is a condition in which a person, from birth, cannot discriminate between certain colours, most commonly red and green. The defect is genetic, and it lies in the cones, which are the cells responsible for distinguishing colours.
It is far commoner in males than in females, in whom it rarely occurs. The gene concerned is carried on the X chromosome, and since a male has only one X chromosome, a single altered copy is enough to produce the condition.
A person with colour blindness sees shapes, movement and brightness perfectly well. What is missing is the ability to tell certain colours apart, which is why the condition is often discovered only when it is tested for.
11.5.8 Corneal Opacities
In corneal opacity, the cornea becomes scarred and turns opaque and non-functional. Because the cornea is the window of the eye, the effects range widely — from minor irritation, through vision problems, to blindness, depending on how much of the cornea is affected and where the scarring lies.
Here also the retina is usually healthy, and this is why the condition can be treated. The defective cornea is replaced by a healthy cornea taken from a donated eye, by the grafting operation described earlier in this chapter.
11.5.9 Squint
Squint is a defect of alignment rather than of focusing. The two eyes do not point at the same object: they may converge somewhat, giving the condition commonly called cross eye, or diverge, giving the opposite condition called wide eye.
Either way, the two eyes send the brain two views that do not match, and the brain cannot combine them into one. This may cause double vision, or diplopia. Squint can be corrected by surgery and by suitable exercises of the eye muscles.
11.6 Stereoscopic Vision And After-Images
Two features of ordinary seeing are easy to overlook, because they work so smoothly that we never notice them. One comes of having two eyes instead of one. The other comes of the fact that the retina does not switch off the instant the light stops.
11.6.1 Stereoscopic (Binocular) Vision
Earlier in the chapter you saw that both orbits face forward, side by side. That position has a consequence: the two eyes look at very nearly the same scene, and their fields of view overlap over most of their width (Fig. 11.6-1).
Because the eyes are set a little apart, each of them views the object from a slightly different angle, so the two images are not identical. Both eyes focus on the object at the same time, and the brain overlaps the two slightly different images into a single view. That overlapping is what gives the three-dimensional effect and allows us to judge depth — how far away an object is, and which of two objects is nearer.
This ability is called stereoscopic vision, or binocular vision. All monkeys and apes possess it, and it is particularly well developed in human beings.
Diagram
Fig. 11.6-1 — Overlapping Fields Of Vision Of The Two Eyes View of a human head from above with an object placed in front; show the field of view of the left eye and that of the right eye as two shaded fans that cross over a wide region in front of the face. Label left eye, right eye, field of the left eye, field of the right eye, overlapping (binocular) field, object.
11.6.2 After-Images And Motion Pictures
Look at a bright object for a moment and then close your eyes, and the sensation of light persists for a short period afterwards. In the same way, if you look at a brightly coloured object and then at a dark surface, an image of that object in the same colour will persist for a moment.
This lingering impression is called a persistence image or an after-image. It lasts for about one-tenth of a second — long enough to matter, because it means the eye is always reporting an instant that has just passed.
Diagram
Fig. 11.6-2 — A Chart For Experiencing An After-Image Two panels side by side on a black ground. Left panel: a bold white silhouette of an open hand with a small mark at the centre of the palm. Right panel: plain black with a small mark at the same height as the first. No labels or lead lines.
The after-image is the basis of motion pictures. In a cinema, still pictures are projected on the screen at the rate of about 24 pictures per second. We never see the separate frames, because the after-image of one is still present in the eye when the next arrives, and each impression runs into the one that follows. The life-like continuous movement on the screen is therefore an illusion; what is really there is a rapid series of still photographs.
Television works in a similar way. The scanning beam that builds up a picture frame moves across the viewing screen so rapidly that the eye cannot keep pace with it, and we see a steady, complete picture instead of a moving point of light.
Both illusions are worth a second thought. The eye supplies a stream of brief impressions; it is the brain that turns them into a moving world. The same brain can produce vivid sights in a dream, with the eyes closed and no light entering them at all — a reminder that we see with the brain, and use the eyes as its instruments.
11.7 The Ear — Organ Of Hearing And Balance
The ear is not one organ doing one job. It is concerned with two quite different functions — hearing, and body balance — and the structures that carry them out lie side by side, deep in the bone of the skull, sharing a single nerve to the brain.
The ear has three main divisions: the outer ear, the middle ear and the inner ear. Fig. 11.7-1 shows all three in one view, and it is worth noticing at the outset how the physical form of the message changes as it passes along. In the outer ear the message travels as sound waves in air; in the middle ear it travels as movements of bone; in the inner ear it travels as ripples in a fluid.
Diagram
Fig. 11.7-1 — Structure Of The Human Ear Section through the human ear, drawn to show the three divisions in one continuous view, with the outer, middle and inner regions marked off by brackets across the top. Label pinna, auditory canal, ear drum (tympanum), malleus, incus, stapes, oval window, round window, eustachian tube, cochlea, semicircular canals, vestibule, auditory nerve. Labels on straight, non-crossing lead lines.
11.7.1 The Outer Ear
The outer ear consists of the projecting part, the pinna, which is also called the auricle, and the passage leading inwards from it, the auditory canal. The canal ends at a thin stretched membrane, the ear drum or tympanum.
The pinna is the only part of the whole organ that is visible, and it is made of cartilage covered with skin, which is why it can be bent without harm. Its curved, funnel-like shape gathers sound waves from the air and directs them into the canal.
The ear drum closes the inner end of the canal completely, so that the outer ear is a blind passage. Anything travelling further has to do so through the drum, by setting it vibrating.
11.7.2 The Middle Ear
Beyond the ear drum lies a small air-filled cavity, the middle ear. It contains three tiny bones — the malleus, the incus and the stapes, known in popular terms as the hammer, the anvil and the stirrup. Together the three are called the ear ossicles, from the Latin osseus, meaning bone, and ossicle, a little bone.
The three are linked in a chain, and the order of the links matters. The handle of the hammer is attached to the inner surface of the ear drum. Its opposite end is connected to the anvil, and the anvil in turn is joined to the stirrup. Whatever the drum does, therefore, the whole chain does.
The flat part of the stirrup fits on the oval window, a membrane-covered opening that leads into the inner ear. A second opening, the round window, also covered by a thin membrane, connects the middle ear with the inner ear.
The Eustachian Tube
The middle ear cavity is not sealed. A narrow passage called the eustachian tube connects it with the throat, so that air can move between the two.
The reason becomes clear if you think about the ear drum. A membrane can vibrate freely only when the pressure on both its faces is the same. Air outside presses on the drum through the auditory canal; air inside the middle ear presses on it from the other side. The eustachian tube keeps the two pressures equal, and the drum stays free to move.
11.7.3 The Inner Ear — The Membranous Labyrinth
The inner ear is a set of fluid-filled tubes and cavities of complicated shape, which is why it is also called the membranous labyrinth. It has three parts — the cochlea, the semicircular canals and the vestibule (Fig. 11.7-2). The first is for hearing; the other two are for balance.
Diagram
Fig. 11.7-2 — The Membranous Labyrinth Of The Inner Ear Whole view of the membranous labyrinth of the right ear, showing the three semicircular canals set at right angles to one another, the swelling at one end of each, the central chamber and the spirally coiled cochlea, with the nerve leaving it. Label semicircular canals, ampulla, utriculus, sacculus, vestibule, cochlea, oval window, round window, auditory nerve.
11.7.3.1 The Cochlea
The cochlea is spiral-shaped and looks like a snail shell, with two and a half turns. Its inner winding cavity is not a single space: it is divided lengthwise by membranes into three parallel canals, which run side by side all the way along the coil (Fig. 11.7-3).
The median canal, also called the cochlear canal, is the middle one of the three. It is filled with a fluid called endolymph, while the other two canals are filled with a fluid called perilymph.
The middle canal is the one that matters for hearing. It contains areas possessing sensory cells, forming a spiral organ called the organ of Corti. These sensory cells rest on a membrane called the basilar membrane, and the nerve fibres arising from them join the auditory nerve, which carries their impulses to the brain.
Diagram
Fig. 11.7-3 — Transverse Section Of One Turn Of The Cochlea T.S. through a single turn of the cochlea, showing the winding cavity divided by membranes into three canals lying side by side. Label vestibular canal, median (cochlear) canal, tympanic canal, basilar membrane, tectorial membrane, organ of Corti with its sensory hair cells, auditory nerve fibres. Mark which canals hold perilymph and which holds endolymph.
11.7.3.2 The Semicircular Canals
The other part of the inner ear is a set of three semicircular canals, which are crucial for maintaining balance and spatial orientation — that is, for knowing which way your body is placed and which way it is moving.
Their arrangement is the key to what they do. The three are set at right angles to each other in three different planes, so that one lies horizontal and the other two lie vertical. Between them the three planes cover every direction in which the head can turn — nodding, shaking and tilting sideways.
One end of each canal is widened to form a swelling called the ampulla, which contains sensory cells. These cells are concerned with dynamic balance, the balance of the body while it is in motion, and the nerve fibres from them join the auditory nerve.
11.7.3.3 The Vestibule — Utriculus And Sacculus
The short stem that joins the bases of the semicircular canals to the cochlea shows two parts, the utriculus and the sacculus. Together these two are termed the vestibule.
These parts also contain sensory cells, but of a different kind of usefulness. They are concerned with static balance — the balance of the body when it is stationary, as when you are standing still. Between the canals and the vestibule, therefore, the inner ear reports both the movements of the head and its resting position.
Table 11.5 gathers the three divisions of the ear together.
11.8 Functions Of The Ear
The internal ear has two sensory functions: hearing and body balance. They are carried out by different structures, they respond to different stimuli, and only the nerve to the brain is shared. It is worth taking them one at a time.
11.8.1 Hearing — The Course Of Perception Of Sound
Sound reaches us as waves travelling through air. The problem the ear has to solve is that the sensory cells which detect sound lie in fluid, deep inside the skull — and a wave in air is far too weak to set a fluid moving. Nearly everything in the outer and middle ear exists to overcome that difficulty.
The pinna collects the sound waves and conducts them through the external auditory canal. They finally strike the ear drum, which is set into vibration. The drum can move freely only because the eustachian tube has equalised the air pressure on either side of it.
The vibrating ear drum also sets the three ossicles into vibration, one after another, since they are joined in a chain. The vibration of the last ossicle, the stirrup, is amplified by the lever-like action of the first two ossicles.
Amplification is not a luxury here. The ear drum is a wide membrane and the oval window is a small one, so the force gathered over the whole drum is concentrated on a much smaller area — and it is this concentrated push, delivered by a lever, that is strong enough to move fluid.
The vibrating stirrup transmits the vibration to the membrane of the oval window, which in turn sets the fluid contained in the cochlear canals into vibration. These vibrating movements of the fluid stimulate the hair-like processes of the sensory cells of the cochlea, in the spiral organ, and the impulses so generated are transmitted to the brain by way of the auditory nerve. The whole course is shown in Fig. 11.8-1 and set out step by step in FC 11.5.
Diagram
Fig. 11.8-1 — The Course Of Perception Of Sound In The Human Ear Section through the ear as in Fig. 11.7-1, with a series of arrows tracing the path of the message: sound waves entering the pinna, along the auditory canal, striking the ear drum, through malleus, incus and stapes, at the oval window, as ripples through the fluid of the cochlear canals, and out along the auditory nerve. Label sound waves, ear drum, ear ossicles, oval window, round window, cochlea, organ of Corti, auditory nerve.
Flowchart
FC 11.5 — The Course Of Perception Of Sound Sound waves → pinna → auditory canal → ear drum vibrates → malleus → incus → stapes → membrane of the oval window → fluid of the cochlear canals set into vibration → hair-like processes of the sensory cells of the organ of Corti stimulated → nerve impulses → auditory nerve → brain → sensation of hearing
Pitch And The Range Of Hearing
Sounds differ not only in loudness but in pitch — how high or low a note sounds, which depends on how rapidly the source vibrates. Different areas of the cochlear canal are suited to sounds of different pitches, so the position along the canal at which the fluid disturbs the sensory cells tells the brain what pitch has arrived. Most of the sounds we hear are combinations of vibrations at many different rates, that is, of many different pitches at once.
The ear cannot pick up vibrations of all frequencies. Our sensory endings detect sounds from 20 to 20,000 hertz (Hz), the most sensitive range being 1000 to 4000 Hz. It is no coincidence that ordinary speech falls within that most sensitive band. Dogs can perceive sounds of even higher frequencies, which is why a whistle that sends a dog running may seem silent to you.
11.8.2 Balancing — Dynamic And Static Equilibrium
The second function of the inner ear has nothing to do with sound. As the head is turned in different directions, the fluid inside the semicircular canals is shaken. The moving fluid pushes against the sensory hair cells in the ampullae, and the nerve fibres attached to them send impulses to the brain by way of the auditory nerve (Fig. 11.8-2).
Because the three canals lie in three different planes, a turn in any direction disturbs the fluid in at least one of them. The brain compares what arrives from all three and works out which way the head has moved. These sensory cells are concerned with dynamic equilibrium, that is, with balance while the body is in motion.
Diagram
Fig. 11.8-2 — How A Semicircular Canal Detects Movement Two simple diagrams of one semicircular canal with its ampulla. A — head at rest: fluid still, sensory hair cells upright. B — head turned to the left: the wall of the canal has moved but the fluid lags behind, and the hair cells are bent in the opposite direction. Label semicircular canal, fluid, ampulla, sensory hair cells, nerve fibres, direction of head movement.
Similar sensory patches are also located in the utriculus and the sacculus. These register static balance — the position of the head with respect to gravity — and they report even when the body is perfectly still. This is how you can tell, with your eyes closed, whether you are upright or lying down.
11.9 Summary
Sense organs make us aware of the conditions of the external environment outside the body and of the internal environment within it.
The major sense organs are the eyes, ears, tongue, nose and skin, sensitive respectively to light, sound, chemical substances (taste and smell) and touch. There are also the senses of balance, body movements, hunger, thirst and pain.
A receptor is any specialised tissue or cell sensitive to a specific stimulus. The sensation itself is felt only when the impulse reaches the brain.
Receptors are of five kinds: photoreceptors (rods and cones of the retina, for vision), phonoreceptors (inner ear, for hearing and balance), chemoreceptors (tongue and nose, for taste and smell), thermoreceptors (skin, for hot and cold) and mechanoreceptors (skin, for touch, pressure and vibration).
Each eye lies in a bony socket, the orbit, and is rotated by six muscles.
The eye is protected by the bony orbit, the eyebrows, the eyelids with their eyelashes, the tear glands and the conjunctiva.
Tears are secreted by the lacrimal glands. They lubricate the surface of the eye, wash away dust particles, kill germs by means of the enzyme lysozyme, and communicate emotions.
Used tear fluid drains at the inner angle of the eye into a sac, and from there through the nasolacrimal duct into the nasal cavity — which is why eye drops are soon tasted in the throat.
The conjunctiva is a thin transparent membrane covering the front of the eyeball and continuous with the inner lining of the eyelids. Over the cornea it is reduced to a single layer of transparent epithelium.
The wall of the eyeball has three concentric layers: the outer sclerotic layer, the middle choroid and the inner retina.
The sclerotic layer is tough, fibrous and white, and gives the eyeball its shape. In front it bulges out as the transparent cornea, through which light enters.
The choroid is rich in blood vessels, which nourish the eye, and contains the black pigment melanin, which prevents light rays from reflecting and scattering inside the eye.
In front, the choroid forms the ciliary body, whose muscles alter the shape of the lens, and the iris, which encloses the circular opening called the pupil. Radial and circular muscles of the iris adjust the size of the pupil and so regulate the light entering the eye. The pattern of these muscles is unique to each individual.
The retina holds two kinds of sense cells. Rods are sensitive to dim light, do not respond to colour, and contain rhodopsin or visual purple. Cones are sensitive to bright light, are responsible for colour vision, and contain iodopsin or visual violet.
The yellow spot, or macula lutea, carries the greatest number of sensory cells, particularly cones, and is the area of best vision. The blind spot, where the nerve fibres leave the eyeball as the optic nerve, has no sensory cells and is the area of no vision.
The lens is a transparent, flexible, biconvex body of lens fibres, held by the suspensory ligament which attaches it to the ciliary body.
The lens divides the cavity of the eyeball into the aqueous chamber in front, filled with watery aqueous humour, and the vitreous chamber behind, filled with jelly-like vitreous humour. The aqueous humour keeps the lens moist, protects it from shock and refracts light; the vitreous humour keeps the eyeball in shape and protects the retina.
Seeing has four steps: entry of light through the transparent structures, focusing of the image by the cornea and lens, transmission of impulses along the optic nerve to the visual area of the cerebrum, and interpretation by the brain.
The image formed on the retina is inverted and real; the brain interprets it and we see objects upright.
Accommodation is the focusing of the eye for objects at different distances by a change in the curvature of the lens. For distant vision the lens is thinner; for near vision the ciliary muscles contract, the suspensory ligament slackens, and the elastic lens becomes thicker and more convex.
In dark adaptation, rhodopsin is regenerated and the pupil dilates. In light adaptation, rhodopsin is bleached and the pupil constricts.
Colour vision is possible only through the cones, which are stimulated only in bright light; in dim light only the rods function and colours cannot be made out.
In myopia the image of distant objects falls in front of the retina, because the eyeball is too long or the lens too curved, and it is corrected by a concave lens. In hyperopia the image of near objects falls behind the retina, because the eyeball is too short or the lens too flat, and it is corrected by a convex lens.
Astigmatism results from uneven curvature of the cornea and is corrected by cylindrical lenses. Presbyopia results from loss of flexibility of the lens with age and is corrected by a convex lens.
Cataract is opacity of the lens, treated by removing the lens and replacing its focusing power with a highly convex spectacle lens or an implanted lens. Corneal opacity is scarring of the cornea, treated by grafting a cornea from a donated eye.
Night-blindness follows non-formation of visual purple in the rods, usually from a deficiency of vitamin A. Colour blindness is a genetic condition, commoner in males, in which certain colours such as red and green cannot be distinguished.
Squint is a defect of alignment of the two eyes and may cause double vision, or diplopia; it is corrected by surgery and suitable exercises.
Stereoscopic or binocular vision is the perception of depth, produced when both eyes focus the same object and the brain overlaps their two slightly different images.
An after-image persists for about one-tenth of a second. Because of it, the 24 pictures per second projected in a cinema are seen as continuous movement.
The ear has three divisions. The outer ear consists of the pinna, auditory canal and ear drum. The middle ear contains the ear ossicles — malleus, incus and stapes — the eustachian tube, the oval window and the round window.
The eustachian tube connects the middle ear with the throat and equalises the air pressure on either side of the ear drum, allowing it to vibrate freely.
The inner ear, or membranous labyrinth, consists of the cochlea, the semicircular canals and the vestibule. The cochlea has two and a half turns and three parallel canals; the median canal holds endolymph and the other two hold perilymph, and the sensory cells of the organ of Corti rest on the basilar membrane.
In hearing, sound waves collected by the pinna vibrate the ear drum, the vibration passes along the three ossicles and is amplified by their lever-like action, the stirrup moves the membrane of the oval window, the fluid of the cochlear canals is set vibrating, the sensory cells of the organ of Corti are stimulated, and impulses travel to the brain along the auditory nerve.
Different areas of the cochlear canal answer to different pitches. The human ear detects sounds from 20 to 20,000 Hz, being most sensitive between 1000 and 4000 Hz.
Balance is registered by the inner ear. The sensory cells in the ampullae of the three semicircular canals, which lie in three different planes, give dynamic balance during motion, while the sensory patches of the utriculus and sacculus give static balance with respect to gravity.
11.10 Key Terms
- Accommodation — the focusing of the eye for objects at different distances by a change in the curvature of the lens.
- After-image (persistence image) — the impression of an object that lingers for about one-tenth of a second after the object is no longer seen.
- Ampulla — the widened end of a semicircular canal, containing the sensory cells for dynamic balance.
- Aqueous chamber — the front chamber of the eye, between the cornea and the lens.
- Aqueous humour — the clear watery fluid filling the aqueous chamber.
- Astigmatism — a defect in which parts of an object are in focus while others are blurred, caused by uneven curvature of the cornea.
- Auditory canal — the passage leading from the pinna to the ear drum.
- Auditory nerve — the nerve carrying impulses of hearing and balance from the inner ear to the brain.
- Basilar membrane — the membrane of the cochlea on which the sensory cells of the organ of Corti rest.
- Blind spot — the point on the retina where the optic nerve leaves the eyeball, having no sensory cells and therefore no vision.
- Cataract — a condition in which the lens turns opaque and vision is cut down.
- Chemoreceptors — receptors of the tongue and nose, sensitive to chemical substances.
- Choroid — the middle layer of the eyeball wall, rich in blood vessels and containing melanin.
- Ciliary body — the ring of tissue formed by the choroid in front, whose muscles alter the shape of the lens.
- Cochlea — the spirally coiled part of the inner ear that is responsible for hearing.
- Colour blindness — a genetic condition in which certain colours, commonly red and green, cannot be distinguished.
- Cones — retinal sense cells sensitive to bright light and responsible for colour vision.
- Conjunctiva — the thin transparent membrane covering the front of the eyeball and lining the inner surface of the eyelids.
- Cornea — the transparent bulging front part of the sclerotic layer, through which light enters the eye.
- Corneal opacity — scarring of the cornea that turns it opaque and non-functional.
- Dark adaptation — the adjustment of the eyes on moving from a brightly lit area to a dimly lit one.
- Diplopia — double vision, in which one object is seen as two.
- Dynamic balance (dynamic equilibrium) — the sense of the body’s position while it is in motion.
- Ear drum (tympanum) — the thin membrane at the inner end of the auditory canal that is set vibrating by sound waves.
- Ear ossicles — the three small bones of the middle ear, the malleus, incus and stapes, taken together.
- Endolymph — the fluid filling the median canal of the cochlea.
- Eustachian tube — the tube connecting the middle ear with the throat, which equalises air pressure on either side of the ear drum.
- Eyebrow — the arch of hair above the eye that diverts rain water and perspiration away from it.
- Eyelashes — the outward-curving hairs on the edge of each eyelid that keep larger particles out of the eye.
- Eyelids — the upper and lower folds of skin that cover and protect the front of the eye and can shut out light.
- Hyperopia — long-sightedness, in which the image of near objects falls behind the retina.
- Incus — the middle ear ossicle, the anvil, lying between the malleus and the stapes.
- Iodopsin (visual violet) — the light-sensitive pigment of the cones.
- Iris — the coloured ring of the eye, an extension of the choroid, enclosing the pupil.
- Lacrimal glands (tear glands) — the glands at the upper outer part of each orbit that secrete tears.
- Lens — the transparent, flexible, biconvex body behind the pupil that focuses light on the retina.
- Lens fibres — the long thin transparent cells, without nuclei, of which the lens is built.
- Light adaptation — the adjustment of the eyes on moving from a dimly lit area to a brightly lit one.
- Lubricant — a fluid that allows two surfaces to slide over each other without rubbing.
- Lysozyme — the enzyme in tears that kills germs on the surface of the eye.
- Malleus — the ear ossicle, the hammer, attached to the inner surface of the ear drum.
- Mechanoreceptors — receptors of the skin, sensitive to touch, pressure and vibration.
- Melanin — the black pigment of the choroid that prevents light from reflecting and scattering inside the eye.
- Membranous labyrinth — the inner ear as a whole, consisting of the cochlea, semicircular canals and vestibule.
- Myopia — short-sightedness, in which the image of distant objects falls in front of the retina.
- Nasolacrimal duct — the duct that carries tear fluid from the inner angle of the eye into the nasal cavity.
- Night-blindness — difficulty in seeing in dim light, caused by non-formation of visual purple in the rods.
- Optic nerve — the nerve formed by the fibres of the retinal sensory cells, carrying impulses of sight to the brain.
- Orbit — the deep bony socket of the skull in which the eyeball lies.
- Organ of Corti — the spiral sensory organ of the cochlea, containing the sensory cells for hearing.
- Oval window — the membrane-covered opening on which the stapes rests, leading into the inner ear.
- Perilymph — the fluid filling the two outer canals of the cochlea.
- Phonoreceptors — receptors of the inner ear, sensitive to sound.
- Photoreceptors — receptors sensitive to light, namely the rods and cones of the retina.
- Pinna — the projecting part of the outer ear that collects sound waves.
- Pitch — how high or low a sound is, depending on the rate at which the source vibrates.
- Presbyopia — difficulty in seeing near objects in older people, due to loss of flexibility of the lens.
- Pupil — the circular opening at the centre of the iris through which light enters the eye.
- Receptor — any specialised tissue or cell sensitive to a specific stimulus.
- Retina — the innermost, light-sensitive layer of the eyeball wall, containing the rods and cones.
- Rhodopsin (visual purple) — the light-sensitive pigment of the rods.
- Rods — retinal sense cells sensitive to dim light but not to colour.
- Round window — the second membrane-covered opening between the middle ear and the inner ear.
- Sacculus — the lower part of the vestibule, containing sensory cells for static balance.
- Sclerotic layer (sclera) — the tough white outer layer of the eyeball wall.
- Semicircular canals — the three fluid-filled canals of the inner ear, set in three planes, concerned with dynamic balance.
- Sense organ — an organ that detects changes outside or inside the body and informs the brain of them.
- Squint — a defect in which the two eyes are not aligned on the same object.
- Stapes — the ear ossicle, the stirrup, whose flat part fits on the oval window.
- Static balance — the sense of the body’s position when it is stationary.
- Stereoscopic vision (binocular vision) — the perception of depth produced when the brain combines the slightly different images of the two eyes.
- Stimulus — any change in the surroundings or in the body that a receptor can detect.
- Suspensory ligament — the ring of fibres holding the lens in position and attaching it to the ciliary body.
- Thermoreceptors — receptors of the skin, sensitive to relative changes in temperature.
- Utriculus — the upper part of the vestibule, containing sensory cells for static balance.
- Vestibule — the part of the inner ear between the cochlea and the semicircular canals, made of the utriculus and sacculus.
- Vitamin A — the vitamin required for the synthesis of visual purple, whose deficiency causes night-blindness.
- Vitreous chamber — the larger cavity of the eyeball behind the lens.
- Vitreous humour — the transparent jelly-like fluid filling the vitreous chamber.
- Yellow spot (macula lutea) — the small area of the retina crowded with cones, giving the sharpest vision.
11.11 Points To Ponder
Both the eye and the ear send their messages to the brain as nerve impulses, and one impulse is much like another. How, then, can the brain tell a sound from a sight?
Every retina has a blind spot, yet nobody walking about sees a hole in the world. What might the second eye, and the retina surrounding the blind spot, be contributing that keeps the gap from being noticed?
The choroid of the human eye is lined with black pigment that absorbs stray light, while many animals active at night have a reflecting layer in the same position. What is being gained, and what given up, in each of the two arrangements?
Rods regenerate their pigment rapidly and cones far more slowly. Think about an animal that hunts at dawn and dusk, when the light is changing every few minutes. Which of the two would serve it better, and why?
You stop noticing the feel of your clothes within minutes of dressing, and the smell of a room soon after entering it, although the stimulus has not gone away. What advantage could there be for an animal whose receptors report change rather than steady conditions?
The middle ear is filled with air and the inner ear with fluid, and the whole chain of ossicles exists to carry a vibration across that boundary. Fishes hear without any middle ear at all. What difference in their surroundings makes that possible?
11.12 Objective Questions
Select the most appropriate option in each case.
- The layer of the eyeball that prevents light rays from reflecting and scattering inside it is the: (1 mark)
- conjunctiva
- sclerotic layer
- choroid
- retina
- When the eye is focused on a nearby object: (1 mark)
- the ciliary muscles relax and the lens becomes thinner
- the ciliary muscles contract and the lens becomes thicker
- the suspensory ligament tightens and the lens becomes thinner
- the suspensory ligament tightens and the lens becomes thicker
- In a person with hyperopia, the image of a near object is formed: (1 mark)
- on the yellow spot
- on the blind spot
- in front of the retina
- behind the retina
- The ear ossicle whose flat part fits on the oval window is the: (1 mark)
- malleus
- incus
- stapes
- tympanum
- Dark adaptation is brought about by: (1 mark)
- bleaching of rhodopsin and constriction of the pupil
- regeneration of rhodopsin and dilation of the pupil
- synthesis of iodopsin and dilation of the pupil
- bleaching of iodopsin and constriction of the pupil
- The part of the inner ear that registers the position of the body while it is standing still is the: (1 mark)
- cochlea
- ampulla of a semicircular canal
- utriculus and sacculus
- organ of Corti
The following questions are assertion–reason based. Choose your answer from the codes given below. (a) A is true and R is false (b) A is false and R is true (c) Both A and R are true (d) Both A and R are false
Assertion (A): A person with cataract may lose their sight although the retina is perfectly healthy. Reason (R): The lens turns opaque, so light can no longer pass clearly through it to the retina. (1 mark)
Assertion (A): The blind spot contains neither rods nor cones. Reason (R): It is the point at which the nerve fibres of the retina converge and leave the eyeball as the optic nerve. (1 mark)
Assertion (A): Swallowing relieves the blocked feeling in the ears while an aeroplane is descending. Reason (R): The eustachian tube opens and equalises the air pressure on either side of the ear drum. (1 mark)
Match the terms in Column I with those in Column II and write down the matching pairs. (5 marks)
| Column I | Column II |
|---|---|
| a. Lysozyme | i. Holds the lens in position |
| b. Melanin | ii. Sensory cells for hearing |
| c. Ampulla | iii. Kills germs on the surface of the eye |
| d. Organ of Corti | iv. Sensory cells for dynamic balance |
| e. Suspensory ligament | v. Equalises air pressure across the ear drum |
- Fill in the blanks. (4 marks)
- The light-sensitive pigment present in the rods is ____________.
- The chamber lying between the cornea and the lens is filled with ____________.
- The eye defect corrected by a cylindrical lens is ____________.
- The three semicircular canals of the inner ear lie in ____________ different planes.
- State whether the following statements are true or false. (3 marks)
- The image formed on the retina is inverted and real.
- Cones are more numerous in the retina than rods.
- The pinna is a part of the middle ear.
- Name the following. (5 marks)
- The membrane of the cochlea on which the sensory cells rest.
- The layer of the eyeball that nourishes it.
- The light-sensitive pigment of the cones.
- The duct that carries tear fluid into the nasal cavity.
- The condition in which a single object is seen as two.
- Pick the odd one out in each set and give a reason for your choice. (3 marks)
- Malleus, incus, stapes, cochlea
- Cornea, lens, aqueous humour, choroid
- Myopia, hyperopia, presbyopia, night-blindness
11.13 Check Your Concepts
Define accommodation. (2 marks)
State the two functions of the vitreous humour. (2 marks)
Give a reason why medicine dropped into the eye is soon tasted at the back of the throat. (2 marks)
Name the two pigments found in the sensory cells of the retina, and state the cell in which each occurs. (2 marks)
Give a reason why a person who walks into a dark hall from bright sunlight can see nothing for a short while. (2 marks)
Differentiate between the yellow spot and the blind spot, stating the basis of difference as the presence of sensory cells. (3 marks)
Why are the colours of flowers not made out on a moonlit night, although the flowers are as coloured then as they are by day? (3 marks)
What would happen to a person’s hearing if the three ear ossicles became fixed and could no longer move? Explain your answer. (3 marks)
Give a reason for each of the following. (3 marks)
- The inner surface of the choroid is black.
- The eyelids blink every few seconds.
- The blind spot is described as the area of no vision.
Describe the changes that take place in the ciliary muscles, the suspensory ligament and the lens when a person shifts their gaze from a distant object to a nearby one. (4 marks)
Differentiate between light adaptation and dark adaptation with reference to (i) the pigment that is regenerated and (ii) the size of the pupil. (4 marks)
Name the eye defect produced by each of the following, and the type of lens used to correct it. (4 marks)
- The eyeball is lengthened from front to back.
- The lens becomes too flat.
- The lens loses its flexibility with age.
- The cornea is unevenly curved.
Explain the four steps involved in seeing an object, from the entry of light to the sensation of sight. (5 marks)
Describe the course of perception of sound in the human ear, from the pinna to the brain. (5 marks)
11.14 Figure-Based Questions
Study the vertical section of the eyeball shown in Fig. 11.3-1. (5 marks)
- Redraw the outline of the eyeball and label the three layers of its wall.
- Label the lens, the suspensory ligament and the optic nerve on your drawing.
- State one function of the layer you have labelled as the innermost.
In Fig. 11.3-1, name the layer that corresponds to the black lining inside a camera, and state the function it performs in the eye. (2 marks)
Fig. 11.5-1 shows an eye defect and the lens used to correct it. (4 marks)
- Name the defect shown.
- State where the image of a distant object is formed in such an eye.
- Give the two possible reasons for this defect.
- Name the type of lens used to correct it, and state how it acts on the light rays.
Fig. 11.4-2 shows the eye adjusted for two different distances. (3 marks)
- Which of the two diagrams shows the eye focused on a near object?
- Give one reason from the figure for your answer.
- State the condition of the ciliary muscles and of the suspensory ligament in that diagram.
Fig. 11.7-2 shows the membranous labyrinth of the inner ear. (5 marks)
- Draw a neat labelled diagram of the membranous labyrinth.
- Name the structure responsible for hearing.
- Name the part that contains the sensory cells for static balance.
- State why the three canals shown are set in three different planes.
Fig. 11.8-1 traces the path of sound through the ear. (3 marks)
- Name, in order, the structures through which the vibration passes between the ear drum and the fluid of the cochlea.
- Name the structure at which the vibration is passed from bone to fluid.
- Name the nerve that carries the impulses to the brain.